The retinoblastoma protein induces apoptosis directly at the mitochondria

Abstract

The retinoblastoma protein gene RB-1 is mutated in one-third of human tumors. Its protein product, pRB (retinoblastoma protein), functions as a transcriptional coregulator in many fundamental cellular processes. Here, we report a nonnuclear role for pRB in apoptosis induction via pRB's direct participation in mitochondrial apoptosis. We uncovered this activity by finding that pRB potentiated TNFa-induced apoptosis even when translation was blocked. This proapoptotic function was highly BAX-dependent, suggesting a role in mitochondrial apoptosis, and accordingly, a fraction of endogenous pRB constitutively associated with mitochondria. Remarkably, we found that recombinant pRB was sufficient to trigger the BAX-dependent permeabilization of mitochondria or liposomes in vitro. Moreover, pRB interacted with BAX in vivo and could directly bind and conformationally activate BAX in vitro. Finally, by targeting pRB specifically to mitochondria, we generated a mutant that lacked pRB's classic nuclear roles. This mito-tagged pRB retained the ability to promote apoptosis in response to TNFa and also additional apoptotic stimuli. Most importantly, induced expression of mito-tagged pRB in Rb -/-;p53 -/-tumors was sufficient to block further tumor development. Together, these data establish a nontranscriptional role for pRB in direct activation of BAX and mitochondrial apoptosis in response to diverse stimuli, which is profoundly tumor-suppressive.

RB1: a prototype tumor suppressor and an enigma

“…Indeed, in some assay systems, pRB-mediated cell cycle exit correlates better with its effects on p27 levels than with changes in proteins expressed from E2F-regulated genes (Ji et al 2004). A pool of pRB has been detected at mitochondria, where it suppresses apoptosis (Ferecatu et al 2009;Hilgendorf et al 2013), providing further support for the view that pRB has effects on cell proliferation that extend beyond transcription.…”

“…Other studies have shown that a subpopulation of pRB is located at the outer mitochondrial membrane, where it physically interacts with Bax and promotes apoptosis (Hilgendorf et al 2013), suggesting that pRB loss impacts multiple aspects of mitochondrial function. This observation adds to a large number of studies showing that, under specific conditions, pRB can cooperate with other factors to promote the transcriptional activation of differentiation programs and regulate the expression of apoptotic regulators (for examples, see Thomas et al 2001;Ianari et al 2009;Calo et al 2010; for review, see Attardi and Sage 2013).…”

Anti-proliferative and pro-apoptotic activity of whole extract and isolated indicaxanthin from Opuntia ficus-indica associated with re-activation of the onco-suppressor p16INK4a gene in human colorectal carcinoma (Caco-2) cells

“…Besides being major controllers of the cell cycle, p16 INK4a and RB possess multiple antitumor functions [44] and their activity in regulating cell death has been shown. Over-expression of p16 INK4a has been associated with apoptosis of various transformed cells with down-regulation of the anti-apoptotic bcl-2 protein [45][46][47], and involvement of activated RB in apoptosis has been reported [48,49]. Early epigenetic changes during carcinogenesis can be induced, but also reverted, by external effectors, including food components.…”

RB localizes to DNA double-strand breaks and promotes DNA end resection and homologous recombination through the recruitment of BRG1

“…The main driver of cellular transformation in the absence of RB is thought to be unchecked cell proliferation. RB loss may also promote tumor development through inhibition of differentiation or impaired apoptosis (Ianari et al 2009;Dick and Rubin 2013;Flowers et al 2013;Hilgendorf et al 2013). In addition, RB has transcription-independent functions on chromosome condensation and maintenance of telomeric heterochromatin (Gonzalo et al 2005;Coschi et al 2010;Manning et al 2010).…”