Overexpression of CYP27 in hepatic and extrahepatic cells: role in the regulation of cholesterol homeostasis

In the liver, sterol 27-hydroxylase (CYP27) participates in the classic and alternative pathways of bile acid biosynthesis from cholesterol (Chol). In extrahepatic tissues, CYP27 converts intracellular Chol to 27-hydroxycholesterol (27OH-Chol), which may regulate the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA-R). This study attempts to better define the role of CYP27 in the maintenance of Chol homeostasis in hepatic and extrahepatic cells by overexpressing CYP27 in Hep G2 cells and Chinese hamster ovary (CHO) cells through infection with a replication-defective recombinant adenovirus encoding for CMV-CYP27. After infection, CYP27 mRNA and protein levels increased dramatically. CYP27 specific activity also increased two- to fourfold in infected cells (P < or = 0.02), with a marked increase in conversion of [(14)C]Chol to [(14)C]27OH-Chol (approximately 150%; P < or = 0.01). Accumulation of 27OH-Chol in CHO cells was associated with a 50% decrease in HMG-CoA-R specific activity (P < or = 0.02). In infected Hep G2 cells, the significant increase in bile acid synthesis (46%; P < or = 0.006), which prevented the accumulation of intracellular 27OH-Chol, resulted in increased HMG-CoA-R activity (183%; P < or = 0.02). Overexpression of CYP27 in Hep G2 cells also increased acyl CoA-cholesterol acyltransferase (71%, P < or = 0.02) and decreased cholesteryl ester hydrolase (55%, P < or = 0.02). In conclusion, CYP27 generates different physiological responses depending on cell type and presence or absence of bile acid biosynthetic pathways.

Positive and negative regulation of JNK1 by protein kinase C and p42(MAP kinase) in adult rat hepatocytes

The role of protein kinase C (PKC) and p42(MAP kinase) signaling in the regulation of proliferation and apoptosis was investigated in freshly isolated and primary cultured rat hepatocytes. Acute treatment of freshly isolated hepatocytes with phenylephrine and EGF caused rapid phasic activations of p42(MAP kinase) and JNK1. Acute pre-treatment of hepatocytes with the PKC inhibitors sphingosine, chelerythrine and bis-indolylmaleimide abolished the ability of phenylephrine, but not EGF, to activate p42(MAP kinase) and JNK1. Acute pretreatments with all of the PKC inhibitors alone increased JNK1 basal activity approximately 2-fold. Acute treatments of primary cultures of hepatocytes with an inhibitor of MEK1 activation (PD98059) also caused inhibition of p42(MAP kinase) and a approximately 2-fold activation of JNK1. These data demonstrate that PKC can function as both a proximal activator and a distal inhibitor of signaling through the JNK1/SAP kinase pathway. Treatments (4 h) of primary cultured hepatocytes with sphingosine, chelerythrine, bis-indolylmaleimide and PD98059 did not induce apoptosis as judged by propidium iodide staining. Similar acute treatments of HepG2 cells rapidly induced cell death. These data demonstrate that acute inhibition of either PKC or p42(MAP kinase) function is sufficient to rapidly induce apoptosis in transformed, but not in non-transformed hepatocytes.

Studies on the link between HMG-CoA reductase and cholesterol 7 alpha-hydroxylase in lymph-fistula rats: evidence for both transcriptional and post-transcriptional mechanisms for down-regulation of the two enzymes by bile acids

The mechanism of down-regulation of hepatic HMG-CoA reductase and cholesterol 7 alpha-hydroxylase by cholic acid (CA) and chenodeoxycholic acid (CDCA) was investigated in rats with a lymph fistula. In accordance with the contention that bile acids are able to down-regulate HMG-CoA reductase by a mechanism independent of their effect on cholesterol absorption, inclusion of the above bile acids in diet (1%, wt/wt) reduced the HMG-CoA reductase specific activity by about 80%. The levels of mRNA were reduced by about 50% and the transcription rate (nuclear run-on) by 27% and 35%, respectively. Thus, HMG-CoA reductase appears to be regulated at both a transcriptional and a post-transcriptional level by bile acids. The cholesterol 7 alpha-hydroxylase activity as well as the corresponding mRNA levels were both reduced by about 50% by the same treatments. The transcription rate of the cholesterol 7 alpha-hydroxylase was not significantly affected by the treatment with CDCA, whereas the CA treatment reduced the transcription rate by about 24%. It is evident that bile acids regulate these two enzymes by different mechanisms in this model, in spite of the fact that the levels of mRNA were affected to about the same degree. The most obvious difference was the marked suppression of HMG-CoA reductase activity by cholic acid with relatively modest effects on transcription rate and level of mRNA. The latter finding is interesting in view of recent reports that dietary cholesterol also seems to down-regulate HMG-CoA reductase predominantly by a post-transcriptional mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of intestinal formation of deoxycholic acid from cholic acid in humans: evidence for a 3-oxo-delta 4-steroid intermediate

12 alpha-Hydroxy-3-oxo-4-cholenoic acid coupled to an adenosine nucleotide has been shown to be a metabolite of cholic acid in the intestinal anaerobic bacteria, Eubacterium species VPI 12708 (1987. J. Biol. Chem. 262: 4701-4707) and it has been suggested that this may be an intermediate in the conversion of cholic acid into deoxycholic acid. The possibility that the intestinal conversion of cholic acid into deoxycholic acid involves a 3-oxo-delta 4-steroid as an intermediate has been studied in the present work by use of [3 beta-3H]- and [5-3H]-labeled cholic acid. Whole cells as well as cell extracts of Eubacterium sp. VPI 12708 catalyzed conversion of [3 beta-3H] + [24-14C]cholic acid into deoxycholic acid with loss of about 50% of 3H label. When unlabeled chenodeoxycholic acid (20 microM) was added along with [3 beta-3] + [24-14C]cholic acid, then approximately 85% of the [3 beta-3H]-labeled was lost from deoxycholic acid. After administration of the same mixture to two healthy volunteers, deoxycholic acid could be isolated that had lost 81 and 84%, respectively, of the 3H label. Conversion of a mixture of [5-3H]- and [24-14C]labeled cholic acid by the above intestinal bacteria or cell extracts led to loss of 79-94 of the [5-3H] label.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamics, oxygen consumption and serum catecholamine changes in progressive, lethal peritonitis in the dog

A progressive, lethal canine peritonitis model was developed using the intraperitoneal injection of previously processed, frozen human stool. Of 15 dogs with fecal contamination, seven died within 24 hours and eight died over two to nine days, with a mean of five days. The model was associated with an initial hyperdynamic state which became quite variable as sepsis progressed. The cardiac index in the septic dogs correlated directly with the arteriovenous oxygen content difference. Evidence of multiple system organ failure and a significant increase in norepinephrine levels was noted in the septic dogs. The serum catecholamine levels were inversely correlated with the systemic blood pressure but did not correlate with the systemic vascular resistance or the cardiac index. This implies a lack of responsiveness of resistance vessels or the heart to catecholamines in this severely septic dog model.