Rb-Raf-1 Interaction as a Therapeutic Target for Proliferative Disorders

• Main Author: Kinkade, Rebecca
• Format: Others
• Published: Scholar Commons 2008
• Subjects: Cancer; Tumor suppressor; Cell-cycle; Small-molecule; Inhibition
• Online Access: https://scholarcommons.usf.edu/etd/335; https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=1334&context=etd

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 TNFα could induce Rb-Raf-1 interaction in vascular smooth muscle cells. Since TNFα-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.