Abstract
The retinoblastoma protein (RB) suppresses cell proliferation and apoptosis. We have previously shown that RB degradation is required for tumor necrosis factor alpha (TNF-α) to induce apoptosis. We show here the identification of two apoptotic effectors, i.e., c-ABL tyrosine kinase and p73, which are activated by TNF-α following RB degradation. In cells expressing a degradation-resistant RB protein (RB-MI), TNF-α does not activate c-ABL. RB-MI also inhibits TNF-α-mediated activation of p73. Genetic deletion and pharmacological inhibition of c-ABL or p73 diminish the apoptotic response to TNF-α in human cell lines and mouse fibroblasts. Thymocytes isolated from RbMI/MI, Abl−/−, or p73−/− mice are resistant to TNF-α-induced apoptosis compared to their wild-type counterparts. This is in contrast to p53−/− thymocytes, which exhibit a wild-type level of apoptosis in response to TNF-α. Thus, c-ABL and p73 contribute to apoptosis induced by TNF-α, in addition to their role in promoting DNA damage-associated cell death.
We thank members of the Wang lab for their critical comments throughout the course of this work. We are particularly indebted to Sewon Ki for reconstituting HA-p73α expression in p73−/− fibroblasts and to Irina Hunton for generating Rb−/−Abl−/− and Rb−/−p53−/− fibroblasts.
B.N.C. is a Robert Black Fellow of the Damon Runyon Cancer Research Foundation (DRG-1725-02). This work was supported by NIH grant CA58320 to J.Y.J.W. and American Cancer Society grant RSG LIB-101051 to J.D.