Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model

Abstract

This study addresses the question of whether the level of expression of SR-BI (an HDL receptor) is linked to the expression of selective lipoprotein-cholesteryl ester delivery in a steroidogenic cell model. Rat ovarian granulosa cells are physiologically normal cells which show no selective uptake of HDL-cholesteryl esters and no progestin production until luteinized by trophic hormones or adenylate cyclase stimulators, after which expression of the selective cholesterol pathway and production of steroid hormone is dramatically up-regulated. The current study demonstrates that at every cell stage studied, the protein content and level of expression of SR-BI mRNA are linked to changes that occur in HDL-cholesteryl ester uptake; i.e., SR-BI is not present in basal (non-luteinized) cells, develops slowly (from 6-9 h) after hormone treatment, increases robustly from 9-48 h after stimulation, and remains high after incubation with HDL. In contrast, another structural protein, caveolin, did not follow this patter n; caveolin expression showed an inverse relationship to selective cholesteryl ester uptake, and was most prominent in basal cells and least prominent in luteinized, HDL-incubated cells. Morphologically, SR-BI appears to be associated with cell surface sites showing high levels of cholesteryl ester uptake (after luteinization and/or incubation with HDL labeled with fluorescent cholesteryl esters), and at the electron microscope level, SR-BI is most clearly associated with microvillar regions on the cell surface which also bind HDL-labeled with colloidal gold. Thus, induction of the SR-BI receptor system and induction of the HDL-selective cholesterol uptake pathway in rat granulosa cells appear to be linked morphologically, biochemically, and functionally.-Azhar, S., A. Nomoto, S. Leers-Sucheta, and E. Reaven. Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model.

Follicle growth, ovulation, and luteal formation in primates and rodents: A comparative perspective

“…Thereafter, LH/hCG-induced scavenger receptor-BI (SR-BI) serves to replenish the cholesterol supply, primarily from LDL, 69,70 and human SR-BI variants with lower activity lead to impairment of LH/hCG-induced progesterone and are associated with infertility, 71,72 In contrast, HDL may be the more relevant cholesterol source for both theca and granulosa cells in rats. 73,74 The LH/hCG-induced increase in SR-BI, as well as in HSD3B and StAR, is due to the rapid (<3 h) reduction of liver X receptors (LXRα, β; NR1H2, NR1H3) following an ovulatory stimulus, which also serves to reduce reverse cholesterol transport. 75,76 The regulation of LXRs in the rodent ovary has not yet been reported, although there is clear evidence that LXR regulates similar genes involved in cholesterol homeostasis in mice, 77 so it would be highly surprising if LH/hCG down-regulation of LXR was not an early event leading to progesterone synthesis in rodents.…”

Differentiation-dependent expression and localization of the class B type I scavenger receptor in intestine

“…e-mail: Huidy@email.uc.edu cells and Leydig cells of the testis, SR-BI levels are elevated after hormone desensitization (8,22,23). SR-BI in these cells is localized to surface microvilli and in complex channel systems within the cytoplasm, which are specialized components for cholesterol uptake by steroidogenic cells (22)(23)(24). The expression of SR-BI was also detected in undifferentiated monocytes as well as in differentiated macrophages (5,25,26).…”

Hormonal regulation of adrenal microvillar channel formation

“…Long acting adrenocorticotropin hormone (ACTH) gel preparation (HP Acthar gel) was the product of Armour Pharmaceuticals, (Kankakee, IL). The anti-rat SR-BI and anti-rat apolipoprotein (apo)A-I antibodies used have been described and characterized in this laboratory (12,31). All other reagents used were of analytical grade.…”

“…The expression of SR-BI protein in whole adrenal tissues and cultured adrenocortical cells was assessed by Western blot analysis using previously described methodology (12,13). Briefly, ad-renal tissues were homogenized in 10 vol of buffer (20 mM Tris-HCl, pH 7.5, 2 mM MgCl 2 , 0.25 M sucrose, 1 mM phenylmethylsulfonylfluoride, 10 g/ml leupeptin, 20 g/ml aprotinin, and 5 g/ml pepstatin), centrifuged (800 ϫ g ) for 10 min, and supernatant centrifuged for 60 min at 100,000 ϫ g .…”

“…Although details of the molecular mechanisms by which cells can selectively internalize large quantities of neutral lipids are not yet clear, some aspects of the pathway have been established. Most impor-tantly, it is known that a regulatable HDL receptor protein scavenger receptor class B type I (SR-BI) is found on the cell surface of a variety of rodent high cholesterol-requiring tissues (steroidogenic tissues, liver) (9)(10)(11)(12)(13)(14)(15)(16), and that cells that express high levels of SR-BI can efficiently obtain lipoprotein derived CEs for use in hormone or product synthesis (7,8,12,14,15,(17)(18)(19). Indeed, work from several laboratories using different adrenal, ovarian, testicular, or liver tissue models show an exceptionally tight association between expression of SR-BI protein and the direct measure of selective CE uptake under changing physiological conditions, including hormone stimulation, hormone inhibition, lipid levels, SR-BI over-expression, or genetic ablation (11,12,15,(20)(21)(22)(23)(24)(25)(26)(27).…”