Rb and Tumorigenesis examines how recent advances have demonstrated the interaction of Rb with chromatin remodeling enzymes. This new title explores the potential roles of these interactions in Rb functions and provides some evidence that distinct Rb co-repressor may target different genes in different phases of the cell cycle. This book will interest cell biologists, graduate students and researchers.

ABSTRACT: The retinoblastoma tumor suppressor protein, Rb, is a key regulator of the mammalian cell cycle and its inactivation facilitates S-phase entry. Rb is inactivated through multiple waves of phosphorylation, mediated mainly by kinases associated with D and E type cyclins in the G1 phase of the cell cycle. Our earlier studies had shown that the signaling kinase Raf-1 (c-Raf) physically interacts with Rb upon growth factor stimulation and initiates the phosphorylation cascade. We had shown that an 8 amino acid peptide derived from Raf-1 could disrupt the Rb-Raf-1 interaction leading to an inhibition of Rb phosphorylation, cell proliferation and tumor growth in nude mice. Here, we describe a newly identified orally-active small molecule, RRD-251 (Rb - Raf-1 Disruptor 251), that disrupts potently and selectively the binding of Raf-1 to Rb; it had no effect on Rb-HDAC1, Rb-Prohibitin, Rb-Ask1, Rb-cyclin E, or Raf-1-Mek interactions. RRD-251 inhibited anchorage-dependent and -independent growth of human cancer cells; it could also potently inhibit angiogenesis both in vitro and in vivo. Oral or intra-peritoneal administration of RRD-251 resulted in a significant suppression of growth of tumors xenotransplanted into athymic nude mice; the tumor suppressive effects were restricted to tumors carrying a wild-type Rb gene. Thus, selective targeting of Rb-Raf-1 interaction appears to be a promising approach for developing novel anti-cancer agents. In addition to mitogens, tobacco components like NNK and nicotine can induce cell proliferation and angiogenesis, contributing to lung cancer. Induction of cell proliferation by tobacco components required the binding of Raf-1 to Rb and RRD-251 could prevent nicotine induced cell proliferation. Our studies also show how nicotine not only promotes tumor growth in vivo, it also increases chance of tumor recurrence and metastasis. In addition to growth factors and tobacco components, cytokines like TNF could induce Rb-Raf-1 interaction in vascular smooth muscle cells. Since TNF-induced proliferation of vascular smooth muscle cells contributes to growth of atherosclerotic plaques, RRD-251 could be beneficial in controlling atherosclerosis as well. Thus, it appears that drugs that can disrupt the Rb-Raf-1 interaction might have beneficial effects in a wide spectrum of human diseases.

The retinoblastoma tumor suppressor RB is a central cell cycle regulator. Here we demonstrate that RB can be methylated by SMYD2 at lysine 860, a highly conserved and novel site of modification. This methylation event occurs in vitro and in cells and is regulated during cell cycle progression, cellular differentiation, and in response to DNA damage. Furthermore, we show that RB mono-methylation at lysine 860 provides a direct binding site for the methyl-binding domain of the transcriptional repressor L3MBTL1. This modification may be crucial for regulating the function of RB as a tumor suppressor. Inactivation of RB and activation of the MYC family of oncogenes are frequent events in a large spectrum of human cancers. Here, we sought to also test whether loss of RB function would affect cancer development in a mouse model of c-MYC-induced hepatocellular carcinoma (HCC). We found that RB inactivation has minimal effects on the cell cycle, cell death, and differentiation features of liver tumors driven by increased levels of c-MYC. However, combined loss of RB and activation of c-MYC led to an increase in polyploidy in mature hepatocytes before the development of tumors and a trend for decreased survival after cancer initiation.