Calcium deficiency-induced and TRP channel-regulated IGF1R-PI3K-Akt signaling regulates abnormal epithelial cell proliferation

Calcium deficiency causes abnormal colonic growth and increases colon cancer risk with poorly understood mechanisms. Here we elucidate a novel signaling mechanism underlying the Ca2+ deficiency-induced epithelial proliferation using a unique animal model. The zebrafish larval yolk sac skin contains a group of Ca-2+ transporting epithelial cells known as ionocytes. Their number and density increases dramatically when acclimated to low [Ca2+] environments. BrdU pulse-labeling experiments suggest that low [Ca2+] stimulates pre-existing ionocytes to re-enter the cell cycle. Low [Ca2+] treatment results in a robust and sustained activation of IGF1R-PI3K-Akt signaling in these cells exclusively. These ionocytes specifically express Igfbp5a, a high-affinity and specific binding protein for insulin-like growth factors (IGFs) and the Ca2+ selective channel Trpv5/6. Inhibition or knockdown of Igfbp5a, IGF1 receptor, PI3K, and Akt attenuates low [Ca2+]- induced ionocyte proliferation. The role of Trpv5/6 was investigated using a genetic mutant, targeted knockdown, and pharmacological inhibition. Loss-of-Trpv5/6 function or expression results in elevated pAkt levels and increased ionocyte proliferation under normal [Ca2+]. These increases are eliminated in the presence of an IGF1R inhibitor, suggesting that Trpv5/6 represses IGF1R-PI3K-Akt signaling under normal [Ca2+]. Intriguingly, blockade of Trpv5/6 activity inhibits the low [Ca2+]- induced activation of Akt. Mechanistic analyses reveal that the low [Ca2+]- induced IGF signaling is mediated through Trpv5/6- associated membrane depolarization. Low extracellular [Ca2+] results in a similar amplification of IGF-induced PI3K-PDK1- Akt signaling in human colon cancer cells in a TRPV6-dependent manner. These results uncover a novel and evolutionarily conserved signaling mechanism that contributes to the abnormal epithelial proliferation associated with Ca2+ deficiency.