The retinoblastoma gene product (Rb) is well known as a tumor suppressor and is either absent or mutated in many human tumors. Retrovirus-mediated gene transfer of the wild-type Rb gene into several Rb mutant neoplastic cell lines suppresses their tumorgenicity. Rb is a 110-kDa nuclear phosphoprotein that undergoes differential phosphorylation during the cell cycle. During G1 phase, Rb is predominantly in a hypophosphorylated state. It becomes increasingly phosphorylated throughout the cell cycle until late mitosis, when substantial dephosphorylation occurs. Hypophosphorylated Rb interacts with a number of cellular proteins including the E2F transcription factor, several cyclins, RBP-1, RBP-2, c-Abl, c-myc, N-myc, and p46. Phosphorylation of Rb at various sites, by Cyclin-dependent protein kinases, inhibits the binding of Rb to these proteins. Rb is thought to mediate its effects, in part, via the repression of genes required for proliferation. For example, Rb is specifically recruited to promoters containing E2F sites and actively represses E2F mediated transcription. Rb also stimulates the activity of other transcription factors, although the mechanisms are less clearly defined. Thus, Rb appears to regulate transcription in its aim to control cell growth.
The J112-906 monoclonal antibody recognizes Rb phosphorylated at serines 807 and 811 (pS807/pS811), which regulate c-Abl binding and cell cycle progression. The orthologous phosphorylation sites in mouse and rat Rb are pS800/pS804 and pS778/pS782, respectively.