The cell cycle inhibitor p57Kip2 promotes cell death via the mitochondrial apoptotic pathway

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Abstract
The p57Kip2 gene belongs to the Cip/Kip family of cyclin-dependent kinase (CDK) inhibitors and has been suggested to be a tumor suppressor gene, being inactivated in various types of human cancers. However, little is known concerning p57Kip2 possible interplay with the apoptotic cell death machinery and its possible implication for cancer. Here, we report that selective p57Kip2 expression sensitizes cancer cells to apoptotic agents such as cisplatin, etoposide and staurosporine (STS) via a mechanism, which does not require p57Kip2-mediated inhibition of CDK. Translocation of p57Kip2 to mitochondria occurs within 20 min after STS application. In fact, p57Kip2 primarily promotes the intrinsic apoptotic pathways, favoring Bax activation and loss of mitochondrial transmembrane potential, consequent release of cytochrome-c into cytosol, caspase-9 and caspase-3 activation. In accordance, Bcl2 overexpression or voltage-dependent anion channel (VDAC) inhibition is able to inhibit p57Kip2 cell death promoting effect. Thus, in addition to its established function in control of proliferation, these results reveal a mechanism whereby p57Kip2 influences the mitochondrial apoptotic cell death pathway in cancer cells.