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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.

Polycystin, the PKD1 gene product mutated in autosomal dominant polycystic kidney disease, is a large membrane protein which is important in the differentiation of epithelial tubular structure. Furthermore, PKD1 mRNA is expressed in various tissues and in neoplastic cell lines particularly, suggesting that polycystin might be involved in differentiation and/or proliferation of other cell types. Therefore, in order to investigate such a possible role, polyclonal antibodies against a recombinant polycystin peptide were raised and used to study polycystin expression in human leukemia cell lines committed to differentiation. Using Western blot and laser scanning confocal microscopy analyses, we demonstrated expression of polycystin in erythroleukemia K562 cells as a membrane-associated polypeptide of approximately 450 kDa, mainly localized in cell-cell contacts. Protein size and subcellular distribution were similar to those found in the kidney epithelial KJ29 cell line. In addition, K562 cell erythroid differentiation induced by hemin was characterized by a reduction in polycystin expression, as measured by Western blot and Northern blot analyses. Cytofluorimetric analysis indicated that upon hemin treatment there was a progressive reduction in the number of polycystin-expressing cells as well as in proliferation rate. Furthermore, reduction in proliferating and polycystin-expressing cells was also observed in K562 cells after serum starvation. When serum was added to the serum-deprived cells an increase in cell number as well as in number of polycystin-positive cells was observed. In addition, polycystin, also expressed in promyelocytic leukemia HL60 cells, was downregulated when macrophage differentiation in HL60 was induced by TPA. Therefore, in these leukemic cells downregulation of polycystin appeared to be closely related to reduction in cell proliferation and to induction of differentiation. This suggests that polycystin may play a relevant role in these cell processes., (Copyright 1998 Academic Press.)