The Role of G-Protein-Coupled Receptor Proteolysis Site Cleavage of Polycystin-1 in Renal Physiology and Polycystic Kidney Disease
Polycystin-1 (PC1) is a large transmembrane protein important in renal differentiation and defective in most cases of autosomal dominant polycystic kidney disease (ADPKD), a common cause of renal failure in adults. Although the genetic basis of ADPKD has been elucidated, molecular and cellular mechanisms responsible for the dysregulation of epithelial cell growth in ADPKD cysts are still not well defined. We approached this issue by investigating the role of the carboxyl cytoplasmic domain of PC1 involved in signal transduction on the control of kidney cell proliferation. Therefore, we generated human HEK293 cells stably expressing the PC1 cytoplasmic tail as a membrane targeted TrkA-PC1 chimeric receptor protein (TrkPC1). We found that TrkPC1 increased cell proliferation through an increase in cytoplasmic Ca2+ levels and activation of PKC alpha, thereby upregulating D1 and D3 cyclin, downregulating p21waf1 and p27kip1 cyclin inhibitors, and thus inducing cell cycle progression from G0/G1 to the S phase. Interestingly, TrkPC1-dependent Ca2+ increase and PKC alpha activation are not constitutive, but require serum factor(s) as parallel component. In agreement with this observation, a significant increase in ERK1/2 phosphorylation was observed. Consistently, inhibitors specifically blocking either PKC alpha or ERK1/2 prevented the TrkPC1-dependent proliferation increase. NGF, the TrkA ligand, blocked this increase. We propose that in kidney epithelial cells the overexpression of PC1 C-terminus upregulates serum-evoked intracellular Ca2+ by counteracting the growth-suppression activity of endogenous PC1 and leading to an increase in cell proliferation.