The role of plasma lipoproteins in steroidogenic response of rat luteal cells during gonadotropin-induced refractory states

The role of cyclic AMP response element binding protein in transactivation of scavenger receptor class B type I promoter in transfected cells and in primary cultures of rat theca-interstitial cells

In the ovary, lutropin (LH) stimulates the selective uptake and transport of cholesterol for steroid biosynthesis from HDL particles via the scavenger receptor class B type I (SR-BI). Furthermore the expression of SR-BI mRNA in the ovary is stimulated by LH and cyclic AMP (cAMP). Since the promoter of the rat SR-BI gene is devoid of consensus cyclic AMP response element (CRE) sequences, this study examined if cAMP response element binding protein (CREB) plays a role in the transactivation of SR-BI promoter (SR-BIpr). The transactivation of SR-BIpr was examined in transfected 293T cells and human granulosa SVOG-4o cells, and in primary cultures of rat theca-interstitial cells infected with adenoviral constructs containing the SR-BIpr and a luciferase reporter gene. Dose-related increases in SR-BRpr activity ranging from 2- to 4-fold was induces by 293T cells co-transfected with the catalytic subunit of protein kinase A (cPKA). Co-transfections with CREB and cPKA produced a concentration-dependent increase ranging from 6- to 32-fold. The cAMP-mediated transactivation was significantly attenuated by co-transfection with CREB M1, a non-phosphorylatable, dominant-negative form of CREB. An increase in transactivation of SR-BIpr activity was also seen in SVOG-4o cells co-transfected with CREB. In primary cultures of rat theca-interstitial (T-I) cells infected with an adenoviral construct of SR-BIpr, forskolin produced a marked increase in promoter activity. These data indicate that stimulation of the cAMP-PKA-CREB pathway enhances rat SR-BIpr activity and substantiate the role of CREB as an intermediary in this process. The absence of canonical CRE sequences in the rat SR-BIpr suggests that the activation of SR-BI by CREB may occur either through non-canonical CRE sequences or through additional transcription factors that cooperate with CREB in the activation of SR-BI promoter activity.

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.

Presence of CLA-1 and HDL binding sites on syncytiotrophoblast brush border and basal plasma membranes of human placenta

It is now known that rapid placental and fetal development is associated with elevated levels of circulating high density lipoprotein (HDL) in pregnant women. The main structure implicated in the maternal-fetal exchange is the syncytiotrophoblast, composed of a brush border membrane (BBM), facing the mother, and a basal plasma membrane (BPM), facing the fetus. In order to understand the mechanisms controlling the placental and fetal supplies of cholesterol, we purified both BBM and BPM and verified the presence of HDL binding sites in these membranes. Binding studies using(125)I-HDL(3)show a single affinity binding site on BPM which has a dissociation constant (K(d)) of 3.45+/-0.43 microg protein/ml and a maximal binding capacity (B(max)) of 5.46+/-1.69 microg protein/mg membrane proteins. In BBM, we observed two affinity binding sites, one with a K(d)of 0.62+/-0.03 microg protein/ml and another one with a K(d)of 6.57+/-0.87 microg protein/ml. Their B(max)values were 0.54+/-0.11 and 2.34+/-0.39 microg of HDL(3)/mg membrane proteins, respectively. CLA-1, a putative HDL-receptor of 85 kDa, was detected on both BPM and BBM, together with two apo A-l binding sites of 110 and 96 kDa on BPM and BBM, respectively. These results provide further evidence that human placenta possesses specific sites for HDL binding, underlining the important role of maternal HDL in the transfer of cholesterol from the maternal circulation to the placenta and the fetus.

IRS-I expression on the luteinized rat ovary: IGF-I and cyclic AMP effects on IRS-I tyrosine phosphorylation

The expression of insulin receptor substrate-I (IRS-I) mRNA was demonstrated in rat luteal cells by Northern blot analysis, in situ hybridization as well as by reverse transcriptase polymerase chain reaction. Western blot with a polyclonal anti IRS-I antibody showed the presence of a 183 kDa protein which corresponds to the size of IRS-I reported in other tissues. Further studies were performed to determine whether human chorionic gonadotropin (hCG) can interact with the insulin-like growth factor-I (IGF-I) signaling pathway to increase tyrosine phosphorylation of IRS-I. While hCG alone was ineffective in stimulating the phosphorylation of IRS-I, IGF-I mediated phosphorylation of IRS-I was increased by prior exposure to hCG. These results were further confirmed by the immunoprecipitation of IRS-I from the lysate of hCG- and IGF-I-treated luteal cell cultures followed by Western blotting with anti-phosphotyrosine antibody. Similarly, pretreatment with forskolin also increased IGF-I stimulated IRS-I phosphorylation. The increased tyrosine phosphorylation of IRS-I seen in response to IGF-I stimulation following treatment with either hCG or forskolin was not due to an increase in IRS-I content. Furthermore, IGF-I receptor tyrosine kinase activity was not affected by forskolin, suggesting that the increase in IRS-I tyrosine phosphorylation was not the result of an increase in its activity. Thus, we conclude that hCG/LH and IGF-I signaling pathways 'cross-talk' to increase the levels of IRS-I tyrosine phosphorylation. The observed increase in IRS-I tyrosine phosphorylation may be the result of an increase in the stability of the phosphorylated form of IRS-I.

Metabolism of high-density lipoproteins in cultured rat luteal cells

The uptake of cholesterol from high-density lipoproteins (HDL) labeled with 125I and [3H]cholesterol was examined in cultured rat luteal cells. Luteal cells were incubated with labeled HDL, following which the metabolic fate of the apolipoproteins and cholesterol moieties of the receptor-bound HDL were examined. About 50% of the originally bound HDL apolipoproteins were released into the medium in 24 h by a temperature-dependent process while only 5% of the HDL cholesterol was released unmetabolized. Inclusion of unlabeled HDL in the chase incubation resulted in increased release of apolipoprotein-derived radioactive products without significant change in the release of unmetabolized cholesterol. 60% of the apolipoprotein-derived radioactivity could be precipitated with trichloroacetic acid; the remaining trichloroacetic acid-soluble radioactive fraction was identified as [125I]iodotyrosine. Gel filtration chromatography of the chase-released material showed that the trichloroacetic acid-precipitable products, which contained no detectable amounts of cholesterol, eluted over a range of molecular sizes (9-80 kDa). No intact HDL was retroendocytosed. About 80% of trichloroacetic acid-precipitable products could be immunoadsorbed on anti-apolipoprotein A-I antibody immobilized on CNBr-activated Sepharose, suggesting the presence of fragments containing apolipoprotein A-I. This material was also capable of reassociating with native HDL. Lysosomal inhibitors were partially effective in inhibiting the amount of trichloroacetic acid-soluble products formed. The lysosomal degradation appeared to have no role in the uptake of HDL-derived cholesterol. These studies demonstrate preferential and total uptake of HDL cholesterol by luteal cells, with concomitant degradation of the lipoprotein.

Effect of plasma lipoproteins in gonadotropin stimulation of 17 beta-estradiol production in the ovarian follicle of rainbow trout (Salmo gairdneri)

The effect of trout plasma lipoproteins on the production of 17 beta-estradiol by trout ovarian follicles is investigated in vitro. 17 beta-Estradiol secretion into the medium was assayed as a function of follicular diameter in the presence of lipoproteins with and without salmonid gonadotropin (SGA-GTH). The presence of very low-density lipoproteins (VLDL) + chylomicrons (Chy), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) amplified the SGA-GTH effect at the lowest concentrations tested (less than 50 micrograms protein/ml). HDL is the most effective for increasing hormone accumulation on a microgram lipoprotein sterol basis. Autoradiography of 125I-labeled LDL showed that they were preferentially bound by thecal cells. Kinetics of 17 beta-estradiol release indicated that lipoprotein amplification occurred especially after 15 hr and subsequent metabolism of 17 beta-estradiol by follicular layers also led to an equilibrium. At the end of vitellogenesis apoprotein B lipoproteins (VLDL + Chy, LDL) apparently inhibited SGA-GTH stimulation. N',O'-Dibutyryl cAMP (10 mM) considerably stimulated 17 beta-estradiol production but lipoprotein amplification did not occur. Chloroquine (30 microM) inhibition of LDL and HDL amplification indicates that this process requires lysosomal degradation. Plasma lipoproteins in trout modulate SGA-GTH stimulation of 17 beta-estradiol production during exogenous vitellogenesis. Due to the ease and frequency with which the experiments can be carried out, the ovarian follicle of salmonids is an excellent model for the study of the role of lipoproteins in the regulation of ovarian steroids biosynthesis.