The Retinoblastoma Protein Is Required for Ras-Induced Oncogenic Transformation

Abstract

Most human cancers involve either mutational activation of the Ras oncogenic pathway and/or inactivation of the retinoblastoma tumor suppressor (RB) pathway. Paradoxically, tumors that harbor Ras mutations almost invariably retain expression of a wild-type pRB protein. We explain this phenomenon by demonstrating that Ras-induced oncogenic transformation surprisingly depends on functional pRB protein. Cells lacking pRB are less susceptible to the oncogenic actions of H-Ras V12 than wild-type cells and activated Ras has an inhibitory effect on the proliferation of pRB-deficient human tumor cells. In addition, depletion of pRB from Ras-transformed murine cells or human tumor cells that harbor Ras pathway mutations inhibits their proliferation and anchorage-independent growth. In sharp contrast to pRB ؊/؊ 3T3 cells, fibroblasts deficient in other pRB family members (p107 and p130) are more susceptible to Ras-mediated transformation than wild-type 3T3 cells. Moreover, loss of pRB in tumor cells harboring a Ras mutation results in increased expression of p107, and overexpression of p107 but not pRB strongly inhibits proliferation of these tumor cells. Together, these findings suggest that pRB and p107 have distinct roles in Ras-mediated transformation and suggest a novel tumor-suppressive role for p107 in the context of activated Ras.Human tumors arise from an accumulation of genetic changes that result in a disruption of the normal control of several cellular processes, including proliferation, apoptosis, and differentiation. These genetic changes largely include lossof-function mutations in tumor suppressor genes and gain-offunction mutations in proto-oncogenes. The retinoblastoma (Rb) gene was the first tumor suppressor gene to be cloned (22,43) and loss of the Rb gene, originally described as the ratelimiting step in the development of retinoblastomas, has since been found in a variety of other tumor types (28,34,40,74).The retinoblastoma protein (pRB) has been implicated in the regulation of a multitude of cellular processes, such as cell cycle progression and cell death, as well as differentiation (44,68,73). The most clearly established role for pRB may be as a regulator of proliferation (10,16,20,31,36,52,56,71). pRB is thought to control progression through the G 1 phase of the cell cycle by regulating the E2F transcription factor family and the recruitment of chromatin-remodeling complexes to promoter regions (4,6,26,27,65). The ability of pRB to repress E2F-mediated transcription is affected by phosphorylation of pRB by cyclin-dependent kinases, linking pRB to the intricate network that regulates the cell cycle (63). Significantly, alterations in several proteins (loss of the p16 cyclin-dependent kinase inhibitor and amplification of cyclin D or mutation of cdk4) that have been implicated in the regulation of pRB function are also commonly observed in a broad spectrum of tumor types, suggesting that deregulation of the normal pathway in which pRB functions is a common and important event in the development ...

Modeling the effect of the RB tumor suppressor on disease progression: dependence on oncogene network and cellular context

“…Thus, these findings suggest that there are intrinsic physiological conditions under which loss of RB does not provide a clear advantage to tumor cells. Interestingly, recent literature has suggested a surprising requirement for RB in Rasmediated transformation (Williams et al, 2006). This role for RB seems counterintuitive given the previously documented role for RB deficiency in promoting escape from facets of Ras-mediated senescence (Serrano et al, 1997).…”

“…It has been shown that oncogenic transformation through Ras in cell culture and animal models is dependent on cyclin D1 (Liu et al, 1995;Peeper et al, 1997). Therefore, it was suggested that these two pathways function in parallel, although recent studies have proposed that loss of RB actually compromises Ras-mediated transformation and proliferation of tumor cells (DeGregori, 2006;Williams et al, 2006). Thus, these contradictory findings suggest that further probing the intersection of oncogenic signaling and RB function may yield important insights into cooperation between coordinated tumorassociated events in cancer.…”

Surprising Dependency for Retinoblastoma Protein in Ras-Mediated Tumorigenesis

“…Still, it is unclear why different paths to cancer preferentially select for mutations in one component over another. A paper in this issue of Molecular Cellular Biology by Williams, Classon, and colleagues reveals a surprising requirement for Rb in proliferation and transformation mediated by the Ras oncogene (50). This study provides a rationale for why mutational activation of Ras and genetic disruption of Rb are rarely found together in human cancers.…”

“…Williams and colleagues now shed light on this issue (50). They demonstrate that, quite in contrast to Rb's usual tumor suppressor role and in marked contrast to p107/p130 loss, Rb loss actually impedes the transformation of mouse NIH 3T3 fibroblasts by Ras*.…”

“…Thus, a mutual antagonism between Rb and Ras appears well established. However, as discussed below, Williams et al now contribute another twist, a functional dependency of Ras* for Rb (50).…”