Hence, developing BH3 mimetics could be a feasible approach to inhibit Mcl 1 function. Unfortu Lenalidomide nately, none of the BH3 mimetics under current devel opment are potent and specific Mcl 1 antagonists. Indeed, many pan Bcl2 inhibitors suffer from a lack of specificity or are simply too weak to compete with native high affinity BH3 only proteins for pro survival BH3 binding pockets. Further, such pan Bcl2 family protein inhibitors might well damage normal tissues. Hence, BH3 mimetics specific for single pro survival targets could have greater clinical utility. Pertinently, GDC 0199, a novel BH3 mimetic developed by Abbott and Genentech that is specific for Bcl 2, and which is now entering clinical trials for lymphoid malignancies, should avoid the dose limiting thrombocytopenia associated with the navitoclax.

For these reasons, designing an Mcl 1 specific inhibitor or searching for alternative tar gets for Mcl 1 antagonism has become popular. Our current research suggests that USP9X regulates Mcl 1 expression in cancer cells. Deubiquitinases have been demonstrated previously to antagonize specific oncogenic and tumor suppressive E3 ligases and are viewed as emerging targets for cancer therapeutics. USP9X can now be added to this list due to its role in deubiquitination and in stabilizing Mcl 1, a bona fide oncogene. In our current analyses, USP9X expression was found to be strongly associated with Mcl 1 expres sion in the human cancer tissue samples we tested. Recent reports have suggested also that USP9X enhances Mcl 1 stability by preventing its proteasomal destruction through de ubiquitination.

The balance between ubiquitination and deubiquitination determines Mcl 1 stability and expression. Ubiquitination of Mcl 1 pro motes USP9X Mcl 1 binding leading to Mcl 1 deubiqui tination and disassociation of these two proteins. Hence, and as shown from our current data, increasing Mcl 1 ubiquitination via PS341 promotes the association of USP9X with Mcl 1. Since Mcl 1 proteins are constantly ubiquitinated, their association with USP9X appears to be a steady state condition. This activity and upregula tion of USP9X as well as Mcl 1 have been associated with a poor prognosis and with Cilengitide chemoresistance in a number of cancers. To determine the impact of USP9X inhibition on cancer cell survival in our present experi ments, we used its inhibitor WP1130 and found that the treated cells showed Mcl 1 downregulation which increased their sensitivity to ABT 737 as well as to other chemotherapeutic agents. In light of the importance of USP9X in the control of Mcl 1 levels, compounds such as WP1130 or other more specific inhibitors may be useful in overcoming the apoptotic resistance associated with USP9X activity and Mcl 1 protection.