Abstract

Although diagnosis and treatment of prostate cancer has improved, prostate cancer remains the second leading cause of cancer-related deaths in men (http://www.cdc.gov/cancer/Prostate/publications/decisionguide/). For this reason, researchers seek new biomarkers for the disease to aid in diagnosis, staging and management. Identifying potential molecular targets helps to create new therapeutics for the clinic. Protein translation is a target for a number of inhibitors currently in clinical trials. HIP/RPL29 is a ribosomal protein with extra-ribosomal functions. In many cells lines it is found on the cell surface and can modulate growth factor binding. However, in the LNCaP model of prostate cancer, its localization is intracellular and most likely its main function is that of a ribosomal protein. Nonetheless, it is overexpressed in more aggressive prostate cancer cell lines and could make an excellent therapeutic target. As predicted, reduction of HIP/RPL29 decreased prostate cancer cell growth in culture and soft agar assays. Because reduction in HIP/RPL29 expression decreased growth, cells in culture which inactivated the ribozyme quickly gained a growth advantage. Nonetheless, an efficient means to continuously reduce HIP/RPL29 in prostate cancer would be expected to be growth static and perhaps even eventually cancer cell death promoting.

mTOR pathway inhibitors, specifically rapamycin and its derivatives also are promising therapeutics that target downstream pathways including protein translation. The effects of a series of inhibitors targeting several pathways in prostate cancer cell lines were assessed for their ability to alter cell cycle distribution, global translation, polysome formation and the association of specific transcripts with the ribosome. The metastatic C4-2B cell line was more sensitive to rapamycin treatment compared with the weakly tumorigenic LNCaP cell line. The decrease of cells in S phase with rapamycin treatment increased in the progression model. LNCaP cells have the smallest decrease and C4-2B cells demonstrated the largest decrease in the percentage of cells in S phase after 24 hours of rapamycin treatment. When the cells were grown with serum, the changes in the percentage of cells in S phase were greater than under serum starved conditions. A corresponding increase in the percentage of cells in G0/G1 accompanied the observed decrease of cells in S phase. Treatment of C4-2B cells with either rapamycin (a mTOR inhibitor), PD98059 (a MEK1/2 inhibitor) or LY294002 (a PI3K inhibitor) failed to change the distribution of polysomes in sucrose gradients. Although there was no change in the accrual of heavy polysomes, there was an overall decrease in the rate of translation with rapamycin or LY294002 treatment. Inhibiting the MAPK pathway with PD98059 had no effect on the translation of mRNAs containing a 5' terminal oligopyrimidine tract (TOP) sequence or complex 5' UTR but did decrease overall translation by about 20%. Treatment with rapamycin for 24 hours decreased overall translation by about 45% and significantly affected the translation of mRNAs with complex 5' UTRs, specifically VEGF, cyclin D1 and HIF1α. LY294002 treatment decreased overall translation by 60% after 24 hours and affected the translation of mRNAs with complex 5' UTRs. While the effects of LY294002 on translation were more significant that those observed with rapamycin, the use of LY294002 in prostate cancer therapy is limited due to the fact that it inhibits this pathway upstream, providing the cancer cells with several opportunities to find ways around the inhibition. However, targeting this pathway futher downstream with rapamycin or targeted agents against HIP/RPL29 would not provide the cancer cells with many opportunities to bypass their effects. Thus, inhibiting downstream targets such as mTOR or targets of mTOR will provide rational approaches to developing new combination therapies focused on reducing growth of prostate cancer after arrival in the bone environment.