Identification and characterization of cis-acting elements conferring insulin responsiveness on hamster cholesterol 7alpha-hydroxylase gene promoter

Hepatic bile acid metabolism in the neonatal hamster: expansion of the bile acid pool parallels increased Cyp7a1 expression levels

Intraluminal concentrations of bile acids are low in newborn infants and increase rapidly after birth, at least partly owing to increased bile acid synthesis rates. The expansion of the bile acid pool is critical since bile acids are required to stimulate bile flow and absorb lipids, a major component of newborn diets. The purpose of the present studies was to determine the mechanism responsible for the increase in bile acid synthesis rates and the subsequent enlargement of bile acid pool sizes (BAPS) during the neonatal period, and how changes in circulating hormone levels might affect BAPS. In the hamster, pool size was low just after birth and increased modestly until 10.5 days postpartum (dpp). BAPS increased more significantly ( approximately 3-fold) between 10.5 and 15.5 dpp. An increase in mRNA and protein levels of cholesterol 7alpha-hydroxylase (Cyp7a1), the rate-limiting step in classical bile acid synthesis, immediately preceded an increase in BAPS. In contrast, levels of oxysterol 7alpha-hydroxylase (Cyp7b1), a key enzyme in bile acid synthesis by the alternative pathway, were relatively elevated by 1.5 dpp. farnesyl X receptor (FXR) and short heterodimeric partner (SHP) mRNA levels remained relatively constant at a time when Cyp7a1 levels increased. Finally, although simultaneous increases in circulating cortisol and Cyp7a1 levels occurred, precocious expression of Cyp7a1 could not be induced in neonatal hamsters with dexamethasone. Thus the significant increase in Cyp7a1 levels in neonatal hamsters is due to mechanisms independent of the FXR and SHP pathway and cortisol.

The pharmacological exploitation of cholesterol 7alpha-hydroxylase, the key enzyme in bile acid synthesis: from binding resins to chromatin remodelling to reduce plasma cholesterol

Mammals dispose of cholesterol mainly through 7alpha-hydroxylated bile acids, and the enzyme catalyzing the 7alpha-hydroxylation, cholesterol 7alpha-hydroxylase (CYP7A1), has a deep impact on cholesterol homeostasis. In this review, we present the study of regulation of CYP7A1 as a good exemplification of the extraordinary contribution of molecular biology to the advancement of our understanding of metabolic pathways that has taken place in the last 2 decades. Since the cloning of the gene from different species, experimental evidence has accumulated, indicating that the enzyme is mainly regulated at the transcriptional level and that bile acids are the most important physiological inhibitors of CYP7A1 transcription. Multiple mechanisms are involved in the control of CYP7A1 transcription and a variety of transcription factors and nuclear receptors participate in sophisticated regulatory networks. A higher order of transcriptional regulation, stemming from the so-called histone code, also applies to CYP7A1, and recent findings clearly indicate that chromatin remodelling events have profound effects on its expression. CYP7A1 also acts as a sensor of signals coming from the gut, thus representing another line of defence against the toxic effects of bile acids and a downstream target of agents acting at the intestinal level. From the pharmacological point of view, bile acid binding resins were the first primitive approach targeting the negative feed-back regulation of CYP7A1 to reduce plasma cholesterol. In recent years, new drugs have been designed based on recent discoveries of the regulatory network, thus confirming the position of CYP7A1 as a focus for innovative pharmacological intervention.

Insulin regulation of cholesterol 7alpha-hydroxylase expression in human hepatocytes: roles of forkhead box O1 and sterol regulatory element-binding protein 1c

Bile acid synthesis and pool size increases in diabetes, whereas insulin inhibits bile acid synthesis. The objective of this study is to elucidate the mechanism of insulin regulation of cholesterol 7alpha-hydroxylase gene expression in human hepatocytes. Real-time PCR assays showed that physiological concentrations of insulin rapidly stimulated cholesterol 7alpha-hydroxylase (CYP7A1) mRNA expression in primary human hepatocytes but inhibited CYP7A1 expression after extended treatment. The insulin-regulated forkhead box O1 (FoxO1) and steroid regulatory element-binding protein-1c (SREBP-1c) strongly inhibited hepatocyte nuclear factor 4alpha and peroxisome proliferator-activated receptor gamma coactivator-1alpha trans-activation of the CYP7A1 gene. FoxO1 binds to an insulin response element in the rat CYP7A1 promoter, which is not present in the human CYP7A1 gene. Insulin rapidly phosphorylates and inactivates FoxO1, whereas insulin induces nuclear SREBP-1c expression in human primary hepatocytes. Chromatin immunoprecipitation assay shows that insulin reduced FoxO1 and peroxisome proliferators-activated receptor gamma-coactivator-1alpha but increased SREBP-1c recruitment to CYP7A1 chromatin. We conclude that insulin has dual effects on human CYP7A1 gene transcription; physiological concentrations of insulin rapidly inhibit FoxO1 activity leading to stimulation of the human CYP7A1 gene, whereas prolonged insulin treatment induces SREBP-1c, which inhibits human CYP7A1 gene transcription. Insulin may play a major role in the regulation of bile acid synthesis and dyslipidemia in diabetes.

Insulin stimulates placental leucine aminopeptidase/oxytocinase/insulin-regulated membrane aminopeptidase expression in BeWo choriocarcinoma cells

Placental leucine aminopeptidase (P-LAP), a cystine aminopeptidase that is identical to insulin-regulated membrane aminopeptidase, hydrolyzes oxytocin, which results in the loss of oxytocin activity. We previously isolated genomic clones containing the human P-LAP promoter region, which included two sites homologous to the 10-bp-insulin responsive element (IRE) that was identified on the phosphoenolpyruvate carboxinase gene. We therefore postulated that insulin regulates P-LAP expression via these IREs and investigated this notion using BeWo choriocarcinoma trophoblastic cells cultured in the presence of insulin. Insulin increased P-LAP activity in a time- and dose-dependent manner. Physiological concentrations of insulin at 10(-7) M exhibited the most potent effect on P-LAP activity. Western blotting demonstrated that 10(-7) M insulin increased P-LAP protein levels. Semi-quantitative RT-PCR and Southern blotting showed that insulin also increased P-LAP mRNA, which was abrogated by prior exposure to cycloheximide. Luciferase assay did not reveal any regulatory regions within 1.1 kb upstream of the P-LAP gene that could explain the insulin-induced P-LAP mRNA accumulation. These findings indicate that insulin induces P-LAP expression in trophoblasts, and that it acts via de novo synthesis of other proteins, which partially contradicts our initial hypothesis.

Dietary polyunsaturated fatty acids and regulation of gene transcription

Dietary polyunsaturated fatty acids (PUFAs) are a source of energy and structural components for cells. PUFAs also have dramatic effects on gene expression by regulating the activity or abundance of four families of transcription factor, including peroxisome proliferator activated receptor (PPAR) (alpha, beta and gamma), liver X receptors (LXRs) (alpha and beta), hepatic nuclear factor-4 (HNF-4)alpha and sterol regulatory element binding proteins (SREBPs) 1 and 2. These transcription factors play a major role in hepatic carbohydrate, fatty acid, triglyceride, cholesterol and bile acid metabolism. Non-esterified fatty acids or fatty acid metabolites bind to and regulate the activity of PPARs, LXRs and HNF-4. In contrast, PUFAs regulate the nuclear abundance of SREBPs by controlling the proteolytic processing of SREBP precursors, or regulating transcription of the SREBP-1c gene or turnover of mRNA(SREBP-1c). The n3 and n6 PUFAs are feed-forward activators of PPARs, while these same fatty acids are feedback inhibitors of LXRs and SREBPs. Saturated fatty acyl coenzyme A thioesters activate HNF-4 alpha, while coenzyme A thioesters of PUFAs antagonize HNF-4 alpha action. Understanding how fatty acids regulate the activity and abundance of these and other transcription factors will likely provide insight into the development of novel therapeutic strategies for better management of whole body lipid and cholesterol metabolism.

Insulin injections enhance cholesterol gallstone incidence by changing the biliary cholesterol saturation index and apo A-I concentration in hamsters fed a lithogenic diet

BACKGROUND/AIMS

A link between insulin and cholesterol gallstone disease has often been suspected but never demonstrated. The aim was to evaluate the direct implication of insulin in the gallbladder cholesterol gallstone formation process.

METHODS

Hamsters fed with a soft-inducing lithogenic diet, enriched with sucrose, were injected daily, for 1 week, either with long-acting insulin or saline (controls).

RESULTS

Insulin injections doubled the cholesterol gallstone incidence. The cholesterol saturation index (CSI) of bile significantly increased (+19%) and biliary apolipoprotein A-I (apo A-I) decreased, both in concentration (-71%) and the proportion relative to the total biliary proteins (-25%). No modifications in the biliary bile acid composition were noticed. Hepatic HMGCoA reductase activity was higher (+341%), CYP7A1 activity was lower (-52%), whereas CYP27A1 and CYP7B1 were not affected. The hepatic low-density liprotein (LDL)-receptor and SR-BI masses did not vary. The hepatic total cholesterol content increased (+42%). Fasting plasma phospholipid and triglyceride concentrations significantly decreased (-15 and -60%, respectively), but the cholesterol concentration remained constant.

CONCLUSIONS

These results suggest that insulin injections enhance cholesterol gallstone incidence by increasing the CSI of bile and decreasing the concentration and proportion of a biliary anti-nucleating protein, apo A-I. Insulin modulates the major enzymes of cholesterol and bile acid metabolisms in vivo.