Men whose genomes contain a polymorphism in the gene for manganese superoxide dismutase (MnSOD) that results in an alanine (A) instead of a valine (V) at the corresponding codon 16 exhibit a 10-fold increase in aggressive prostate cancer risk when those in the lowest tertile of anti-oxidant intake are compared to those in the highest. The MnSOD encoded by the A-expressing allele partitions more efficiently to the mitochondria where it may promote cancer development by increasing the production of H2O2 from superoxide or due to its inherent ability to function as a peroxidase to generate oxidative damage. Increased cancer risk among MnSOD(A) men is also associated with the presence of a polymorphism in the anti-oxidant selenium-dependent glutathione peroxidase-1 (GPx-1) protein and the selenium transport protein SEPP1. Based on these human epidemiological data, it is proposed that MnSOD exists at the center of an axis of a small number of anti-oxidant molecules that together contribute to the risk of prostate cancer, and perhaps also contribute to determining whether that cancer is of the more dangerous aggressive type.
The first aim of the proposal is to use human prostatic cells in culture to recapitulate the human data by manipulating MnSOD and GPx-1 genotype and levels, and determining the consequences to reactive oxygen species generation and damage to biomolecules under controlled experimental conditions. In the second aim, the effects of MnSOD and GPx-1 levels in promoting carcinogenesis will be investigated using human primary prostatic cells that will be engineered to over-express these proteins, allowed to form prostaspheres in 3D culture and implanted under the renal capsule of nude mice. These structures ultimately develop into tumors. Xenograft studies with human prostate tumor cells engineered to over-express different MnSOD and GPx-1 alleles injected into nude mice will also be conducted to evaluate the effect of these proteins on tumor growth. The same endpoints investigated in the first aim will be assessed in the mouse prostatic tissues.
The third aim i s to determine whether MnSOD genotype predicts risk of recurrence/progression in two independent case series of patients with clinically organ-confined prostate cancer, 400 cases from the Cooperative Prostate Cancer Tissue Resource (CPCTR) using a case vs. non-recurrent control study design and 200 cases from an ongoing NIH-supported prospective cohort study of lifestyle determinants of prognosis. The secondary goal is to explore the interaction of MnSOD genotype with GPx-1 and SEPP1 genotype and with antioxidant intake with respect to recurrence/progression risk. It is anticipated that the results f these studies will elucidate some of the important determinants of prostate cancer risk, allowing for better prediction as to whose prostate cancer will progress, and providing new targets for individualized preventive and therapeutic interventions.
Human epidemiology has revealed a remarkable impact of dietary antioxidant intake, as well as polymorphisms in the gene for the GPx-1 anti-oxidant protein, in the risk of aggressive, clinically significant prostate cancer among carriers of a common allelic variant of the MnSOD gene. By studying the mechanism of this interaction in model systems and determining the predictive value of MnSOD allelic identity, novel approaches may be developed to reduce prostate cancer mortality.
