Kidney Biology

Polycystin-1: A Double-Duty Activator

Science Signaling  17 May 2011:
Vol. 4, Issue 173, pp. ec142
DOI: 10.1126/scisignal.4173ec142

Mutations in the gene encoding polycystin-1 (PC1) are associated with autosomal-dominant polycystic kidney disease (ADPKD). PC1 is a transmembrane protein implicated in ciliary mechanotransduction. PC1 can be cleaved either in the extracellular portion or the intracellular portion, and cleavage of the intracellular domain in response to cessation of luminal fluid flow results in the accumulation of the PC1 C-terminal tail in the nucleus. The PC1 tail enhances the transcriptional regulatory activity of STAT6, and overexpression of PC1 also promotes the activity of STAT1 and STAT3, but whether this involves the cleaved C-terminal domain is unknown. Talbot et al. used either a C-terminal PC1 construct (FLS-PC1) or a membrane-anchored full-length PC1 construct (FLM-PC1) to show that the membrane-anchored form activated a reporter gene responsive to either STAT1 or STAT3 in HEK293T cells (a fibroblast cell line). Introduction of various ADPKD-associated mutations resulted in the identification of two mutants that had opposite effects on the ability of FLM-PC1 to activate the STAT1/3 reporter. Analysis in Madin-Darby canine kidney (MDCK) cells expressing FLM-PC1 in the presence or absence of various inhibitors of Janus kinase (JAK) signaling or dominant-negative inhibitors of STAT1 or STAT3 suggested that STAT-reporter activation by the membrane-bound PC1 required JAK-mediated phosphorylation of the STAT protein and that FLM-PC1 stimulated STAT3, but not STAT1, activation. FLM-PC1 interacted with JAK2, suggesting that membrane-bound PC1 may promote JAK2-mediated phosphorylation of STAT3. FLS-PC1 stimulated the activity of the STAT1/3 reporter in transfected HEK293T cells, and this effect was substantially increased by stimulation of the cells with interferon-γ (IFN-γ). Unexpectedly, FLS-PC1 did not increase the amount of phosphorylated STAT1 or STAT3, suggesting that the cleaved form of PC1 serves as a coactivator of previously phosphorylated STAT. The pathogenic mutations that altered FLM-PC1–mediated activation of STAT3 did not affect the coactivating activity of FLS-PC1. Forced expression of FLS-PC1 in MDCK cells stimulated cell proliferation under basal conditions in which phosphorylated STAT3 was present and amplified the responses to cytokines that activated either STAT1 or STAT6, suggesting that this cleaved nuclear form of PC1 sensitizes renal epithelial cells to STAT signaling. Consistent with this model, kidney lysates of mouse models of ADPKD exhibited increased basal phosphorylated STAT3 and strongly increased STAT1 activation after IFN-γ treatment of the mice compared with basal activity and with the IFN-γ response of the kidneys of wild-type animals. Mechanistically, this work suggests a dual mechanism of regulation of STAT activity by PC1. At the membrane, PC1 promotes JAK-mediated phosphorylation of STAT3, whereas in the nucleus, the cleaved PC1 C-terminal tail appears to promote the transcriptional regulatory activity of multiple STATs.

J. J. Talbot, J. M. Shillingford, S. Vasanth, N. Doerr, S. Mukherjee, M. T. Kinter, T. Watnick, T. Weimbs, Polycystin-1 regulates STAT activity by a dual mechanism. Proc. Natl. Acad. Sci. U.S.A. 108, 7985–7990 (2011). [Abstract] [Full Text]