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

Abstract

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 Ca2+ -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. © 2014 Macmillan Publishers Limited All rights reserved.