Combined depletion of cell cycle and transcriptional cyclin-dependent kinase activities induces apoptosis in cancer cells

Selective cyclin-dependent kinase (cdk) 2 inhibition is readily compensated. However, reduced cdk2 activity may have antiproliferative effects in concert with other family members. Here, inducible RNA interference was used to codeplete cdk2 and cdk1 from NCI-H1299 non-small cell lung cancer and U20S osteosarcoma cells, and effects were compared with those mediated by depletion of either cdk alone. Depletion of cdk2 slowed G(1) progression of NCI-H1299 cells and depletion of cdkI slowed G(2)-M progression in both cell lines, with associated endoreduplication in U20S cells. However, compared with the incomplete cell cycle blocks produced by individual depletion, combined depletion had substantial consequences, with G(2)-M arrest predominating in NCI-HI299 cells and apoptosis the primary outcome in U20S cells. In U20S cells, combined depletion affected RNA polymerase II expression and phosphorylation, causing decreased expression of the antiapoptotic proteins Mcl-1 and X-linked inhibitor of apoptosis (XLAP), effects usually mediated by inhibition of the transcriptional cdk9. These events do not occur after individuaI depletion of cdk2 and cdk1, suggesting that reduction of cdk2, cdki, and RNA polymerase II activities all contribute to apoptosis in U2OS cells. The limited cell death induced by combined depletion in NCI-H1299 cells was significantly increased by codepletion of cdk9 or XILAP or by simultaneous treatment with the cdk9 inhibitor flavopiridol. These results show the potency of concomitant compromise of cell cycle and transcriptional cdk activities and may guide the selection of clinical drug candidates.