The Mechanism of Action of the Tumor Suppressor HRSL3

Science's STKE  10 Apr 2007:
Vol. 2007, Issue 381, pp. tw126
DOI: 10.1126/stke.3812007tw126

Loss of tumor suppressor function is a critical step in the development of tumors. Two classes of tumor suppressors are known. Loss of function of class I suppressors, such as p53, is achieved through genetic alterations, whereas inhibition of class II suppressors is reversible and leaves their genomic structures unchanged. Nazarenko et al. investigated the mechanism of action of the class II suppressor HRSL3. HRSL3 protein is found in normal tissues but is undetectable in human tumors. The authors performed a yeast two-hybrid screen for binding partners for HRSL3 and detected PR65α, the regulatory subunit Aα of protein phosphatase 2A (PP2A), a serine/threonine phosphatase. Using HRSL3 mutants, the authors determined that the N-terminal region of HRSL3 was necessary for binding to PR65α. Coimmunoprecipitation studies in a transfected human ovarian carcinoma cell line (OVCAR-3) demonstrated that HRSL3 did not bind to the catalytic subunit of PP2A, PR36α. The catalytic activity of PP2A isolated from cells that contained HRSL3 was much reduced compared with that of PP2A isolated from cells that contained a noninteracting mutant of HRSL3. Through flow cytometric analysis of nuclear DNA and Western blotting analysis, the authors demonstrated that either increased amounts of HRSL3 or knockdown of PP2A with a PR65-specific siRNA induced apoptosis of OVCAR-3 cells. The presence of HRSL3 in OVCAR-3 cells resulted in increased amounts of phosphorylated PKCζ, a target of PP2A. Blocking PKCζ activity with a pseudosubstrate peptide abrogated HRSL3-induced apoptosis of the OVCAR-3 cells, suggesting a role for PKCζ in regulating cell death in ovarian cancer.

I. Nazarenko, R. Schäfer, C. Sers, Mechanisms of the HRSL3 tumor suppressor function in ovarian carcinoma cells. J. Cell. Sci. 120, 1393-1404 (2007). [Abstract] [Full Text]