Knockdown of Slingshot 2 (SSH2) serine phosphatase induces Caspase3 activation in human carcinoma cell lines with the loss of the Birt–Hogg–Dubé tumour suppressor gene (FLCN)

Birt–Hogg–Dubé (BHD) syndrome, is a dominantly inherited familial cancer syndrome associated with susceptibility to renal cell carcinoma (RCC) caused by inactivating mutations in the folliculin (FLCN) gene. The precise functions of the FLCN gene product are still under investigation but RCC from BHD patients show loss of the wild-type allele consistent with a tumor suppressor gene function. In a search for potential synthetic-lethal targets for FLCN using a phosphatase siRNA library screening approach, we found that knockdown of SSH2 serine phosphatase (one of the three members of Slingshot family and previously implicated in actin reorganization) specifically induced Caspase3/7 activity in a dose-dependent manner (up to six-fold increase, 10 nM, 72 h) in two human FLCN-deficient cell lines (BHD-origin renal cell carcinoma UOK257 and thyroid carcinoma FTC133) but not in their folliculin expressing isogenic cell lines. SSH2 siRNA-induced knockdown was accompanied by increased expression of SSH1 and SSH3 (suggesting a compensatory regulatory mechanism among members of SSH family). FLCN-null cells exhibited evidence of dysregulated cofilin de/phosphorylation pathways. Knockdown of SSH2 in FLCN-null cells was associated with an alteration in cell cycle kinetics (20% increase in G1, 30% and 40% decrease in S and G2M, respectively). Combination treatment of multiple SSH family (SSH2 plus SSH1 and/or SSH3) siRNAs potentiated induction of Caspase3/7 activity and changes in the cell cycle kinetics. These data indicate that: (a) apoptotic cell death in FLCN-null cells can be triggered by SSH2 knockdown through cell cycle arrest; (b) SSH2 represents a potential therapeutic target for the development of agents for the treatment of BHD syndrome and, possibly, related tumors.