Oviparous vertebrates, such as the freshwater turtle, with their heavy reliance on estrogen dependent lipid mobilization (vitellogenesis) for oocyte nutrients, provide excellent model systems for study of cholesterol transport and homeostasis. In addition to apoproteins B and E, the ligands for the LDL receptor, two other protein ligands are involved in the delivery of cholesterol and lipoproteins to the tissues. Most recently, apoprotein A, a component of HDL, has been demonstrated to be the ligand for the scavenger receptor (SR-BI), and is believed to be of particular importance in steroidogenic tissues. In addition, in non-mammals, the direction of vitellogenin molecules to the oocyte via a specific receptor, the VLDL/vtg-receptor (structurally related to LDL-receptor), further complicates the picture. Our hypothesis is that apoprotein A plays a role in the seasonal redistribution of cholesterol associated with the ovarian growth cycle. In this study we have begun to investigate the tissue distribution of the expression of SR-BI in the turtle using an antiserum to murine SR-BI provided by Millennium Pharmaceuticals. In addition, we have measured seasonal changes in the ligand for SR-BI, apoprotein A, which is found in HDL in this species. Our Western blot studies show that SR-BI is quite widely distributed in Chrysemys picta , in both steroidogenic and non-steroidogenic tissues, and in ovarian follicles. Values of apoprotein A determined by ELISA vary over the ovarian cycle from 15 to 75 mg/dL, and are higher in females than males at all times (p<0.001). We believe that changing levels of apoprotein A, and probably its receptor, SR-BI, are physiologically relevant to the demands of the ovarian cycle and cholesterol distribution, and are probably under hormonal control as indicated by the effects of steroid treatment in this species.
This abstract is being presented on Sunday, August 1 at 8:00 AM to 10:15 AM at CUB 2nd Floor Ballroom.