Transcriptional Basis of Ca2+ Remodeling Reversal Induced by Polyamine Synthesis Inhibition in Colorectal Cancer Cells

Simple Summary The common activation of the c-Myc oncogene in colorectal cancer (CRC) induces the overexpression of ornithine decarboxylase (ODC), the limiting step in polyamine synthesis, which is a process blocked by alpha-Difluoromethylornithine (DFMO), an ODC suicide inhibitor and potential CRC treatment. We showed previously that intracellular Ca2+ homeostasis is remodeled in CRC and contributes to cancer hallmarks. We investigated whether polyamine synthesis inhibition induced by DFMO may reverse this remodeling in CRC and, if so, the molecular basis for this phenotypic reversal. To this end we used calcium imaging and transcriptomic analysis in both normal and CRC cells. We found that CRC cells showed enhanced resting Ca2+ and store-operated Ca2+ entry (SOCE) but decreased Ca2+ store content relative to normal cells. Polyamine synthesis inhibition reversed not only this Ca2+ remodeling but also reversed the changes in the transcription of a dozen genes involved in Ca2+ transport in CRC cells, including genes modulating Ca2+ entry into the cells as store-operated channels and TRP channels as well as Ca2+ extrusion systems in the plasma membrane and mitochondria. These results provide a molecular basis for the role of polyamine synthesis in Ca2+ remodeling in cancer. Colorectal cancer (CRC) is associated with mutations in APC/Wnt leading to c-myc activation and the overexpression of ODC1, the limiting step in polyamine synthesis. CRC cells also display a remodeling of intracellular Ca2+ homeostasis that contributes to cancer hallmarks. As polyamines may modulate Ca2+ homeostasis during epithelial tissue repair, we investigated whether polyamine synthesis inhibition may reverse Ca2+ remodeling in CRC cells and, if so, the molecular basis for this reversal. To this end, we used calcium imaging and transcriptomic analysis in normal and CRC cells treated with DFMO, an ODC1 suicide inhibitor. We found that polyamine synthesis inhibition partially reversed changes in Ca2+ homeostasis associated with CRC, including a decrease in resting Ca2+ and SOCE along with an increased Ca2+ store content. We also found that polyamine synthesis inhibition reversed transcriptomic changes in CRC cells without affecting normal cells. Specifically, DFMO treatment enhanced the transcription of SOCE modulators CRACR2A; ORMDL3; and SEPTINS 6, 7, 8, 9, and 11, whereas it decreased SPCA2, involved in store-independent Orai1 activation. Therefore, DFMO treatment probably decreased store-independent Ca2+ entry and enhanced SOCE control. Conversely, DFMO treatment decreased the transcription of the TRP channels TRPC1 and 5, TRPV6, and TRPP1 while increasing TRPP2, thus probably decreasing Ca2+ entry through TRP channels. Finally, DFMO treatment enhanced the transcription of the PMCA4 Ca2+ pump and mitochondrial channels MCU and VDAC3 for enhanced Ca2+ extrusion through the plasma membrane and mitochondria. Collectively, these findings suggested the critical role of polyamines in Ca2+ remodeling in colorectal cancer.

Automated summary

In colorectal cancer (CRC), the activation of c-Myc oncogene leads to overexpression of ornithine decarboxylase (ODC), a key enzyme in polyamine synthesis. The ODC suicide inhibitor DFMO can potentially be used as a treatment for CRC. Our study showed that DFMO can reverse the remodeling of intracellular Ca2+ homeostasis and the changes in the transcription of Ca2+ transport-related genes in CRC cells.