Abstract #3317: JAZ directly interacts with Rb/E2F to mediate G1 cell cycle arrest

The Rb/E2F pathway plays a central role in the control of cellular proliferation and tumor suppression. The tumor suppressor retinoblastoma protein (Rb) negatively regulates G1/S cell cycle transition by directly binding to E2F in a mechanism regulated by Rb phosphorylation. We previously identified a novel double-stranded RNA-binding zinc finger protein JAZ (Just Another Zinc Finger Protein) and reported that JAZ could directly bind and positively regulate p53 sequence-specific transcriptional activity with increased expression of p53 target genes including p21 and induction of G1 cell cycle arrest. We now find that JAZ may also negatively regulate G1 cell cycle progression in a novel mechanism downstream of p21 that involves its direct interaction with Rb/E2F proteins. Results of glutathione S-transferase (GST) \#8220;pull-down\#8221; and co-immunoprecipitation assays demonstrate that JAZ can interact with Rb and, like E2F-1, JAZ interact exclusively with the hypophosphorylated form of Rb that is known to bind and inactivate E2F. E2F is a heterodimeric transcription factor composed of two structurally related subunits, E2F and DP, and data show that JAZ co-immunoprecipitates with both E2F-1 and DP-1, suggesting a direct role for JAZ in regulating E2F activity. In support of this possibility, results from a luciferase reporter assay show that ectopic expression of JAZ can repress E2F transcriptional activity in association with G1 cell cycle arrest. Importantly, siRNA-mediated \#8220;knock-down\#8221; of endogenous JAZ increases E2F-mediated luciferase activity. It has been reported that Rb requires and cooperates with Brg-1,a componentof the SWI/SNF family of the ATP-dependent chromatin remodellingcomplexes, to repress the activation functions of E2F-1. We now find that JAZ also co-immunoprecipitates with Brg-1, suggesting a possible regulatory role. These data suggest that JAZ may be a component in a regulatory protein \#8220;complex\#8221; containing Rb/Brg-1/E2F1/DP-1 to inhibit E2F activity, at least in part. Thus, in addition to functioning as a direct positive regulator of p53, JAZ may have a dual role in the Rb/E2F pathway to mediate G1 cell cycle arrest. This dual function of JAZ could represent a potentially powerful therapeutic target in tumors.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3317.