KSHV encoded Rta induces cell cycle arrest

The Kaposi\u27s sarcoma-associated herpesvirus (KSHV) encoded immediate early gene, replication and transcription activator (K-Rta) is a key viral protein that serves as the master regulator for viral lytic replication. In this study, we investigated the role of K-Rta in cell cycle regulation and found that the expression of K-Rta in doxycycline (Dox)-inducible BJAB cells induced cell cycle arrest in G0/G1 phase. Western blot analysis of key cell cycle regulators revealed that K-Rta mediated cell cycle arrest was associated with a decrease in Cyclin A and phosphorylated Rb (pS807/ pS811) protein levels, both markers of S phase progression and an increase in protein levels for p27, a cyclin-dependent kinase inhibitor. Further, we found that K-Rta does not affect the transcription of p27 but regulates p27 at the post-translational level by inhibiting its proteosomal degradation. Immunofluorescence staining and cell fractionation experiments revealed largely nuclear compartmentalization of p27 in K-Rta expressing cells demonstrating that K-Rta not only stabilizes p27 but also modulates its cellular localization. Finally, shRNA knockdown of p27 significantly abrogates cell cycle arrest in K-Rta expressing cells supporting its key role in K-Rta mediated cell cycle arrest. Our findings are consistent with previous studies which showed that expression of immediate early genes of several herpes viruses including HSV, EBV and CMV results in cell cycle arrest at the G0/G1 phase, possibly to avoid competition of resources needed for host cell replication during the S phase