Navitoclax is an effective pro-apoptotic agent against cancer cells. Uncontrolled cell survival is a hallmark of pathological angiogenesis in cancer and could promote plaque instability that contributes to atherosclerosis progression owing to intraplaque neovascularization. Cancer cell inhibition by navitoclax can restrain metastasis; therefore, it is possible to reduce endothelial cells survival and is expected to confer a novel therapeutic strategy for advanced atherosclerosis in regards to plaque instability. However, regulation of endothelial cell activity by navitoclax is yet to be examined. This study will analyze navitoclax efficacy in modulating human umbilical vein endothelial cells (HUVEC) viability, proliferation, migration and angiogenesis. Navitoclax concentrations ranging from 0.2 to 3.0µM at four-time points; 18-, 24-, 48- and 72-hours were used for MTT assay. The IC50 value for 18-hours post-treatment was undefined due to low efficacy at a limited time. While for 24-, 48- and 72-hours, the IC50 values were 0.91µM, 0.72µM, and 0.12µM, respectively. Navitoclax potency to inhibit HUVEC viability increased as the treatment time elevated. 0.9µM navitoclax for 24 hours treatment was selected for subsequent experiments. Next, 25 ng/ml IL-3 was used to induce the in-vitro angiogenesis model within 6 hours. Expectedly, navitoclax reduced the tube formation and migration of HUVEC induced by IL-3 in consistent with CXCL-8 released and MMP-3 expression in the cell. However, HUVEC proliferative activity was not affected by navitoclax treatment, as well as the BCL-2 gene expression. Therefore, an anti-angiogenic effect of navitoclax on HUVEC by preventing the cell motility through CXCL-8 and MMP-3 mechanism is determined.
Research Article
Navitoclax mediates Interleukin-3 induced human umbilical vein endothelial cells survival and angiogenesis
https://doi.org/10.21203/rs.3.rs-2759691/v1
This work is licensed under a CC BY 4.0 License
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ABT-263
angiogenesis
cell viability
HUVEC
migration
Neovascularization or new blood vessel development occurs through either angiogenesis, arteriogenesis or vasculogenesis. Angiogenesis is an expansion of the existing vasculature through the sprouting of endothelial cells that drives neovascularization. Activation, migration, proliferation and maturation of unique precursor cells are the underlying processes of this angiogenic sprouting [1]. This dynamic process is modulated by various growth factors, cytokines as well as extracellular matrix (ECM) activity. Under the physiological situation, angiogenesis helps to repair the damaged tissues [2, 3], re-establish the blood supply to restore nutrients delivery [4], and aids the placenta development [5, 6]. In certain circumstances, however, new vessel regeneration drives the pathogenesis of various life-threatening diseases, including cancer and diabetic retinopathy [7, 8]. Additionally, recent findings reported that intimal neovascularization within atherosclerotic plaques might promote plaque destabilization and lead to fatal cardiovascular events such as stroke and heart attack [9, 10]. This is due to abnormal vascular branching, immature and “leaky” endothelial tube linings that will permit inflammatory mediators as well as blood constituents’ infiltration into the lesion area [11]. Regulation of cell survival and proliferation is fundamental in stimulating other signaling proteins and growth factors for new vessel development.
Proliferation and migration of endothelial cells contribute to angiogenesis, and this process is regulated by various cytokines, including interleukin-3 (IL-3) [12, 13]. IL-3 is produced by active T cells, natural killer cells, mast cells and megakaryocytes, where at some point IL-3 will bind to the receptor on the endothelial cells [14]. Initially, the ability of IL-3 was identified to stimulate the production, development and function of hemopoietic cells, including mast cells, basophils, neutrophils, eosinophils, macrophages and erythrocytes [12]. Nevertheless, the role of IL-3 beyond hematopoiesis was reported, whereby it could regulate endothelial cells proliferation [15], motility and angiogenic response [13] that contributes to chronic inflammation that is related with endothelial cells [16]. This evidence was supported by discovering IL-3 receptor (IL-3 R) alpha and beta chains within the plasma membranes of endothelial cells [17, 18]. IL-3 was able to sustain immunologically regulated chronic inflammatory response in pathological conditions via endothelial-leukocyte adhesion molecule 1 (ELAM-1) activation [15] and endothelial cells motility mediated by IL-3 was demonstrated to recruit platelet-activating factor (PAF) activity [13]. Additionally, IL-3 and its receptors play an essential role in cancer metastasis as it is secreted by leukemia cells that enhance the survival and proliferation of cancer cells, ultimately contributing to the development of cancer pathology [12, 19]. Besides, tumor-derived endothelial cells induced by IL-3 autocrine signal exhibited rapid turnover rate and high expression of inflammatory genes that promote vessel growth compared to normal endothelial cells [13, 20]. Apart from that, IL-3 is involved in cardiovascular disease (CVD) pathology due to its ability to act as pro-inflammatory and pro-angiogenic agents. Evidence reported a potential paracrine signal of IL-3 in CVD via endothelial cell-derived extracellular vesicles release may hinder the cardioprotective effect due to changes in protein cargo [16]. Furthermore, a previous study showed high expression of IL-3 R alpha within intraplaque neovessels in advanced human carotid plaques, hence revealing the involvement of IL-3 in atherogenesis [18]. Ergo, IL-3 is utilized in this study to establish the in-vitro angiogenesis model that mimics the intraplaque neovascularization process.
Interleukin-8 (IL-8, or CXCL-8) is a chemokine with a distinguishing CXC amino acid pattern that was initially identified for its leukocyte chemotactic activity [21]. CXCL-8 is reported to induce tumorigenic and proangiogenic activities. CXCL-8 has biological activities independent from and in addition to its well-known role in controlling inflammatory reactions. Particularly relevant to cancerCXCL-8 is a potent angiogenesis mediator. There is growing evidence that inflammation and fibroproliferation have a role in the etiology of atherosclerosis [22, 23]. Angiogenesis has also been observed within atherosclerotic plaques, suggesting that it may contribute to the pathophysiology of plaque formation [11, 24]. CXCL-8 is over-expressed in human coronary artery plaque samples compared to control samples from internal mammary arteries without atherosclerosis, where it co-localized with factor VIII-related antigen expression on endothelial cells in coronary atherectomy specimens and is the major mediator of net angiogenic activity of the plaque in the rat cornea micro-pocket assay [25]. Therefore, association of CXCL-8 expression with navitoclax effect on the in-vitro angiogenesis model is going to be determined in this study.
The expression of matrix metalloproteinase (MMP) has been proposed to involve in the angiogenesis that links to the advancement of plaque growth in atherosclerosis. MMPs are a family of structurally related proteinases that widely known for their ability to degrade extracellular matrix (ECM) and can also process bio-active molecules such as growth factors. However, MMP expression is not conventionally present but is usually controlled by: (1) cytokines, growth factors, and cell ± cell and cell ± matrix interactions that control gene expression; (2) activation of its proenzyme form; and (3) the presence of MMP inhibitors namely tissue metalloproteinases (TIMP) inhibitors [26]. One study has suggested that human stromelysin promoter variation is associated with the development of coronary atherosclerosis[27]. Further studies have reported that a stromelysin-1 promoter also known as MMP-3 plays an important role in regulating stromelysin-1 gene expression and may be involved in the pathological development of atherosclerosis [28]. Furthermore, MMP-3 expression has been associated with cell activity such as migration and causing the development of angiogenesis [18]. MMPs are suggested as a new biomarker to atherosclerotic plaque instability. Based on the previous findings reported, navitoclax effect on the expression of MMP-3 which is one of the stromelysin group members is worth to be observed.
BCL-2 family proteins have been reported as the intrinsic key modulator of cell survival or death [29, 30]. A multicomplex interaction among BCL-2 family proteins, comprised of pro- and anti-apoptotic mediators, will determine cell fate [31]. Navitoclax displays a pro-apoptotic response towards cancer cells by targeting several BCL-2 family proteins, including BCL-2, Bcl-xL and Bcl-w [32, 33]. Navitoclax has entered human clinical trials for treating small cell lung cancer [34, 35], chronic lymphocytic leukemia [36], and other lymphoid malignancies [37]. Recently, it shows promising outcomes in preclinical studies of breast cancer [38] and oral tumors [39]. Apart from that, navitoclax potency as a single agent and in combination with other chemotherapeutic agents was reported to effectively ameliorate cancerprogression in our published review [40]. In cancer, excessive proliferation of tumor cells and neo-angiogenesis lead to tumor metastasis, thus worsening the situation. Similarly, in atherosclerosis, a new blood vessel formation will deteriorate the plaque stability and cause it to rupture, eventually causing myocardial infarction and thrombosis [10]. High expression of BCL-2 anti-apoptotic proteins is reported in cancer cells, making the tumor resistant to conventional chemotherapy [41, 42]. Plus, a study on cardiovascular disease demonstrated an abundant expression of BCL-2 pro-survival proteins that contributes to myocyte replication [43]. Due to that, they are potentially an attractive target for drug development to obstruct cell survival.
The atherosclerosis pathogenesis is comparable to cancer in abnormal cell proliferation, leading to intraplaque angiogenesis [44, 45]. Thus, an initiative to include navitoclax in atherosclerosis treatment development is predicted to produce a promising therapeutic outcome. However, most published reports demonstrated the pharmacology and clinical applications of navitoclax only on cancer cells. Hence, there is limited evidence of navitoclax potency in reducing primary cells such as endothelial cells viability mainly. Considering that uncontrolled endothelial cell proliferation is the underlying process of angiogenesis, which leads to atherosclerotic plaque instability, a study to evaluate the navitoclax potency to inhibit the survival of human endothelial cells will be carried out. Subsequently, navitoclax inhibitory effect on endothelial cell angiogenesis is conducted in the presence of IL-3. Additionally, modulation of endothelial cell proliferation and motility by navitoclax is being carried out further to elucidate navitoclax mechanisms in deteriorating in-vitro blood vessel formation. Lastly, the gene expressions of CXCL-8, MMP-3 and BCL-2 are investigated to determine the association of navitoclax effect with the target gene on each cell biological assays; angiogenesis (CXCL-8), migration (MMP-3), proliferation (BCL-2). We hypothesize that the ability of navitoclax to inhibit human endothelial cells survival is augmented with increasing dosage. Besides, the angiogenesis is diminished by navitoclax through the downregulation of endothelial cell motility and proliferation.
- Materials and chemical reagent
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Cell culture: Basal endothelial cell medium (ECM) (Cat Number: 1001-b; Sciencell, USA), endothelial cell growth supplement (ECGS) (Cat Number: 1001-b; Sciencell, USA), M199 media (Cat Number: 31100-027; Gibco, USA), collagenase I (Cat No: SCR103; Merk, Jerman), fetal bovine serum (FBS) (Cat No: F7524; Sigma Aldrich, non-USA), penicillin-streptomycin (pen/strep) (Cat No: LM-A4118; Biosera, France), phosphate-buffered saline (PBS) tablet (Cat No: P4417-100; Sigma Aldrich, USA), trypsin-EDTA (0.25% trypsin, 1 mM EDTA) with phenol red (Cat No: 25200-056; Gibco, USA).
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Cell assay: Thiazolyl blue tetrazolium bromide (MTT) powder (Cat No: T-030-1; Gold BioTechnology, USA), navitoclax (Cat No: FN16901; Carbosynth, USA), dimethyl sulfoxide (DMSO) (Cat No: D4540; Sigma Aldrich, USA). Matrigel ™ reduced with growth factor (Cat. No. 354230, Scientific Lab Supplies), IL-3 (Cat no: AF1418061; R&D systems, USA), basal media of endothelial cells (Cat. No. 354230; Sciencell, USA). The Calbiochem® BrdU Cell Proliferation Assay (Cat no. QIA58-200TEST; Merck, Germany)
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RT-qPCR: innuPREP RNA mini kit 2.0 (Cat no: AJG#845-KS-2040050; Jena Analytics, Germany), ReverTra Ace qPCR Rt Master Mix with gDNA Remover (Cat no: FSQ 301; Toyobo, Japan), Thunderbird SYBR qPCR Mix kit (Cat no: QPS 201; Toyobo, Japan), forward and reverse primers; MMP-3, BCL-2 and CXCL-8.
i. Primary endothelial cells isolation
Endothelial cells of human umbilical cord veins were detached by following the procedures from previous studies [46, 47]. The umbilical cords were obtained from the labor ward of the Department of Obstetrics and Gynaecology, Hospital Canselor Tuanku Mukhriz (HCTM). Ethical approval for this study was given by the Ethical Research Committee of Universiti Kebangsaan Malaysia (Reference number: UKM PPI/111/8/JEP-2020-006), and written consent was obtained from all donors before delivery. The cells were cultured in sterile complete ECM consisting of basal medium, 1% ECGS, 1% pen/strep and 5% FBS. They were stored in a CO2 incubator at 37˚C containing 5% CO2. The media was replaced when it turned yellowish or within 48 hours until the cells reached 80% confluency. The authenticity of HUVEC was confirmed in our lab by the morphology of cobblestone-shaped endothelial cells and CD31 marker expression via immunocytochemistry. HUVEC from passage 3 to passage 5 were used in this study.
ii. Cell viability assay
3-(4,5-Dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was conducted to analyze the number of viable cells after the treatment [48,49]. Initially, 2 x 105 cells/well were seeded in 96 well-plates until 80% confluency. An untreated group and a vehicle group containing 0.01 µM DMSO were included in this study. A range of navitoclax concentrations between 0.2 µM to 3.0 µM was added into the well in triplicates. The cells were placed in a CO2 incubator at 37˚C for different treatment periods; 18, 24, 48 and 72 hours. After that, the cells were incubated with MTT solution for 3 hours to allow the formation of formazan crystals representing viable cells. Next, dimethyl sulfoxide (DMSO) was added to dissolve the remaining crystals. The absorbance was quantified by using the spectrophotometric plate reader at 570 nm. The equation below was applied to calculate the percentage of cell viability:
The line of the best-fit curve was generated by Graphpad Prism 8.4.3 software, using the non-linear regression (curve fit) of dose vs response. The drug dose that reduced 50% of cell viability (IC50) was determined by Prism through the best-fit curve generated.
iii. Angiogenesis assay
Tube formation assay was conducted to study the endothelial cell angiogenesis by using a µ-Slide angiogenesis system. First, 10µl Matrigel ™ reduced with growth factor was placed in a well and was allowed to polymerize at 37°C for an hour. HUVEC were seeded in the well containing Matrigel at a density of 2 x 104 with the treatment of 25ng/ml IL-3 and 0.9 µM navitoclax. Tube formations were allowed to occur for 8 hours, and cell images were captured every 2 hours. The tube formation potential is calculated in each sample as the total length of branching using an Angiogenesis Analyzer plugin from a computer-assisted image analysis system (ImageJ software). Images from at least four random fields at 5x magnification are examined from an independent experiment.
iv. Migration assay
The in-vitro scratch test method was carried out to determine endothelial cell migration after navitoclax treatment by referring to the protocol from previous study [50]. 5 x 104 cells were seeded in a 24-well plate and incubated until they reached 90% confluency. The cells were then scraped by 200µl pipette tips and the floating cells were washed with PBS immediately. After that, new media with treatment was added into the well. A constant pressure was applied during scraping to reduce variation of wound size. Three wound areas in each well were set and captured using a live inverted microscope at 0 hour until 24 hours. The wound areas were determined using an ImageJ software. Then, the area of wound closure percentage was calculated using this formula;
v. Proliferation assay
This assay was conducted using a colorimetric bromodeoxyuridine (BrdU) cell proliferation assay kit. 2 x 104 cells were seeded in 96-well plates and incubated until they reached 90% confluency. Then, new media was added along with the treatment. BrdU Label (1:2000) was added into the well after 4 hours the treatment started to allow 20 hours incubation. After the treatment, the cells were incubated with 200 µl of fixative solution for 30 minutes at room temperature. 1X Anti-BrdU Antibody was incubated with the cells for 1 hour at room temperature. The cells were washed three times with 1X Wash Buffer before adding 1X Peroxidase Goat Anti-Mouse IgG HRP Conjugate for 30 minutes incubation. After that, the washing step was repeated again for three times before flooding the entire plate with distilled water. Substrate Solution was put into the well and incubated for 15 minutes in the dark at room temperature. Lastly, Stop Solution was added before measuring the absorbance at 450–540 nm using spectrophotometric plate reader. The percentage of BrdU positive cells was calculated using excel.
vi. RT-qPCR
3 x 105 HUVECs were seeded in 6 well plates and the treatments were administered once the cell had reached 90% confluency. RNA extraction was performed using innuPREP RNA mini kit 2.0. The concentration and purity of RNA were read using a nanodrop spectrophotometer machine. Next, PCR reverse transcription process was performed using ReverTra Ace qPCR Rt Master Mix with gDNA Remover to convert 500 µg RNA. For DNAse I reaction process, a mixture of 4x DN Master Mix and ‘gDNA Remover’ (2 µl), and a balanced volume between RNA sample and RNA-free water (6 µl) were added into a 200 µl tube and incubated in thermal cycler machine (Bio-Rad, Model CFX Connect, USA) for 5 minutes at a temperature of 37 ℃. Next, 2 µl 5x DN Master Mix was added into the tube for DNAse II reaction process whereby the sample was further incubated at 37 ℃ for 15 min and heated at 98 ℃ for 5 min. Samples were stored at -20℃.
Real-time qPCR was carried out using the Thunderbird SYBR qPCR Mix kit. The primers to be used in this study are MMP − 3, BCL-2, CXCL8 and 36B4 (housekeeping gene). The sequence for the primers can be referred to Table 1. The RT-qPCR reactions consist of 10 µl Thunderbird SYBR qPCR mix, 0.6 µl forward primer (6 pmol), 0.6 µl reverse primer (pmol), 2 µl cDNA (0.5 µg) and 6.8 µl RNA-free water were prepared in 200 µl qPCR tube. The tube was then put into the CFX 96 Real-Time PCR Detection System (Bio-Rad, CA, USA RT-qPCR machine) and the protocol was set as follows; pre-denaturation at 95 ℃ for 60 seconds, 40 cycles of denaturation at 95 ℃ for 15 seconds, annealing at 60 ℃ for 60 seconds; the data collection was set at the annealing step. Lastly, the relative changes in target gene expression were analyzed using the 2− Delta−Delta Ct method (2−∆∆Ct).
|
Primer |
Primer sequence |
Product Size (bp) |
|---|---|---|
|
MMP-3 |
Forward: GATCCTGCTTTGTCCTTTGATGCT Reverse: CTGAAGGAAGAGATGGCCAAAATG |
145 |
|
BCL-2 |
Forward: GGTGGGGTCATGTGTGTGG Reverse: CGGTTCAGGTACTCAGTCATCC |
89 |
|
CXCL8 |
Forward: GAGAGTGATTGAGAGTGGACCAC Reverse: CACAACCCTCTGCACCCAGTTT |
112 |
|
36B4 |
Forward: GCCAGCGAAGCCACGCTGCTGAAC Reverse: CGAACACCTGCTGGATGACCAGCCC |
76 |
vii. Statistical analysis
Results are shown as mean ± SEM. IBM SPSS statistics version 26 was used to carry out statistical analysis. The mean difference between control and treatment groups was compared statistically using an independent sample t-test. One-way analysis of variance (ANOVA) test was run to compare the mean difference between treatment groups, followed by Tukey post-hoc test. The difference was considered significant at p < 0.05.
Navitoclax concentration against cell viability
The navitoclax effect against HUVEC viability at four different time points (i.e., 18, 24, 48 and 72 hours) were investigated via MTT assay. The navitoclax concentrations used were 0.2 µM, 0.4 µM, 0.6 µM, 0.8 µM, 1.0 µM, 1.5 µM, 2.0 µM and 3.0 µM. There was no significant difference in the cell viability between untreated and vehicle control groups, hence the treated groups were normalized to the untreated group. On the other hand, the mean of cell viability between treated and untreated groups at different treatment periods is significantly different, p < 0.05 (Fig. 1). The highest navitoclax concentration which is 3.0 µM reduced the 100% cell viability to 73.65 ± 9.40%, 12.34 ± 6.50%, 0.37 ± 0.16% and 2.09 ± 1.28% after 18, 24, 48 and 72 hours respectively. Besides, at the lowest navitoclax concentration (i.e., 0.2 µM), the 100% of cell viability decreased more than 50% after 72 hours of treatment.
A graph of log dose against cell survival at different time points was plotted to compare the potency of different treatment periods in reducing cell viability (Fig. 2). As the treatment time increased, the log dose against cell survival curves was shifted to the left. There was a statistically significant mean difference in cell viability between treatment times as determined by one-way ANOVA, p < 0.001. Regarding the Tukey post-hoc test, the cell viability was significantly reduced in 48 hours (50.18 ± 11.55%, p < 0.05) and 72 hours (24.45 ± 10.12%, p < 0.001) post-treatment compared to the 18 hrs (85.37 ± 2.26%, p > 0.05) post-treatment group.
From the dose-response graph, IC50 values for each time point were deduced. The IC50 value showed a decreasing trend as the treatment period was increased. There was no IC50 detected after 18 hours treatment; even at the highest concentration used, the cell viability observed was still higher than 50%. However, for 24 hours of treatment, the IC50 value was identified at 0.91 µM, then after 48 hours was 0.72 µM, and lastly for 72 hours was 0.12 µM. The morphological observation was performed to determine the cell changes before and after the treatment, as shown in Fig. 3. All the control groups show the cobblestone-like structure of HUVEC which indicate healthy and normal cells. Whilst at 0.2 µM, the cells’ structure was altered and shrunk, especially after 24, 48 and 72 hours of treatment. On top of that, the number of cell shrinkage was elevated dramatically with 3.0 µM navitoclax after 24, 48 and 72 hours of treatment. These findings indicate more cells were dead with navitoclax treatment. Nevertheless, after 18 hours, the number of normal cells were still visible.
Based on the MTT assay result, 0.9 µM navitoclax for 24 hours treatment has been selected for subsequent experiment to determine the regulation of HUVEC angiogenesis and motility.
Establishment of in-vitro angiogenesis model by IL-3
The influence of various IL-3 concentrations on the induction of HUVECs tube formation after 6 hours, as measured by the mean total branching length can be depicted in Fig. 4. The total branching length is represented by the total segments and branches length [51]. The images displayed more branches and segments were formed by HUVECs with 25ng/ml. 10ng/ml and 25ng/ml IL-3 generated considerably longer branches than the control quantitatively. However, there was no statistically significant change in the mean total branch length between the three concentrations of IL-3. Nonetheless, the total branch length was greatest at 25ng/ml IL-3. This result shows a model of in-vitro angiogenesis generated by IL-3, one of the principal cytokines known to promote intraplaque angiogenesis.
Navitoclax inhibits the in-vitro angiogenesis model
Modulation of IL-3-induced HUVEC tube formation by 0.9 µM navitoclax was observed for 8 hours. All groups began to develop tube formation after two hours of incubation. The blockage of the branches was then observed after 4 hours in the IL-3 with navitoclax treated group and continued to deteriorate until the end of the incubation period (24 hours). Quantification of total branching length was performed after 8 hours treatment since at 24 hours the branches and segments were unlikely to be recognized by imageJ – Angiogenesis Analyzer. In the presence of navitoclax, as seen in Fig. 5(A), the number of segments and associated branches decreases. In addition, numerous isolated branches were observed, indicating the degeneration of the tube development. Similar to the data shown in Fig. 4A, 25 ng/ml IL-3 greatly increased tube formation, whereas navitoclax considerably reduced the overall branching length compared to the inducer group (Fig. 5B).
Regulation of navitoclax on IL-3 induced HUVEC proliferation and migration
To observe the cell behavior related to in-vitro angiogenesis, a migration and proliferation assay was undertaken. After 24 hours of incubation, the wound area in the control and IL-3 groups was not completely covered by cells, as evidenced in Fig. 6A. Possibly, by extending the incubation period, the entire wound will heal. Quantification of the data (Fig. 6B) reveals that the control group generated 42% wound closure, whereas IL-3 demonstrated the highest percentage which was 49%. Despite this, there was no statistically significant difference between the control and IL-3 groups, p > 0.05. After 24 hours of treatment with navitoclax, as depicted in Fig. 6A, a small number of cells had moved to the wound site. Around 17% of wound closure was impacted by navitoclax treatment, as seen in Fig. 6B, whereby p < 0.05 indicates statistical significance in comparison to the control and IL-3 groups. The data demonstrate that after 24 hours of treatment, navitoclax inhibits cell motility, as measured by a decrease in the percentage of wound closure area.
BrdU assay was done to quantify proliferating cells by permitting BrdU marker incorporation into newly produced DNA of actively developing cells. As seen in Fig. 7, the data were not statistically significant across all groups. IL-3 increased by 16% after 24 hours of treatment, while navitoclax reduced BrdU-positive cells by just 2% compared to the control group. This result indicates navitoclax had minimal effect on the reduction of proliferating cells.
Modulation of target genes related to HUVEC angiogenesis and motility by navitoclax
Cell lysate was used to assess mRNA expression levels. Three mRNA targets were chosen as markers for angiogenesis (CXCL-8), migration (MMP-3) and proliferation (BCL-2). In angiogenesis, endothelial cells secrete CXCL-8, a pro-angiogenesis chemokine, to facilitate the formation of tubes. In agreement with the angiogenesis result, CXCL-8 expression in the IL-3 cell lysate was slightly lower than the control, however in the presence of navitoclax, the CXCL-8 level was dramatically enhanced, p < 0.001. This may indicate that navitoclax degenerates the branches by inhibiting the secretion of CXCL-8. Subsequently, the activation of MMP-3 levels was correlated with cell migration data, in which the expressions increased significantly in response to IL-3 treatment and were inhibited by navitoclax. Finally, BCL-2 expressions were evaluated to determine the impact of navitoclax on HUVEC survival. Unexpectedly, the BCL-2 concentrations in IL-3 with or without navitoclax were comparable, and slightly lower than the control.
Our study conducted several optimizations in order to discover an appropriate set of navitoclax concentrations against proliferating endothelial cells (data not shown) with a finalized range between 0.2 µM to 3.0 µM. Different treatment times were performed to evaluate the potency of navitoclax in mediating HUVEC survival over time. This study included two control groups: untreated cells and cells incubated with 0.01 µM DMSO (vehicle control). The vehicle control group did not show a significant effect on cell death. This result would validate the anti-survival impact of navitoclax alone in the treatment group. After 18 hours of navitoclax treatment, more intact cells were still visible. Even though the statistical analysis showed a significant mean difference output between untreated and treated groups, from the dose-response curve plotted, the cell viability was reduced below 80% only at the highest navitoclax concentration used. We proposed it is due to a minimal time for the navitoclax to have an effect on HUVEC and a low range of concentration used. Nevertheless, our study did not proceed with higher navitoclax concentration; thus, the IC50 for 18 hours treatment could not be identified. An investigation to determine a relevant concentration range of navitoclax for 18 hours treatment is worth conducting. The result can further disclose that the time-dependent navitoclax activity is influenced by the concentration. However, as the treatment time increased with a constant concentration range, the number of intact cells was diminished. After 24, 48 and 72 hours of treatment, the IC50 values of navitoclax were successfully determined.
Our findings are supported by a previous study that reported approximately 50% reduction of HUVEC viability after 72 hours of 0.1 µM navitoclax treatment though they did not report on the morphological changes [52]. Our studies also showed a comparable IC50 value after 72 hours of treatment which was 0.12 µM. After all, this current study demonstrated that the IC50 value decreases as the treatment time increases. This indicates that the potency of navitoclax in reducing HUVEC viability is escalated over time. The data from the MTT assay conducted in this study would represent the number of living cells that can be detected after navitoclax treatment. This may indirectly indicate the cytotoxicity effect of navitoclax on living cells and illustrate that navitoclax can inhibit metabolically active cells. Yet, endothelial cell apoptosis induced by navitoclax is not clearly determined through this assessment. Hence, an investigation to ascertain the navitoclax apoptotic effect and its mechanism on endothelial cells is worth proceeding.
Most of the navitoclax cytotoxicity studies have been conducted on various cancer cells, including small-cell lung cancer (SCLC) cell lines, leukemia cells, breast cancer cells and neuroblastoma. This drug has been demonstrated to block anti-apoptotic proteins of the Bcl-2 family, which then triggers the downstream signaling of cell death. A different range of navitoclax concentrations were used in cytotoxicity assay against cancer cells subject to navitoclax efficacy. As demonstrated by a previous study, navitoclax treatment was highly efficacious towards head and neck squamous cancer cell lines; hence low dosage range was used starting from 0.0 to 1.5 µM [53]. In contrast, other tumor cells were less potent against navitoclax treatment; thus, a high concentration range was applied (i.e. 5 µM to 10 µM) for breast cancer cells [38], human oral squamous cell carcinoma-derived cell lines [39] and leukemia cells [54].
There is growing evidence for the therapeutic outcome of pro-apoptotic and anti-angiogenic agents on pathological angiogenesis other than cancer, such as advanced atherosclerosis. Angiogenesis in advanced atherosclerosis is primarily regulated by endothelial cells and smooth muscle cells survival as well as motility[11]. Therefore, a study of navitoclax cytotoxicity via cell viability assay to determine its IC50 value on primary cells (i.e., endothelial cells) is worth being conducted before further investigating its mechanism and pharmacological characteristics. A previous study has reported 10 µM navitoclax exhibited a significant but moderate pro-apoptotic effect against a primary cell which was human mesenchymal stromal cells [55]. Their results hence reassured the potential of navitoclax treatment to mediate a therapeutic effect against primary cells.
The in-vitro angiogenesis model would represent the neovascularization that occurs at the intraplaque region hence causing plaque instability and rupture. High expression of IL-3 receptor was detected upon pro-inflammatory agent stimulation such as IL-3, and these receptors were accumulated in coronary atherosclerotic plaques with neovessels development [18]. The discovery suggested that IL-3 – induced inflammation is strongly associated with angiogenic response. Besides, IL-3 was shown to mediate the angiogenic response in wound healing and tumor vasculature by activating endothelial cell-derived extracellular vesicles via STAT5 [56] and β-catenin signaling pathway [19, 20] respectively. In view of this evidence, the in-vitro angiogenesis model induced by IL-3 is justified to imitate the pathological neovascularization that contributes to the progression of atherosclerosis and cancer. The establishment of this model is crucial prior to navitoclax treatment in order to mimic the pathological condition of the disease. Therefore, 25 ng/ml IL-3 was administered with navitoclax to observe the efficacy of navitoclax in ameliorating the disease model.
Based on the MTT assay result, 0.9 M navitoclax was used in the subsequent tests to observe the regulation of navitoclax on the in-vitro intraplaque angiogenesis model. All studies were conducted with an incubation period of 24 hours, hence the IC50 value at 24 hours was utilized. In addition, the IC50 value employed in this study demonstrates the effectiveness of navitoclax in blocking the formation of in-vitro blood vessels by half-maximal inhibitory dose. Hence, it is possible to conclude that the inhibition of the subsequent cell biological test was not due to the absence of live cells. As illustrated by the tube formation assay, in the presence of navitoclax, the in-vitro angiogenesis induced by IL-3 was remarkably suppressed within 8 hours. This finding reveals a novel navitoclax function as anti-angiogenic agent on IL-3 – induced HUVEC neovascularization through cell motility inhibition. In accordance with the inhibitory effect of navitoclax on HUVEC angiogenesis and migration, the release of CXCL-8 and MMP-3 expression in cell lysates was reduced notably in navitoclax-treated group. Our findings revealed a novel function of navitoclax as anti-angiogenic drug through the regulation of CXCL-8 and MMP-3 expressions. Interestingly, CXCL-8 and MMP-3 levels were also highly expressed in the presence of IL-3 which may suggest the involvement of these molecules in the downstream signaling of IL-3 receptor activation. Nevertheless, current study determined anti-proliferative effect of navitoclax on HUVEC to be minimal and BCL-2 protein involvement was insignificant. On top of that, this is the first in-vitro study to employ navitoclax as one the pharmacological strategies to ameliorate intraplaque angiogenesis in atherosclerosis. Several drugs have been proposed for this disease model, however, the outcomes are yet to be elucidated. This is due to a lack of in-vivo disease model and some of the treatments display unappealing effects.
This is among the few studies that reported the navitoclax effect on primary cell viability, which is HUVEC mainly. Navitoclax demonstrated a moderate toxicity result on primary human endothelial cells with a concentration below 3.0 µM indicated by the number of viable cells after 6- and 24-hours treatment time. The potency of navitoclax elevated as the treatment time increased. The action of navitoclax is time- and concentration-dependent on HUVEC. The finding suggested using 0.9 µM navitoclax in 24 hours treatment period is appropriate for further HUVEC in-vitro study. Research on navitoclax effect in less than 24 hours treatment time will require higher navitoclax concentration, whilst for longer treatment time (i.e., > 24 hours), lower navitoclax dose can be used.
Our results provide the first evidence of navitoclax as an anti-angiogenic agent against IL-3 – induced HUVEC angiogenesis. Besides, navitoclax effects on matrix metalloproteinases activities and CXCL-8 released to degenerate blood vessel formation were revealed through in-vitro study. Unexpectedly, this drug showed minimal association with BCL-2 expression and proliferative activity of endothelial cells, as opposed from the cancer study.
As in general, navitoclax has moderate toxicity effect on the metabolically active endothelial cells. Other than that, with half – maximal concentration applied, navitoclax is able to significantly inhibit the in-vitro tube formation and cell migration induced by IL-3 through the downregulation of CXCL-8 and MMP-3 activities. In conclusion, navitoclax is proposed to produce therapeutic outcomes in pathological angiogenesis such as intraplaque angiogenesis in atherosclerosis through its anti-angiogenic properties.
ACKNOWLEDGMENT
This research was supported by a grant from the Ministry of Education, Malaysia (FRGS/1/2019/SKK06/UKM/02/7) and Universiti Kebangsaan Malaysia (UKM). We like to thank the Department of Obstetrics and Gynecology, Universiti Kebangsaan Malaysia Medical Center (PPUKM) for their support in providing human umbilical cord samples. Appreciation also goes to the Department of Physiology, Faculty of Medicine, Universiti Kebangsaan Malaysia for the usage of their equipment and expertise for HUVEC isolation.
Funding
This research was supported by a grant from the Ministry of Education, Malaysia (FRGS/1/2019/SKK06/UKM/02/7) and Universiti Kebangsaan Malaysia (UKM).
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or otherwise.
AUTHOR CONTRIBUTIONS
AU and NNMA designed the study conceptualization and supervised the experiment. MFA assisted the umbilical cord collection and isolation process. NSNH conducted the experiment and drafted the manuscript. KDG and NFR edited and reviewed the manuscript. All authors approved the final version of the manuscript.
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No competing interests reported.