Design and setting of the study
This study aimed to gain insight into the molecular mechanisms allowing cancer cells to survive and proliferate under detached conditions, regardless of both tumor-intrinsic variables and nutrient culture conditions. To this, we used 3D tumor spheroids as in vitro experimental models to mimic the anchorage-independent cancer cell growth as well as to mimic the fluctuation in nutrient and oxygen availability that cells undergo as tumor mass grows and expands in vivo. 3D tumor spheroids derived from LUAD, and breast cancer cell lines were grown in two customized culture conditions: i) the sphere medium (SM), which mimics an environment rich in major nutrients (glucose and L-glutamine) and growth factors (Epidermal Growth Factor, EGF, and basic Fibroblast Growth Factor, bFGF) and ii) the RPMI or DMEM FBSlow, supplemented with only 2%FBS, mimicking a nutrient-restricted culture condition. A wide multi-omic approach, based on the integration of transcriptomic, proteomic, and metabolomic analyses, was used to identify the common molecular changes occurring during all the transitions from adherent 2D to 3D cultures, regardless of the tumor type and nutrient culture availability. Small interfering RNA-mediated loss of function assays were used to validate the role of the identified differentially expressed genes and proteins in LUAD and breast cancer cell lines (Fig. 1).
Rna-seq Analysis Of H460 And Mcf7 Cell Lines Grown In 2d And 3d Culture Conditions
Cancer cells grown in 3D cultures show distinct gene expression patterns when compared to the same parental cells grown in 2D conditions [39–41]. Indeed, differential extracellular interactions with ECM as well as different nutrient availability within 3D models change the intracellular signal transduction, culminating in the activation of a unique set of transcription factors and in the significant changes of the transcriptomic profiles [42, 43]. Here, we first shed light on the transcriptional reorganization associated with 3D cell growth in nutrient-rich or nutrient-restricted culture conditions. H460 and MCF7 cells (1.5 x 104/mL) were grown in non-adherent conditions either in customized nutrient-rich sphere medium (SM) or in customized nutrient-restricted culture medium (FBSlow). After 4 days, which was previously established as the optimal time frame to collect the first generation of tumor spheroids [44], we performed RNA-seq analysis of H460 and MCF7 grown either as a monolayer (2D) or as 3D_SM and 3D_ FBSlow. Overall, differential expression analysis (DEA) highlighted a total of 2169 DEGs in the comparison H460 2D vs 3D_SM and 1478 DEGs in the comparison H460 2D vs 3D_FBSlow (Table S1, in Additional file 1). For MCF7 cells, 1925 DEGs emerged from the comparison between 2D vs 3D_SM while 2222 DEGs resulted from 2D vs 3D_FBSlow (Table S1, in Additional file 1). Then, we sought to find a common transcriptional signature associated with the 3D tumor spheroid growth, namely genes and processes significantly up or down regulated in the transition from 2D to 3D, regardless of the tumor type and the culture media utilized. A signature of 100 genes was found to be commonly regulated in all the systems; among these, 84 genes were commonly up regulated while 16 were commonly down regulated in all 3D vs 2D conditions (Fig. 2A). Interestingly, functional enrichment analysis on the common DEGs highlighted that among the enriched biological processes, most of them (17 out of 20) were associated with cellular metabolism. In particular, Glycolysis/Gluconeogenesis appeared the most consistently enriched metabolic pathway because of the up regulation of 6 out of 10 glycolytic enzymes (hexokinase 2, HK2; pyruvate kinase muscle isozyme, PKM; phophoglycerate kinase 1, PGK1; aldolase C, ALDOC; enolase 2, ENO2; glyceraldehyde 3-phosphate dehydrogenase, GAPDH) and of phosphoglucomutase 1 (PGM1). Notably, both HK2 and PKM encode for muscle-specific isoenzymes involved in the regulation of two irreversible steps of glycolysis [45]. ALDOC and ENO2 encode for neuronal-specific aldolase and enolase isoforms [46, 47]. HK2 catalyses the first priming and irreversible reaction of glycolysis, the conversion of the substrate glucose into glucose-6-phosphate, ALDOC is the key enzyme of the fourth step of glycolysis, during which fructose − 1,6-bisphosphate is converted to gylceraldehydes-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). GAPDH, PGK1, ENO2, and PKM catalyze 4 out of the 5 reactions of the energy-releasing phase of glycolysis. PGM1 belongs to the phosphohexose mutase family and catalyzes the transfer of phosphate between the 1 and 6 positions of glucose; as such, it is involved in both the synthesis and degradation of glycogen [46, 48–50]. As the second most significantly affected biological process in the 2D to 3D transition, the cellular response to hypoxia (HIF-1 signalling pathway) was enriched by the up regulation of BNIP3, BNIP3L, EGLN3, FAM162A, HILPDA, PGK1, RORA, NDRG1 (Fig. 2B-C) (Tables S2-3, in Additional files 2 and 3). In addition to the glycolytic enzyme PGK1, EGLN3 is a member of the 2-oxoglutarate (2OG)–dependent dioxygenases family responsible for the prolyl hydroxylation of HIF-1//2 and for the regulation of cell apoptosis in response to hypoxia [51]. Similarly, BNIP3, BNIP3L, and FAM162A are involved in the regulation of cell death in response to hypoxic conditions [52, 53]. In particular, the BH3-only proapoptotic genes BNIP3 and BNIP3L enhance autophagy and, in particular, mitophagy to overcome cell death and guarantee survival under hypoxic conditions [54]. N-myc downstream-regulated gene-1 (NDRG1) is a hypoxia inducible-protein involved in the p53-mediated activation of the caspase cascade; furthermore, it influences the epithelial to mesenchymal transition (EMT) as it is required for the vesicular recycling of e-cadherin and for the cadherins switching [55, 56]. The hypoxia-inducible and lipid droplet-associated protein HILPDA is known to promote lipid droplets formation in response to hypoxia as well as to autophagic flux induced by nutrient deprivation [57]. RORA is a hypoxia-induced member of the retinoic acid-receptor-related orphan receptor α superfamily; unlike the other members of this family, RORA binds to the promoter of cell cycle-related genes and N-myc, thus affecting cell growth and tumorigenesis [58]. Finally, the GO cell component analyses highlighted that PKM, ALDOC, AMPD3, PGM1, EFEMP2, and RAB3A, are up-regulated in all 3D vs 2D culture conditions, consistently enriched the ficolin-1-rich granule lumen and extracellular vesicles (EVs) (Table S4, in Additional file 4). Together with the already described PKM, ALDOC and PGM1, the Adenosine Monophosphate Deaminase 1 (AMPD3), encoding for the red blood cells (RBC)-specific member of the adenosine monophosphate (AMP) deaminase family, catalyzes the irreversible hydrolytic deamination of AMP to inosine monophosphate (IMP), thus it is involved in purine nucleotide, uric acid, and carbohydrate metabolism [59]. Recent reports indicate that, in RBCs, AMPD3 can be activated by the increased intracellular levels of ROS and calcium, along with decreased intracellular pH [60]. The exact role of AMPD3 in cancer is instead still unclear; however, since it controls the intracellular levels of AMP, it is reasonable to hypothesize that it might affect AMP-activated protein kinase (AMPK). AMPK is largely recognized as a key energy sensor. In response to diverse stressors, such as glucose starvation, hypoxia, and oxidative damage, it activates ATP-producing pathways [61]. In agreement, according to several studies, AMPK deficiency renders cancer cells more vulnerable to the stresses induced by cell detachment [62]. EFEMP2 (EGF Containing Fibulin Extracellular Matrix Protein 2) gene encodes for a member of fibulin glycoprotein family, involved in the stabilization of the ECM structure; indeed, it is necessary for elastic fiber formation, and it is involved in collagen fibril assembly. So far, the role of EFEMP2 in tumorigenesis is found to be “context-specific”; indeed, while in cervical cancer, ovarian cancer, and glioblastoma it has been associated with tumor progression and poor prognosis, in endometrial cancer it has been found to inhibit EMT, tumor invasion and metastasis [63]. Finally, Rab3A belongs to the small Ras-like GTPase superfamily and functions as a key regulator in transporting cellular products into secretory vesicles and lysosomes [64]. Normally Rab3A is predominantly expressed in the neural system; however, it has been found aberrantly overexpressed in breast cancer where it is associated with a more malignant phenotype and in hepatocellular carcinoma where, instead, it inhibits metastasis via enhancing mitochondrial oxidative metabolism [65].
Overall, RNAseq data suggest that the ability of cancer cells to survive and grow in 3D culture conditions requires the rewiring of intracellular metabolic pathways and the control of redox homeostasis most likely in response to the decreased oxygen levels.
Proteomic Analysis Of H460 And Mcf7 Cell Lines Grown In 2d And 3d Culture Conditions
Once identified the gene expression signature associated with 3D tumor spheroid growth, we analyzed the proteomic profiles of H460, and MCF7 3D tumor spheroids grown either in SM or in FBSlow conditions and compared them to their relative 2D cultures. By using an absolute log2 |FC| > 1 and a p-value < 0.01, we identified a total of 534 DEPs in H460 3D_SM vs H460 2D, n = 413 DEPs in H460 3D_FBSlow vs H460 2D, n = 216 DEPs in MCF7 3D_SM vs MCF7 2D, and n = 222 DEPs in MCF7 3D FBSlow vs MCF7 2D (Table S5, in Additional file 5). Among these, 2 proteins (MRPL41 and MRPL24) were down regulated while 7 proteins (ALDOA, ALDOC, NOL3, ENO2, SH3BGRL, DBI, HEBP2) were up regulated in both H460 and MCF7 3D vs 2D conditions (Fig. 3A). Both the commonly down regulated proteins MRPL41 and MRPL24 are component of mitochondrial ribosomes (mitoribosomes) large 39S subunit and are involved in the synthesis of mitochondrial electrons transport chain (ETC) components [66, 67]. Among the commonly up regulated proteins, as already discussed above, the two isoenzymes ALDOA and ALDOC as well as ENO2 are glycolytic enzymes, NOL3 acts as apoptosis repressor, often in response to hypoxia, by inhibiting the release of cytochrome c from mitochondria [68], the Acyl-coA-binding protein DBI is a lipogenic factor that regulates fatty acids metabolism [69], the heme binding protein 2 (HEBP2) is involved in heme metabolism but it also enhances the outer and inner mitochondrial membrane permeabilization, especially under oxidative stress conditions [70]. The SH3 Domain Binding Glutamate Rich Protein Like (SH3BGRL) is located within the extracellular vesicles and as a scaffold protein it mediates many protein-protein interactions; however, its role in cancer is still largely undefined [71]. In agreement, KEGG enrichment analysis revealed that the common DEPs mainly affected metabolic and bioenergetic processes (i.e., GO Generation of precursor metabolites and energy, GO Monosaccharide biosynthetic process, GO Ribose phosphate metabolic process, KEGG Glycolysis and gluconeogenesis), exocytosis, cell adhesion processes (i.e., GO Cell adhesion molecule binding, GO Cadherin binding) and cellular response to oxidative stress (i.e., GO Cell redox homesostasis, GO regulation of response to oxidative stress) (Fig. 3B). Interestingly, when RNAseq and proteomic data were intersected, ALDOC, ENO2, and NOL3 emerged as significantly up regulated with a log2|FC| > 1 and p-value < 0.05 in all 3D vs 2D culture conditions both at gene and protein levels (Fig. 3C). Collectively, proteomic data confirmed that cancer cells reprogram their glucose metabolic to adapt, and thus to survive, to the altered oxygen homeostasis caused by cellular reorganization of within 3D tumor spheroids and that is independent from both cancer type and nutrient availability.
Metabolic Profiling Of H460 And Mcf7 Cells In 2d And 3d Culture Conditions
Prompted by the information arising from RNAseq and proteomic analysis, we decided to investigate the metabolic shift associated with changes in nutrient availability in non-adherent conditions. To this aim, we performed targeted polar metabolomic profiling of H460 and MCF7 cells grown as 2D, as well as 3D_SM and 3D_FBSlow tumor spheroids. Collectively, the LC-MS platform enabled us to detect 80 metabolites (Table S6, in Additional file 6). A total of 66 metabolites were found significantly altered among the three cell culture conditions (2D, 3D_SM and 3D_FBSlow) with log2 |FC| > 1 and a p-value < 0.01. We observed that, as for the transcriptomic and proteomic profiles, the intracellular metabolomic profiles of H460 and MCF7 cells grown as 2D cultures were substantially different, as attested by the net clustering of samples shown in Fig. 4A. According to the literature, lung and breast cancer cells have different inherited metabo-phenotypes (metabotypes) and dependencies caused by the genetic background, the oncogenic evolution, and the interaction with the cellular niche [72]. H460 are primarily glycolytic cells [73]; MCF7, instead, are the most oxidative among the breast cancer cells, and overall display high flexibility in the substrate-driven ATP production [74]. In this regard, our data show that both H460 and MCF7 in 2D culture conditions consume glucose; however, the higher ratio isocitrate/citrate in MCF7 compared to H460 suggests a higher mitochondrial functionality in the breast cancer cell line than in the lung cancer cell line. In addition, as suggested by the higher amount of Ribose-5P, Xylulose-5P and Sedoheptulose-7P, MCF7 cells seem to promote anabolism through PPP for nucleotide synthesis, synthesis of serine and glycerol-3-P (Fig. 4A).
The amount of intracellular polar metabolites significantly diverged along the transition from 2D to 3D models regardless of the cell type. Overall, we identified 66 altered metabolites; among these, 7 showed the same trend of variation in all 3D vs 2D cultures: D-Glucose monophosphate, D-Fructose monophosphate, D-hexose pool, UDP glucose, dIMP, L-Aspartic Acid and L-Serine were significantly down-regulated in 3D vs 2D while L-lactic acid was the only metabolite up-regulated in 3D H460 and MCF7 compared to their relative adherent cells (Fig. 4A-B). It is important to note that, in FBSlow culture condition, MCF7 produced a higher amount of L-lactic acid compared to H460 cells, thus further suggesting the occurrence of a significant shift toward a glycolytic phenotype in the breast cancer cell line compared to the LUAD cell line which instead appeared more glycolytic already in 2D conditions. The increased intracellular ratio L-lactic acid/Glucose monophosphate and D-Fructose monophosphate in all 3D tumor spheroids compared to their relative 2D cultures well agreed with the up-regulation of the glycolytic enzymes ALDOC and ENO2, at both gene and protein levels (Fig. 4C). Thus, we assessed the effects of transient knock down of ALDOC and ENO2 alone or in combination on L-lactic production. As shown in Fig. 4D-E, the knock down of both enzymes together led to the marked reduction of L-lactic acid production and release in the culture media (*p-value < 0.05) in both the cell lines; only in MCF7 3D spheroids grown in SM, the sole ALDOC was sufficient to repress lactate production (*p-value < 0.05). Overall, these results strongly suggested that the increased glucose metabolism and the consequent lactate generation might represent a “universal” hub in 3D tumor cell growth regardless of both the intrinsic cancer cell metabotype and the environmental nutrient availability.
Characterization of H460 and MCF7 3D tumor spheroids growth in nutrient rich- or nutrient-restricted culture conditions
Gene and protein expression reorganization associated with 3D cell culture drive morphological and functional changes, such as proliferation rate and drug resistance [75]. Here, we observed that nutrient restriction had different effects on both tumor spheroids size and number depending on the cell type. Indeed, the FBSlow culture condition caused an increase of H460 tumor spheroids number compared to SM (3230 ± 221 (FBSlow)) vs (2450 ± 158 (SM)) (p-value < 0.05) without significantly affecting their diameter (163.9 ± 30.8 (SM) vs 158.51 ± 25.55 (FBSlow), ns). The number of tumor spheroids deriving from MCF7 cells was instead apparently unaffected by the different culture conditions (1050 ± 24 (FBSlow) vs 1223 ± 320 (SM), ns), but they appeared increased in size when grown in the FBSlow culture medium (156.99 ± 26.59 (SM) vs 180.02 ± 22.43 (FBSlow), p-value < 10− 7) (Fig. 5A-B). Cell viability assay highlighted that while H460 cells suffered from nutrient-restricted culture medium (FBSlow) MCF7 cells, grown in the same culture condition, showed an enhanced cell viability (Fig. 5C). This difference can be attributed to the previously mentioned higher inherited metabolic plasticity of MCF7 cells, which therefore result more adaptable to nutrient restrictions and, overall, less dependent on glucose to produce ATP.
Finally, Scanning Electron Microscopy (SEM) analysis revealed that the plasma membrane ultrastructural features of 3D spheroids appeared morphologically distinguishable depending on culture conditions. Notably, both H460- and MCF7-derived spheroids cultured in SM showed intense plasma membrane blebbing, indicating high membrane dynamics with respect to FBSlow cultured counterpart. Since this activity can be related to microvesicles formation this aspect deserves further investigations. Moreover, H460-derived tumor spheroids grown in FBSlow appeared more compact, provided with a marked roundness, suggesting a different junctional behaviour of SM and FBSlow cultured samples (Fig. 5D). Collectively, these results suggest that both H460 and MCF7 cells survive to harsh nutrient culture conditions and generate tumor spheroids that appear more compact; besides, MCF7 appear favoured in terms of spheroids size and growth rate possibly because of their inherited metabolic plasticity.
Suppression Of Aldoc And Eno2 Restrains 3d Tumor Spheroids Growth Of H460 And Mcf7 Cells
Lastly, based on the presented results we hypothesized that H460 and MCF7 might be dependent on ALDOC- and ENO2-mediated glucose metabolism to survive in non-adherent conditions. Thus, we assessed the effects of transient knock down of ALDOC and ENO2 alone or in combination on the spheroid forming ability of both cell lines. First, we observed that cell viability was markedly reduced in both SM and FBSlow culture media upon ALDOC and ENO2 knock down in both cell lines (*p-value < 0.05, **p-value < 0.01, ***p-value < 0.001) (Fig. 6A). Spheroids forming ability was also found attenuated in both cell types upon ALDOC and ENO2 knock down, as a single entity or in combination, regardless of the culture media conditions. Indeed, as shown in images and relative histograms reported in Fig. 6B, ALDOC and ENO2 knock down caused a significant reduction in tumor spheroids size. These results were further confirmed in additional LUAD (HCC827) and breast cancer (T47D) cell lines. Indeed, as shown in Fig. S1, the combined knock down of the two glycolytic enzymes significantly impaired lactate production (Fig. S1A-B, in Additional file 7) and hampered the growth of HCC827 and T47D cells as 3D tumor spheroid in non-adherent conditions.