The Primate Corpus Luteum: Functional Regression and Cardiovascular Impacts

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DESCRIPTION (provided by applicant): The candidate, Dr. Randy Bogan, is a postdoctoral fellow at the Oregon National Primate Research Center (ONPRC). Dr. Bogan's graduate research utilized sheep as an experimental model to study mechanisms controlling the synthesis of progesterone (P4) by the corpus luteum (CL). During this experience, Dr. Bogan developed an interest in the applications of his research to human health. Therefore, he came to the ONPRC to conduct postdoctoral training that would further his expertise in the regulation of the mammalian CL using a translationally-relevant nonhuman primate model. P4 production from the CL is required to maintain pregnancy in women through the first 6-7 weeks of gestation with the probability of pregnancy loss during this period being inversely related to P4 levels. Therefore, understanding the mechanisms that halt P4 production (termed functional regression) in the primate CL at the end of the luteal phase has implications for preventing early pregnancy loss, and improving the efficiency of assisted reproduction technologies. However, the mechanisms causing functional regression of the primate CL are not known. The candidate previously identified the reverse cholesterol transport (RCT) system as a potential mediator of functional regression and ultimately luteolysis (complete structural degradation) in the primate CL. As study of the RCT system also has implications for cardiovascular disease, he additionally discovered changes in lipoprotein levels in rhesus macaques at distinct phases of the menstrual cycle that are consistent with a reduced risk of developing cardiovascular disease. These changes may be due to direct ovarian effects on lipoprotein metabolism, and/or steroids secreted from the ovary having systemic effects on the liver or other organs. In keeping with the candidate's long term career goal of becoming an independent investigator in reproductive physiology to help improve the reproductive health of humans, the candidate has identified three immediate areas where additional training is needed to ensure success as an independent investigator: 1) further develop research expertise regarding the study of cardiovascular disease and its risk factors, 2) obtain the necessary knowledge and skills to conduct clinical research, and 3) gain sufficient experience to allow for effective laboratory management. This proposal will take advantage of the scientific environment at OHSU, which has one of only eight National Primate Research Centers, as well as a medical campus where clinical research and training can be performed. Experts in the fields of reproduction and cardiovascular physiology, with both nonhuman primate and clinical research experience, have been recruited to oversee the candidate's training and research. Formal classes and training objectives are laid out to meet the aforementioned immediate goals. The research plan will address the novel hypothesis that cholesterol uptake and efflux activities, regulated by liver x receptor (LXR) 1 and/or 2, determine luteal steroidogenic lifespan and reduce the risk of developing cardiovascular disease. To test this hypothesis, the first aim conducted during the K99 mentored phase will use molecular studies to determine the individual importance of LXR1 and LXR2 in inducing the RCT system during regression of the primate CL, and in vitro studies will identify genome-wide changes in gene expression of the primate CL induced by activation of both or individual LXR isoforms. Direct binding of each LXR isoform to promoter regions of known LXR target genes will be determined during spontaneous luteolysis in vivo, and DNA microarray assays will be performed on luteal cells containing both or individual LXR isoforms in the presence of an LXR agonist. The independent phase research includes aims 2 and 3. The second aim will utilize direct infusion of a synthetic LXR agonist within the CL to determine if premature activation of the RCT system can shorten CL lifespan. Also, we will determine if simulating early pregnancy prevents LXR activation and RCT normally observed at the end of non-conception cycles, and whether LXR agonist replacement can reverse the effects of chorionic gonadotropin (CG). Serum levels of estradiol (E2) and P4, as well as the first day of menstruation, will be used as markers of luteal lifespan. Determining the expression of known LXR target genes and lipoprotein receptors, endogenous LXR ligand levels, and lipid localization will provide an assessment of RCT activity, and markers of apoptosis will evaluate CL structural integrity. The third specific aim will utilize rhesus macaques to determine changes in cardiovascular risk factors induced by cyclical levels of E2 and P4, versus those induced by direct intra-ovarian effects. Ovariectomized monkeys will receive E2 and P4 implants to simulate an artificial menstrual cycle and levels of high, low, and very-low density lipoproteins (HDL, LDL, VLDL, respectively) and triglycerides in serum will be measured as indicators of cardiovascular risk. Also, experiments will be performed using women to determine menstrual cycle-induced changes in cardiovascular risk factors and how inhibiting ovarian activity via hormonal oral contraceptives affects these factors. This work is unique as the RCT system has not been investigated in the CL of any species, and it may be the as-yet unidentified signal causing functional regression of the primate CL. Additionally, cross-disciplinary implications of these findings will be explored that may yield important findings with relevance to reducing cardiovascular disease in women and safer administration of hormonal oral contraceptives and hormone replacement therapies.