Paradoxical implication of BAX/BAK in the persistence of tetraploid cells.

Pro-apoptotic multi-domain proteins of the BCL2 family such as BAX and BAK are well known for their important role in the induction of mitochondrial outer membrane permeabilization (MOMP), which is the rate-limiting step of the intrinsic pathway of apoptosis. Human or mouse cells lacking both BAX and BAK (due to a double knockout, DKO) are notoriously resistant to MOMP and cell death induction. Here we report the surprising finding that BAX/BAK DKO cells proliferate less than control cells expressing both BAX and BAK (or either BAX or BAK) when they are driven into tetraploidy by transient exposure to the microtubule inhibitor nocodazole. Mechanistically, in contrast to their BAX/BAK-sufficient controls, tetraploid DKO cells activate a senescent program, as indicated by the overexpression of several cyclin-dependent kinase inhibitors and the activation of ß-galactosidase. Moreover, DKO cells manifest alterations in ionomycin-mobilizable endoplasmic reticulum (ER) Ca2+ stores and store-operated Ca2+ entry that are affected by tetraploidization. DKO cells manifested reduced expression of endogenous sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a (Serca2a) and transfection-enforced reintroduction of Serca2a, or reintroduction of an ER-targeted variant of BAK into DKO cells reestablished the same pattern of Ca2+ fluxes as observed in BAX/BAK-sufficient control cells. Serca2a reexpression and ER-targeted BAK also abolished the tetraploidy-induced senescence of DKO cells, placing ER Ca2+ fluxes downstream of the regulation of senescence by BAX/BAK. In conclusion, it appears that BAX/BAK prevent the induction of a tetraploidization-associated senescence program. Speculatively, this may contribute to the low incidence of cancers in BAX/BAK DKO mice and explain why human cancers rarely lose the expression of both BAX and BAK.

TRPC1 and ORAI1 channels in colon cancer.

Colon cancer cells, like other types of cancer cells, undergo the remodeling of the intracellular Ca2+ homeostasis that contributes to cancer cell hallmarks including enhanced cell proliferation, migration, and survival. Colon cancer cells display enhanced store-operated Ca2+ entry (SOCE) compared with their non-cancer counterparts. Colon cancer cells display an abnormal expression of SOCE molecular players including Orai1 and TRPC1 channels, and the stromal interacting molecule (STIM) 1 and 2. Interestingly, upregulation of Orai1 and TRPC1 channels and their contribution to SOCE are associated with cancer malignancy in colon cancer cells. In a specific cellular model of colon cancer, whereas in non-cancer colon cells SOCE is composed of the Ca2+ release activated (CRAC) currents, in colon cancer cells SOCE is composed of CRAC- and cationic, non-selective store operated (SOC) currents. Former SOCs are mediated by TRPC1 channels. Moreover, colon cancer cells also display dysregulation of the expression of 1,4,5-triphosphate receptors (IP3R) that could contribute to the enhanced SOCE. Another important factor underlying the enhanced SOCE is the differential mitochondrial modulation of the CRAC and SOC currents in non-cancer and colon cancer cells. In colon cancer cells, mitochondria take up more Ca2+ that prevent the Ca2+-dependent inactivation of the SOCs, leading to sustained Ca2+ entry. Notably, the inhibition of SOCE in cancer colon cells abolishes their cancer hallmarks. Robust evidence has shown the efficiency of non-steroidal anti-inflammatory drugs (NSAIDs) and difluoromethylornithine (DFMO) to reverse the enhanced cell proliferation, migration, and apoptosis resistance of cancer cells. In colon cancer cells, both NSAIDs and DFMO decrease SOCE, but they target different molecular components of SOCE. NSAIDs decrease the Ca2+ uptake by mitochondria, limiting their ability to prevent the Ca2+-dependent inactivation of the SOCs that underlie SOCE. On the other hand, DFMO inhibits the expression of TRPC1 channels in colon cancer cells, eliminating their contribution to SOCE. The identification of players of SOCE in colon cancer cells may help to better understand the remodeling of the Ca2+ homeostasis in cancer. Importantly, the use of different pharmacological tools that target different SOCE molecular players in colon cancer cells may play a pivotal role in designing better chemoprevention strategies.

Inhibition of Polyamine Biosynthesis Reverses Ca2+ Channel Remodeling in Colon Cancer Cells.

Store-operated Ca2+ entry (SOCE) is the most important Ca2+ entry pathway in non-excitable cells. Colorectal cancer (CRC) shows decreased Ca2+ store content and enhanced SOCE that correlate with cancer hallmarks and are associated to remodeling of store-operated channels (SOCs). Normal colonic cells display small, Ca2+-selective currents driven by Orai1 channels. In contrast, CRC cells display larger, non-selective currents driven by Orai1 and transient receptor potential canonical type 1 channels (TRPC1). Difluoromethylornithine (DFMO), a suicide inhibitor of ornithine decarboxylase (ODC), the limiting step in polyamine biosynthesis, strongly prevents CRC, particularly when combined with sulindac. We asked whether DFMO may reverse SOC remodeling in CRC. We found that CRC cells overexpress ODC and treatment with DFMO decreases cancer hallmarks including enhanced cell proliferation and apoptosis resistance. Consistently, DFMO enhances Ca2+ store content and decreases SOCE in CRC cells. Moreover, DFMO abolish selectively the TRPC1-dependent component of SOCs characteristic of CRC cells and this effect is reversed by the polyamine putrescine. Combination of DFMO and sulindac inhibit both SOC components and abolish SOCE in CRC cells. Finally, DFMO treatment inhibits expression of TRPC1 and stromal interaction protein 1 (STIM1) in CRC cells. These results suggest that polyamines contribute to Ca2+ channel remodeling in CRC, and DFMO may prevent CRC by reversing channel remodeling.

Mitochondrial control of store-operated Ca2+ channels in cancer: Pharmacological implications.

Intracellular Ca2+ is a pleiotropic second messenger involved in control of different cell and physiological functions including long-term processes such as cell proliferation, migration and survival. Agonist-induced Ca2+ entry in most cells, especially in non-excitable cells including epithelial cells, is mediated by store-operated Ca2+ entry (SOCE), a Ca2+ entry pathway activated by agonist-induced release of Ca2+ from intracellular stores in the endoplasmic reticulum (ER). This pathway is modulated also by mitochondria which, acting as Ca2+ sinks, take up Ca2+, thus limiting Ca2+-dependent inactivation of Ca2+-release activated Ca2+ channels (CRAC). Compelling evidence shows that SOCE is upregulated in a large variety of cancer cells and this change contribute to cancer hallmarks. Mechanisms for enhanced SOCE include changes in expression of members of the Orai, Stromal interaction molecule (STIM) and canonical transient receptor potential channel (TRPc) gene families. Tumor cell mitochondria may contribute to SOCE upregulation in cancer as well. Molecular players involved in enhancing mitochondrial Ca2+ uptake are upregulated in tumor cells whereas negative modulators are repressed. Furthermore, mitochondrial potential, the driving force for mitochondrial Ca2+ uptake, is enhanced in tumor cells due to the Warburg effect. Finally, SOCE in tumor cells may be sustained further by the gain of function of non-selective TRPC channels permeable to Na+ that favour Ca2+ exit from mitochondria in exchange for Na+, thus limiting Ca2+-dependent inactivation of Orai1 channels. Therefore, tumor cell mitochondria may efficiently contribute to enhance and sustain SOCE in cancer. Interestingly, this effect could be counterbalanced by selected non-steroidal anti-inflammatory drugs (NSAIDs) reported to prevent colorectal cancer and other forms of cancer.

Transcriptomic Analysis of Calcium Remodeling in Colorectal Cancer.

Colorectal cancer (CRC) cells undergo the remodeling of intracellular Ca2+ homeostasis, which contributes to cancer hallmarks such as enhanced proliferation, invasion and survival. Ca2+ remodeling includes critical changes in store-operated Ca2+ entry (SOCE) and Ca2+ store content. Some changes have been investigated at the molecular level. However, since nearly 100 genes are involved in intracellular Ca2+ transport, a comprehensive view of Ca2+ remodeling in CRC is lacking. We have used Next Generation Sequencing (NGS) to investigate differences in expression of 77 selected gene transcripts involved in intracellular Ca2+ transport in CRC. To this end, mRNA from normal human colonic NCM460 cells and human colon cancer HT29 cells was isolated and used as a template for transcriptomic sequencing and expression analysis using Ion Torrent technology. After data transformation and filtering, exploratory analysis revealed that both cell types were well segregated. In addition, differential gene expression using R and bioconductor packages show significant differences in expression of selected voltage-operated Ca2+ channels and store-operated Ca2+ entry players, transient receptor potential (TRP) channels, Ca2+ release channels, Ca2+ pumps, Na⁺/Ca2+ exchanger isoforms and genes involved in mitochondrial Ca2+ transport. These data provide the first comprehensive transcriptomic analysis of Ca2+ remodeling in CRC.