205 Background: The E2F family of genes encodes transcription factors that are key to the regulation of a number of target genes, including those encoding cyclins , CDKs , checkpoints regulators, and DNA repair and replication proteins. One of the primary functions of the E2Fs is to control the cell cycle, playing a major role in regulating the G1/S transition. One of the primary regulators of E2F expression is the retinoblastoma gene, RB, a chromatin associated protein that, in its unphosphorylated state, binds to and negatively regulates E2F; hyperphosphorylation of RB releases E2F, allowing cell cycle progression. Many tumor cells have mutant or dysfunctional RB, allowing the aberrant overexpression of the E2Fs and tumor cell proliferation; this aberrant overexpression is better tolerated when p53 is mutated, suppressing subsequent apoptosis. Overexpression of E2F, particularly E2F1, has thus been an attractive target for therapeutic intervention. However, this approach has not yet been successful, most likely due to the redundancy of the E2Fs and the lack of biomarkers for sensitivity. Methods: Using phage display, we have previously identified a novel peptide that, when coupled with penetratin (PEP) to enhance uptake), targets the E2F consensus site in E2F1,2 and 3a, leading to the downregulation of the activating E2Fs and their downstream targets. We have recently enhanced the stability and potency of this peptide by substituting L-Arg within the peptide with D-Arg. Results: Castrate resistant prostate cancer (CRPC) cells, DU-145, lack functional RB, have mutant p53, and are more sensitive to the D-Arg PEP than LnCap or PC-3 cells, with functional RB. Xenograft studies in mice show that the PEP, when encapsulated in PEGylated liposomes (PL-D-Arg PEP) , regresses DU-145 tumors without toxicity. Current studies are examining the combination of the (PL-D-Arg PEP) with taxotere, cisplatin and irradiation in prostate cancer xenografts and organoids from patients. Conclusions: A peptide that inhibits transcription of the activating E2Fs has promise to treat CRPC.
Targeting Amino Acid Transport in Metastatic Castration-Resistant Prostate Cancer: Effects on Cell Cycle, Cell Growth, and Tumor Development
