Novel Apoptosis-Sensitizing E3 Ubiquitin Ligases

Defects in apoptosis play important roles in cancer pathogenesis and treatment failures. A need exists therefore for ways of reducing roadblocks to apoptosis in cancer, placing these malign cells into a more vulnerable state and making them easier to eradicate. Moreover, given the status of most people with cancer, it would be desirable if such therapies were relatively non-toxic. An attractive idea in this regard is to devise strategies for enticing immune responses against the tumor tissue. However, during the evolution of most tumors, defects emerge in the apoptosis pathways that immune effector cells rely upon for cytolytic killing, rendering the immune system impotent to eradicate tumors. The goal of this proposal is to characterize E3 ubiquitin ligases (E3s) that sensitize cancer cells to apoptosis, focusing particularly on the apoptosis pathways upon which the immune system depends. As explained below, two classes of such targets have been identified, including (a) IAPs, representing a family of apoptosis suppressors that directly bind to and inhibit the principal mediators of apoptosis, i.e. caspase-family cell death proteases; and (b) FLIP, an antagonist of caspases activated by TNF/Fas-family death receptors. Strategies for obtaining candidate E3s through siRNA library screening have been devised and lead-E3s are already in hand, which will be further characterized. E3s that demonstrate mechanism-based activity will be advanced into in vitro cell-based and in vivo tumor xenograft models, laying a foundation for further manipulation of tumors and consequently, reduction of treatment failures.