Kaposi sarcoma-associated viral cyclin K overrides cell growth inhibition mediated by oncostatin M through STAT3 inhibition.

DNA viruses have evolved a number of mechanisms to inhibit the major cellular tumor-suppressor pathways. Viral oncogenes can override growth suppressive signals and extend the virus proliferative capacity. The Kaposi sarsoma–associated human herpesvirus 8 (KSHV) encodes a protein, cyclin K, that is similar to cellular cyclin D1 but behaves atypically. Cyclin K resists the actions of the p16 INK4a and p27Kip1 inhibitors and extends the range of cdk6 substrates, thereby inducing cell-cycle progression toward S phase. In this study, we show that cyclin K overrides growth suppressive signals through signal transducer and activator of transcription 3 (STAT3) inactivation. Cyclin K was found to associate with the activation domain of STAT3 to inhibit its DNA-binding and transcriptional activities. Overexpression of cyclin K and inhibition of STAT3 prevents the growth suppressive effect imposed by the interleukin 6–type cytokine, oncostatin M. Altogether, these results suggest that KSHV is able to override growth suppressive effects through multiple mechanisms, and they further indicate that cyclin K plays an important role in the oncogenic activity of these viruses.