Local anesthetics impaired cell survival via downregulation of ZDHHC15 expression
To study the effect of local anesthetics on malignant progression among GSCs, GSCs derived from U251 cells were subjected to in vitro treatment with 11 local anesthetics (prilocaine, benzocaine, procaine, dibucaine, butacaine, oxethazaine, lidocaine, propoxycaine, levobupivacaine, bupivacaine, and ropivacaine) after 24 h and 48 h of treatment under different concentrations (5 µM, 10 µM, and 20 µM); cell counting kit 8 (CCK-8) assays were performed to determine the GSC viability (Figure 1A). After 24 h, prilocaine, lidocaine, procaine, bupivacaine, and ropivacaine inhibited the growth of GSCs in a concentration-dependent manner. After 48 h of treatment, prilocaine, oxethazaine, lidocaine, levobupivacaine, procaine, and ropivacaine significantly suppressed GSC survival in a concentration-dependent manner. Combined with the above results, we found that the local anesthetics prilocaine, lidocaine, procaine, and ropivacaine killed GSCs in a concentration-dependent and time-dependent manner.
Palmitoylation, mediated by the DHHC family, markedly affects tumorigenesis and tumor progression through different substrates, particularly in gliomas [22,23]. We analyzed the expression of all DHHCs in GSCs after treatment with the four local anesthetics mentioned above via reverse transcription-PCR (RT-PCR) assays (Figure 1B). Of the 24 PATs, 19 were detected in U251 GSCs, and ZDHHC1, 9, 11, 19, 22, and 23 were undetectable. ZDHHC15 was significantly downregulated in all groups treated with the local anesthetics. We also observed a significant decrease in ZDHHC15 expression levels after treatment with lidocaine, prilocaine, ropivacaine, or procaine in a concentration-dependent manner (Figure 1C). These results indicate that the local anesthetics prilocaine, lidocaine, ropivacaine, and procaine killed cells via inhibition of ZDHHC15 expression.
Expression pattern of ZDHHC15 isoforms in GSCs
In the tumor cell lines, ZDHHC15 has three isoforms, which are generated by alternative splicing (Figure 2A). Transcript variant 1 encodes the longest isoform, including 12 exons. Compared with variant 1, variant 2 lacks an in-frame coding exon, resulting in a shorter isoform 2 that is missing a 9 amino acid segment. Compared with variant 1, variant 3 is missing an in-frame coding exon and differs at the 3' end, resulting in a shorter isoform 3 with a C-terminus that is distinct from isoform 1. Transcript variant 3 does not code for protein.
First, we analyzed the expression of three isoforms of ZDHHC15 in six human GBM cell lines via RT-PCR (Figure 2B). Isoforms 1, 2, and 3 were undetectable in the H4, A172, and T98G cell lines. In U87 cells, only isoform 3 was detected. However, isoforms 1 and 3 were detected in U251 and LN18 cells, which belong to the classic GBM cell line (Figure S1). U87 cells have a significantly mesenchymal phenotype, and H4, A712, and T98G cells belong to the proneural subtype of cells (Figure S1). We used The Cancer Genome Atlas and Gene Expression Profiling Interactive Analysis databases and investigated the association between the expression of ZDHHC15 and the anatomical distribution (Table S1 and S2). The expression level of ZDHHC15 was positively correlated with the classic GBM subtype.
Compared with monolayer cell culture, ZDHHC15 isoform 2 could be detected in neurosphere formation, similar to that of NSCs (Figure 2C). In particular, the expression of isoform 2 was strongly elevated during GSC self-renewal and then progressively decreased during the differentiation stage (Figure 2D). However, isoforms 1 and 3 were not changed (Figure 2D). Consistent with the above results, western blot analysis showed that isoform 1 was expressed in U251 and LN18 cells (Figure 2E) and that isoform 2 could be detected in the neurospheres from U251 and LN18 cells (Figure 2E). These results indicated that ZDHHC15 might play an important role in GBM, particularly in the classic subtype, and isoform 2 may be essential for the self-renewal of GSCs.
Local anesthetics strongly induce differentiation and impair the self-renewal of GSCs
We first compared ZDHHC15 expression in 60 glioma tissues, including pilocytic astrocytoma (PA, grade I; n=6), oligodendrocytoma (OL, grade II, n=18), anaplastic astrocytoma (AA, grade III; n=15), and GBM (grade IV; n=21), and eight normal brain tissue samples (Figure 3A). ZDHHC15 levels in gliomas were elevated relative to the levels in the normal brain tissue and were positively correlated with the degree of malignancy. The positivity rates of ZDHHC15 were 11.11% in OL, 53.33% in AA, and 66.66% in GBM, and ZDHHC15 showed negative expression in PA.
Considering the elevated levels of ZDHHC15 in GSCs, we investigated whether ZDHHC15 is crucial for GSC self-renewal in a single cell neurosphere formation assay. shRNA-expressing lentiviruses were prepared to target ZDHHC15 expression. Under free-floating neurosphere culture conditions, ZDHHC15 knockdown diminished the capacity of GSCs to form neurospheres (Figure 3B). Consistent with these results, GSC maintenance was impaired by the local anesthetics procaine, prilocaine, lidocaine, or ropivacaine (20 µM each). In the serial dilution assay, U251 GSCs transduced with PBS-treated or control shRNA produced a significantly greater number of neurospheres at each level of dilution compared with the ZDHHC15-deficient cells or GSCs treated with local anesthetics (Figure 3C). These results indicate that ZDHHC15 is required for GSC maintenance, and that self-renewal of GSCs could be impaired via inhibition of ZDHHC15 expression by the local anesthetics procaine, prilocaine, lidocaine, or ropivacaine.
The decreased formation of neurospheres in CSCs treated with procaine, prilocaine, lidocaine, or ropivacaine suggests an alteration in the “stemness” of CSCs. To provide a mechanistic insight into these phenotypic changes, we examined stem and differentiation markers in spheroids treated with the local anesthetics (Figure 3D). Spheroids treated with the local anesthetics, namely, procaine, prilocaine, lidocaine, or ropivacaine (20 µM each) and ZDHHC15 shRNA were stained with antibodies for several stem cells and differentiation markers. The neurosphere CSCs exhibited significant staining for nestin and SOX2, which are both neural stem cell markers, but showed limited expression of GFAP and MAP2, both of which are markers of differentiated cells. Notably, the CSCs transfected with ZDHHC15 shRNA or treated with the local anesthetics exhibited greatly reduced staining of nestin and SOX2, accompanied by an increased expression of the differentiated cell markers. This further confirms our findings that ZDHHC15 silencing or local anesthetic treatment strongly induces differentiation, suggesting the existence of an additional mechanism by which ZDHHC15 inhibition could ameliorate the malignant phenotype in glioma.
Identification of palmitoylation proteins mediated by ZDHHC15
ZDHHC15 belongs to a super-family of PATs that catalyze the attachment of palmitate to other protein substrates [26,27]. To identify the role of ZDHHC15 in protein palmitoylation in GSCs, we performed an ABE assay to identify proteins that are S-acylated by ZDHHC15 (Figure 4A). U251 GSCs were lysed, incubated with the ZDHHC15 antibody, and the immunoprecipitated protein samples were divided into two fractions. In the HAM+ sample, the palmitate residue was cleaved and exchanged with biotin. The HAM- condition served as a negative control. After the ABE reaction was completed, streptavidin beads were used to enrich the biotinylated proteins. Proteins enriched under HAM+ and HAM- conditions were identified using mass spectrometry. Proteins with at least 2-fold greater abundance in the HAM+ sample were considered to be candidate proteins. Using this approach, we identified 74 palmitoylated proteins. Supporting the validity of the assay, we identified 10 previously validated S-acylated proteins, and 28 S-acylated proteins predicted using the CSS-Palm version 4.0 software (The Cuckoo Workgroup, http://csspalm.biocuckoo.org/down.php). GP130, low density lipoprotein receptor-related protein 12 (LRP12), and Rap1 interacting factor 1 (RIF1), as candidate palmitoylated proteins for experimental validation, may be associated with glioma development and malignant progression (Figure 4B). The results of immunoprecipitation further indicated that ZDHHC15 interacted with GP130, but not with LRP12 and RIF1 (Figure 4C).
Local anesthetics repressed GP130 palmitoylation and impaired its membrane localization
Next, we examined palmitoylation levels using an ABE assay after immunoprecipitation of GP130 and found that GP130 was palmitoylated (Figure 5A). A significant reduction in the palmitoylation level of GP130 was observed in ZDHHC15-deficient or 2-bromopalmitate (2BP)-treated cells, compared with that in control cells (Figure 5B). Moreover, we found that treatment with the depalmitoylation inhibitor palmostatin B (1 μm) resulted in the accumulation of palmitoylated GP130 (Figure 5B). We then investigated the effects of the local anesthetics procaine, prilocaine, lidocaine, and ropivacaine on GP130 palmitoylation (Figure 5C). In addition to the decrease in ZDHHC15 expression, the level of GP130 palmitoylation decreased after treatment with the local anesthetics in a concentration-dependent manner. Moreover, the palmitoylation status of GP130 was positively correlated with the Janus kinase/STAT3 signaling activity. Immunofluorescence results also confirmed that STAT3 (Y705) phosphorylation was decreased by ZDHHC15 knockdown or treatment with local anesthetics (Figure 5C). We then assessed whether GP130 palmitoylation influences its cellular distribution/localization in GSCs (Figure 5 D and E). Strikingly, we found that GSCs transfected with ZDHHC15 shRNA or treated with local anesthetics showed reduced expression of GP130 on the membrane surface without affecting overall expression levels, compared with normal conditions. These results indicated that palmitoylation inhibition mediated by the local anesthetics resulted in the disappearance of GP130 in the membrane fractions.
IL-6/STAT3 regulated ZDHHC15 transcripts via positive feedback
Of note, through bioinformatics analysis with JASPAR we discovered three putative STAT3-binding elements within the ZDHHC15 isoform 1 and 3 promoter regions, and four STAT3 responsive elements within isoform 2 (Figure 6A). We further investigated if IL-6/STAT3 could regulate ZDHHC15 expression in turn. STAT3 inhibitor or siRNA significantly decreased ZDHHC15 transcription in U251 cells treated with or without rhIL-6 (Figure 6B). A luciferase assay indicated that the ZDHHC15 promoter was repressed by a STAT3 inhibitor or siRNA. The local anesthetics procaine, prilocaine, lidocaine, or ropivacaine could also inhibit ZDHHC15 transcription (Figure 6C). To clarify which element was necessary for STAT3-mediated ZDHHC15 expression, three or four predicted STAT3-binding sites were individually deleted. We found that STAT3 failed to promote ZDHHC15 transcriptional activity without the E2 element for isoforms 1 and 3, while E1 and E3 absence alone partially downregulated ZDHHC15 promoter activity, indicating that the E2 element was essential for STAT3 to activate transcription of ZDHHC15 isoforms 1 and 3 (Figure 6D). Moreover, the E1 element was essential for the activation of ZDHHC15 isoform 2 transcription by STAT3. To further confirm these results, a chromatin immunoprecipitation (ChIP) assay was performed with p-STAT3 antibody, followed by detection via quantitative RT-PCR with specific primers for the E2 or E1 elements. STAT3 could associate with the ZDHHC15 isoform 1 and 3 promoters and was enriched within the E2 region, and within the E1 region of isoform 2 (Figure 6E). These findings suggest that there is a regulatory feedback loop between ZDHHC15 and IL-6/STAT3 signaling, which may continuously activate their oncogenic functions.
To determine the efficacy of a 5-day in vitro treatment of GSCs with the local anesthetics, namely, procaine, prilocaine, lidocaine, or ropivacaine (20 µM each) on the tumor-initiating potential of GSCs, we subcutaneously injected GSCs into immunocompromised mice. Similar to the results observed with ZDHHC15 depletion, GSCs treated with local anesthetics before injection significantly suppressed tumor growth relative to control (or PBS-pretreated) animals (Figure 6F and G). These results suggest that short-term in vitro treatment with procaine, prilocaine, lidocaine, or ropivacaine sufficiently reduced the number of tumor-initiating cells in all GSC samples, resulting in delayed tumor development.