A Role for pRb and Cdk5 as Regulators of Adherens Junction Assembly and Cell-to-Cell Interactions
Co-leaders:
MCC – Dr. Douglas Cress
PSM – Dr. Pedro Santiago
Our work points to a novel role for the retinoblastoma tumor suppressor protein (pRb), a cell cycle repressor commonly inactivated in cancer, as an inducer of cell-cell adhesion. Our studies show that pRb, acting via the cyclin-dependent kinase 5 (Cdk5), promotes the formation of cadherin-based adherens junctions, which are membrane-associated structures involved in cell-cell adhesion and that are known to be disrupted during oncogenic transformation. A dual role for pRb as both a cell cycle repressor and as inducer of adherens junction assembly raises the extremely exciting prospect that, in at least some tumor types, inactivation pRb’s tumor suppressive capacity could play a significant role not only during initial tumor formation by providing targeted cells with a replicative advantage over normal cells, but also during later stages of tumorigenesis by provoking the disruption of cell-cell interactions consequently leading to metastasis. This proposal is aimed at obtaining a better understanding of the mechanisms by which pRb regulates adherens junction assembly. To accomplish this goal we plan to pursue the following specific aims. First, we propose to elucidate the mechanism by which pRb activates Cdk5, an activation step of central importance for the pRb-mediated assembly of adherens junctions. In this aim we will employ several strategies to determine whether pRb can regulate Cdk5 activity by modulating its phosphorylation status, its intracellular localization, or the expression and/or activity of known Cdk5 regulatory subunits such as p35 and p39. Second, we wish to determine whether pRb can regulate the assembly of adherens junctions by controlling the transcription of genes coding for their constituent proteins. Here we will identify genes coding for adherens junction proteins whose expression is controlled by pRb and dissect their regulatory elements in search for putative binding sites either for pRb or for transcriptional regulators capable of interacting with pRb. In broader terms, this project could contribute to the characterization of a non-traditional role for pRb as a regulator of adherens junction formation and cell-cell interactions and of a signaling pathway that may be involved in the orchestration between cell proliferation itself and the mechanisms that regulate cell proliferation in response to extracellular cues such as cell-cell interactions. A disruption of pRb¢s tumor suppressive capacity with a concomitant breakdown of this orchestration could be at the core of the molecular etiology of cancer. Conceptually, this integrative view of cancer could significantly contribute to what has been described by some cancer researchers as the hope to achieve an understanding of the complex process of neoplastic transformation at the cellular level in terms of a small number of underlying genetic changes (Hahn and Weinberg, 2002). A role for pRb as a regulator of both cell cycle progression and adherens junction formation would be fully consistent with this emerging paradigm of neoplastic transformation.