Expression of low density lipoprotein receptor in cultured human granulosa cells: regulation by human chorionic gonadotropin, cyclic AMP, and sterol

The liver X receptors and sterol regulatory element binding proteins alter progesterone secretion and are regulated by human chorionic gonadotropin in human luteinized granulosa cells

There is increased expression of liver x receptor (LXR) target genes and reduced low density lipoprotein receptor (LDLR) during spontaneous luteolysis in primates. The LXRs are nuclear receptors that increase cholesterol efflux by inducing transcription of their target genes. Transcription of LDLR is regulated by sterol regulatory element binding proteins (SREBPs). Human chorionic gonadotropin (hCG) prevents luteolysis and stimulates progesterone synthesis via protein kinase A (PKA). Thus, our primary objectives are: 1) Determine the effects of LXR activation and SREBP inhibition on progesterone secretion and cholesterol metabolism, and 2) Determine whether hCG signaling via PKA regulates transcription of LXR and SREBP target genes in human luteinized granulosa cells. Basal and hCG-stimulated progesterone secretion was significantly decreased by the combined actions of the LXR agonist T0901317 and the SREBP inhibitor fatostatin, which was associated with reduced intracellular cholesterol storage. Expression of LXR target genes in the presence of T0901317 was significantly reduced by hCG, while hCG promoted transcriptional changes that favor LDL uptake. These effects of hCG were reversed by a specific PKA inhibitor. A third objective was to resolve a dilemma concerning LXR regulation of steroidogenic acute regulatory protein (STAR) expression in primate and non-primate steroidogenic cells. T0901317 induced STAR expression and progesterone synthesis in ovine, but not human cells, revealing a key difference between species in LXR regulation of luteal function. Collectively, these data support the hypothesis that LXR-induced cholesterol efflux and reduced LDL uptake via SREBP inhibition mediates luteolysis in primates, which is prevented by hCG.

Cholesterogenic lanosterol 14alpha-demethylase (CYP51) is an immediate early response gene

Lanosterol 14alpha-demethylase (CYP51) responds to cholesterol feedback regulation through sterol regulatory element binding proteins (SREBPs). The proximal promoter of CYP51 contains a conserved region with clustered regulatory elements: GC box, cAMP-response elements (CRE-like), and sterol regulatory element (SRE). In lipid-rich (SREBP-poor) conditions, the CYP51 mRNA drops gradually, the promoter activity is diminished, and no DNA-protein complex is observed at the CYP51-SRE1 site. The majority of cAMP-dependent transactivation is mediated through a single CRE (CYP51-CRE2). Exposure of JEG-3 cells to forskolin, a mediator of the cAMP-dependent signaling pathway, provokes an immediate early response of CYP51, which has not been described before for any cholesterogenic gene. The CYP51 mRNA increases up to 4-fold in 2 h and drops to basal level after 4 h. The inducible cAMP early repressor (ICER) is involved in attenuation of transcription. Overexpressed CRE-binding protein (CREB)/CRE modulator (CREM) transactivates the mouse/human CYP51 promoters containing CYP51-CRE2 independently of SREBPs, and ICER decreases the CREB-induced transcription. Besides the increased CYP51 mRNA, forskolin affects the de novo sterol biosynthesis in JEG-3 cells. An increased consumption of lanosterol, a substrate of CYP51, is observed together with modulation of the postlanosterol cholesterogenesis, indicating that cAMP-dependent stimuli cross-talk with cholesterol feedback regulation. CRE-2 is essential for cAMP-dependent transactivation, whereas SRE seems to be less important. Interestingly, when CREB is not limiting, the increasing amounts of SREBP-1a fail to transactivate the CYP51 promoter above the CREB-only level, suggesting that hormones might have an important role in regulating cholesterogenesis in vivo.

Sex steroids and lipoprotein metabolism

Lipoprotein metabolism is involved in atherogenesis. Female sex-hormones have substantial effects on both lipoprotein metabolism and the vessel wall. Cholesterol, one of the major lipids in lipoproteins, is both the substrate for, and the target of, the steroidal sex hormones.

Degradation of human apolipoprotein B-100 by apolipoprotein(a)

Human plasma low density lipoproteins (LDL) contain a very high molecular weight protein termed apoB-100 (M(r) = 550,000). In many samples of LDL, minor components designated as apoB-74 (M(r) = 407,000) and apoB-26 (M(r) = 145,000) are present. It has been shown that they can arise as a result of proteolytic degradation of apoB-100. Our earlier studies demonstrated that the active forms of lipoprotein(a) (LP(a)) and apolipoprotein(a) (apo(a)) possess proteolytic activity. In the present study we investigated the possibility of apoB-100 degradation in the presence of activated apo(a). LDL were incubated with the purified apo(a) and analyzed by SDS-polyacrylamide gel electrophoresis. It was found that apoB is cleaved by apo(a) with formation of proteolytic fragments including B-74 and B-26. The physiological significance of apoB degradation under the action of apo(a) is considered.

Enzymatic properties of active form of human apolipoprotein (a)

Lipoproteins (d = 1.05-1.12 g/ml) were obtained from pooled serum by density gradient ultracentrifugation and used as a source for isolation of apolipoprotein (a) (apo(a]. It was found that both these lipoproteins and purified apo(a) possess negligible amidolytic and proteolytic activity. After preincubation of lipoproteins and apo(a) with collagen-Sepharose, the increase in enzymatic activity was observed. The activation of purified apo(a) also occurred upon its storage in the cold. After two week storage at 7 degrees C, the amidase activity, as measured by splitting of the substrate D-Pro-Phe-Arg-pNA, was increased from 0.009 U/mg to 0.85 U/mg. The amidase activity was completely inhibited by phenylmethylsulfonyl fluoride (10(-3) M) and by soybean trypsin inhibitor (10(-5) M); it was not inhibited by aprotinin (10(-6) M). Activated apo(a) did not split azocasein but converted plasma prekallikrein to kallikrein and degraded apolipoprotein B-100.

The role of exogenous calcium for gonadotropin-stimulated progesterone production by human granulosa-luteal cells

Previous studies of cells from various species have indicated that exogenous calcium is necessary for gonadotropic stimulation of steroidogenesis. To determine whether this requirement for exogenous calcium is a universal attribute of steroidogenic cells, we studied baseline and stimulated progesterone (P) production by cultured human granulosaluteal cells obtained at the time of oocyte retrieval for in vitro fertilization (IVF). During 4 hours in culture, both cholera toxin (1.25 micrograms/mL) and human chorionic gonadotropin (hCG, 1 IU/mL) stimulated a significant (P less than 0.05) 2- to 4-times increase in P production. Both baseline and stimulated (cholera toxin or hCG) increases in P were unaffected when cellular uptake of exogenous calcium was inhibited by the calcium channel blocker nitrendipine (10 microM), or by culturing the cells in calcium-free medium or in calcium-free medium with [ethylenebis(oxyethylenenitrilo)]-tetra-acetic acid (EGTA, to chelate any possible free extracellular calcium). At later time points (24 and 48 hours), lack of available exogenous calcium began to have an inhibitory effect on P production, and the hCG effect was more sensitive to the lack of exogenous calcium than was the cholera toxin effect. We speculate that this apparent independence from exogenous calcium over a short culture period is due to the prior stimulation of these cells by exogenous gonadotropins employed in IVF cycles.

8-bromoadenosine cyclic 3',5'-phosphate rapidly increases 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in human granulosa cells: role of cellular sterol balance in controlling the response to tropic stimulation

Exposure of cultured human granulosa cells to 8-bromoadenosine cyclic 3',5'-phosphate (8-bromo-cAMP) resulted in a rapid increase in the content of the mRNA for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme in the de novo synthesis of cholesterol. HMG-CoA reductase mRNA levels increased within 2 h of stimulation and remained elevated for at least 6 h. Treatment of granulosa cells with 25-hydroxycholesterol, a soluble cholesterol analogue, in combination with aminoglutethimide to block conversion of cellular sterols to pregnenolone, resulted in suppression of HMG-CoA reductase mRNA. When cells were stimulated with 8-bromo-cAMP in the presence of 25-hydroxycholesterol and aminoglutethimide, the increase in HMG-CoA reductase mRNA provoked by the tropic agent was markedly attenuated. This indicates that 8-bromo-cAMP raises HMG-CoA reductase mRNA levels indirectly by accelerating steroidogenesis and depleting cellular sterol pools, thus relieving sterol-mediated negative feedback of HMG-CoA reductase gene expression. 25-Hydroxycholesterol in the presence of aminoglutethimide suppressed low-density lipoprotein (LDL) receptor mRNA, but 8-bromo-cAMP effected a significant stimulation of LDL receptor mRNA levels when added with hydroxysterol and aminoglutethimide. These findings reveal differential regulation of HMG-CoA reductase and LDL receptor mRNAs in the presence of sterol negative feedback.

The upstream promoter of the human LDL receptor gene does not contain a cyclic AMP response element

Fusion genes containing segments of the promoter region of the human LDL receptor gene and the coding sequence of the bacterial enzyme, chloramphenicol acetyltransferase (CAT), were introduced into JEG-3 human choriocarcinoma cells. Constructs containing 177 base pairs of 5'-flanking DNA (pLDLR-CAT 234) or 6500 base pairs (pLDLR-CAT 6500) promoted CAT activity in transient expression assays. Although both pLDLR-CAT 234 and pLDLR-CAT 6500 contain sequences related to the recently reported consensus sequence for cyclic AMP responsiveness, treatment of the transfected JEG-3 cells with 8-bromo-cAMP did not increase CAT activity. The cyclic AMP analog did, however, stimulate steroidogenesis and hCG secretion and increase CAT activity in cells transfected with p18X2SV1CAT, which contains two copies of an 18 base pair sequence corresponding to the cAMP-responsive element of the human alpha chorionic gonadotropin gene.

Regulation of low density lipoprotein receptor and cytochrome P-450scc mRNA levels in human granulosa cells

hCG and 8-bromo-cAMP increase the levels of the mRNAs encoding the LDL receptor and cytochrome P-450scc in human granulosa cells. The increment in these mRNAs occurs rapidly after stimulation (within hours). Actin mRNA levels are not changed by hCG and 8-bromo-cAMP treatment. The tropic hormone effects on LDL receptor mRNA are observed even in the presence of 25-hydroxycholesterol and aminoglutethimide, which by themselves suppress LDL receptor mRNA. Actinomycin D blocks the hCG and 8-bromo-cAMP promoted augmentation of LDL receptor mRNA, suggesting that the tropic factors act to increase transcription of this gene. Treatment of cells with cycloheximide increases LDL receptor mRNA levels, and the stimulatory response to hCG and 8-bromo-cAMP is enhanced in the presence of cycloheximide. This indicates that tropic hormones act to increase LDL receptor mRNA levels without requiring synthesis of intermediary proteins and suggests that some short-lived protein may actually be reducing LDL receptors mRNA levels in these cells. We conclude that gonadotropins increase steroidogenesis in human granulosa cells, in part, by increasing expression of mRNAs encoding specific proteins involved in cholesterol metabolism.