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 Ca 2 þ deficiency-induced epithelial proliferation using a unique animal model. The zebrafish larval yolk sac skin contains a group of Ca 2 þ -transporting epithelial cells known as ionocytes. Their number and density increases dramatically when acclimated to low [Ca 2 þ ] environments. BrdU pulse-labeling experiments suggest that low [Ca 2 þ ] stimulates pre-existing ionocytes to re-enter the cell cycle. Low [Ca 2 þ ] 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 Ca 2 þ -selective channel Trpv5/6. Inhibition or knockdown of Igfbp5a, IGF1 receptor, PI3K, and Akt attenuates low [Ca 2 þ ]-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 [Ca 2 þ ]. These increases are eliminated in the presence of an IGF1R inhibitor, suggesting that Trpv5/6 represses IGF1R-PI3K-Akt signaling under normal [Ca 2 þ ]. Intriguingly, blockade of Trpv5/6 activity inhibits the low [Ca 2 þ ]-induced activation of Akt. Mechanistic analyses reveal that the low [Ca 2 þ ]-induced IGF signaling is mediated through Trpv5/6-associated membrane depolarization. Low extracellular [Ca 2 þ ] 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 Ca 2 þ deficiency.

Stanniocalcin-1 Potently Inhibits the Proteolytic Activity of the Metalloproteinase Pregnancy-associated Plasma Protein-A

“…In teleost fish, STC1 inhibits uptake of calcium ions from the environment through the gills (3), and recently, studies in zebrafish have revealed that this involves regulated expression of the epithelial calcium ion channel in gill ionocytes (43,44), although some molecular details are still lacking. In this regard, it is interesting that an apparent link between calcium levels and IGF-induced cell proliferation has been demonstrated for zebrafish skin ionocytes, which have a similar regulatory role in the embryo before the gills are developed: activation of IGF1R-PI3K-Akt signaling was found to result from low levels of extracellular calcium ions (45). The proposed signaling mechanism involves zebrafish IGFBP-5, which is also a PAPP-A substrate (46).…”

The Control of Calcium Metabolism in Zebrafish (Danio rerio)

“…IGF-1 is able to stimulate zebrafish cell proliferation through activating the PI3K-Akt signaling pathway [106]. In zebrafish embryos, a phospho-Akt signal was detected in NaRC by ICC [22], and lower-Ca 2+ water treatment increased the number of phospho-Akt signals and NaRCs [22]. Subsequent pharmacological experiments suggested that the IGF-1R-PI3K-Akt signaling pathway directly regulates Ca 2+ absorption through stimulating NaRC proliferation [22].…”

“…In zebrafish embryos, a phospho-Akt signal was detected in NaRC by ICC [22], and lower-Ca 2+ water treatment increased the number of phospho-Akt signals and NaRCs [22]. Subsequent pharmacological experiments suggested that the IGF-1R-PI3K-Akt signaling pathway directly regulates Ca 2+ absorption through stimulating NaRC proliferation [22]. These findings bring a novel insight into the action of IGF-1 signaling on body fluid Ca 2+ homeostasis in vertebrates.…”

“…However, the function of other hormones (i.e., hydrogen sulfide (H 2 S), calcitonin (CT), isotocin, insulin-like growth factor (IGF), and stanniocalcin-1 (STC-1)) on Ca 2+ regulation was controversial or unclear. In zebrafish, the function of these calciotropic hormones has been explored in recent years [18,19,20,21,22,23,24]. The expression and/or secretion of calciotropic hormones are responsive to extracellular Ca 2+ levels.…”

Transient Receptor Potential Channel, Vanilloid 5, Induces Chondrocyte Apoptosis in a Rat Osteoarthritis Model Through the Mediation of Ca2+ Influx

“…TRPV5 is particularly important in regulating Ca 2+ influx to maintain Ca 2+ homeostasis [7]. Loss of TRPV5 function results in abnormal ionocyte proliferation and increased colon cancer risk [8]. TRPV5 may contribute to the process of estrogen-inhibited osteoclastogenesis and bone resorption activity by mediating extracellular Ca 2+ [9].…”