Intestinal cell kinase, a MAP kinase-related kinase, regulates proliferation and G₁cell cycle progression of intestinal epithelial cells

Abstract

Intestinal cell kinase (ICK), originally cloned from the intestine and expressed in the intestinal crypt epithelium, is a highly conserved serine/threonine protein kinase that is similar to mitogen-activated protein kinases (MAPKs) in the catalytic domain and requires dual phosphorylation within a MAPK-like TDY motif for full activation. Despite these similarities to MAPKs, the biological functions of ICK remain unknown. In this study, we report that suppression of ICK expression in cultured intestinal epithelial cells by short hairpin RNA (shRNA) interference significantly impaired cellular proliferation and induced features of gene expression characteristic of colonic or enterocytic differentiation. Downregulation of ICK altered expression of cell cycle regulators (cyclin D1, c-Myc, and p21(Cip1/WAF1)) of G(1)-S transition, consistent with the G(1) cell cycle delay induced by ICK shRNA. ICK deficiency also led to a significant decrease in the expression and/or activity of p70 ribosomal protein S6 kinase (S6K1) and eukaryotic initiation factor 4E (eIF4E), concomitant with reduced expression of their upstream regulators, the mammalian target of rapamycin (mTOR) and the regulatory associated protein of mTOR (Raptor). Furthermore, ICK interacts with the mTOR/Raptor complex in vivo and phosphorylates Raptor in vitro. These results suggest that disrupting ICK function may downregulate protein translation of specific downstream targets of eIF4E and S6K1 such as cyclin D1 and c-Myc through the mTOR/Raptor signaling pathway. Taken together, our findings demonstrate an important role for ICK in proliferation and differentiation of intestinal epithelial cells.

Ketogenesis contributes to intestinal cell differentiation

“…32 Moreover, HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity in intestinal cells. 46 As activation of mTOR increases c-Myc expression in intestinal cells, 47 it is likely that mTOR inhibition induces HMGCS2 through the alterations of PPARα-and c-Myc-dependent functions in intestinal cells. Our data also showed that treatment with βHB or overexpression of HMGCS2 in vitro, or induction in ketogenesis in vivo, resulted in the inhibition of mTOR signaling in intestinal cells.…”

Intestinal cell kinase, a protein associated with endocrine-cerebro-osteodysplasia syndrome, is a key regulator of cilia length and Hedgehog signaling

“…Furthermore, the role of mTORC1 in regulating ciliary length is mediated by the regulation of ciliary protein synthesis through activation of a ribosomal protein S6k1. Recently, ICK was shown to act as an upstream regulator of the mTORC1 signaling pathway to promote cell proliferation and G1 cell cycle progression (3,33). Thus, it would be worth investigating whether ICK influences ciliary length by the translational regulation of ciliary proteins through the mTORC1 signaling pathway.…”

Regulations and Functions of ICK/MAK/MOK - A Novel MAPK-Related Kinase Family Linked to Human Diseases

“…Taken together, a plethora of emerging evidence indicates that the complex phosphorylation status of Raptor is tightly associated with the activity of mTORC1. Our data indicated that ICK associates with Raptor and phosphorylates Raptor at Thr-908 (Fu, Kim et al 2009). More importantly, knockdown the ICK expression significantly reduced the phosphorylation of S6K1 at Thr-389 targeted by the mTORC1, suggesting that ICK is an upstream regulator of the mTORC1 activity in the regulation of cell growth and proliferation (Fu, Kim et al 2009).…”

“…ICK deficiency also led to a significant decrease in the expression and/or activity of S6K1, indicating that disrupting ICK function downregulates the mTORC1 signaling pathway. Our prior studies also provided biochemical evidence that ICK interacts with the mTOR/Raptor complex in cells and Raptor is an in vitro substrate for ICK (Fu, Kim et al 2009). Recently, we investigated whether and how ICK targets Raptor to regulate the activity of mTORC1.…”

“…Recently, we reported that suppression of ICK expression in cultured colorectal carcinoma and intestinal epithelial cell lines by short hairpin RNA (shRNA) interference significantly www.intechopen.com (Fu, Kim et al 2009). Downregulation of ICK altered expression of cell cycle regulators (cyclin D1, c-Myc, and p21 Cip1/WAF1 ) of G1-S transition, consistent with the G1 cell cycle delay induced by ICK shRNA.…”