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Hepatic HMG-CoA reductase gene expression during the course of puromycin-induced nephrosis
1Department of Medicine, University of California, Irvine, USA.
Abstract:
Increased production and depressed catabolism of lipoproteins play major roles in the pathogenesis of hypercholesterolemia of nephrotic syndrome (NS). However, the effect, if any, of NS on cholesterol biosynthetic capacity is uncertain. We examined the gene expression of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR, the rate limiting step in cholesterol biosynthesis) during the induction and chronic phase of puromycin (PAN)-induced NS in rats. The rats were randomized to NS (given i.p. puromycin aminonucleoside 130 mg/kg on day 1 and 60 mg/kg on day 14) and placebo-treated control groups. Subgroups of animals were sacrificed at days 5, 10, 20 and 30. The liver was harvested between 7 and 9 p.m. for measurements of HMG-CoAR and actin mRNAs, HMG-CoAR enzymatic activity and microsomal cholesterol concentration. In separate experiments, subgroups of animals with chronic NS (day 30) were studied in fed and 20-hour fasting states. A marked but transient rise in hepatic HMG-CoAR mRNA and HMG-CoAR enzymatic activity was observed following the onset and exacerbation of proteinuria within a few days after each puromycin injection. On each occasion, HMG-CoAR fell to the baseline level despite persistent severe hypercholesterolemia. In an attempt to examine the possible acute effect of PAN per se, experiments were repeated before and at short intervals (8 and 24 hr) after puromycin injection when proteinuria was absent and the drug exposure prominent. The HMG-CoAR mRNA and activity were virtually unchanged during this period, suggesting the lack of an acute effect of puromycin. Twenty-hour fasting led to a marked rise in HMG-CoAR mRNA and activity in animals with chronic NS but not in the controls. Microsomal cholesterol remained unchanged and comparable in the two groups at all points. Thus, the marked but transient rise in hepatic HMG-CoAR gene expression observed during the induction phase and with fasting during the chronic phase of PAN-induced NS may contribute to the generation and maintenance of hypercholesterolemia in this animal model.
Insights
Nephrotic syndrome (NS) causes temporary increases in cholesterol synthesis, particularly during fasting, contributing to hypercholesterolemia. This study investigated hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR) gene expression in a rat model.
Area of Science:
- Biochemistry
- Nephrology
- Molecular Biology
Background:
- Nephrotic syndrome (NS) is linked to hypercholesterolemia, primarily due to increased lipoprotein production and decreased catabolism.
- The impact of NS on cholesterol biosynthesis capacity remains unclear.
- Hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR) is the rate-limiting enzyme in cholesterol synthesis.
Purpose of the Study:
- To investigate the effect of puromycin aminonucleoside (PAN)-induced NS on hepatic HMG-CoAR gene expression and enzyme activity in rats.
- To determine if NS influences cholesterol biosynthetic capacity during induction and chronic phases.
- To examine the role of fasting in modulating HMG-CoAR activity in chronic NS.
Main Methods:
- Rats were induced with NS using puromycin aminonucleoside and compared to placebo controls.
- Hepatic HMG-CoAR mRNA levels, HMG-CoAR enzymatic activity, and microsomal cholesterol were measured at various time points.
- Experiments included fed and fasting states in chronic NS models.
Main Results:
- A transient increase in HMG-CoAR mRNA and activity was observed during NS induction, returning to baseline despite persistent hypercholesterolemia.
- Puromycin administration itself did not acutely affect HMG-CoAR levels.
- Twenty-hour fasting significantly increased HMG-CoAR mRNA and activity in chronic NS rats but not controls.
Conclusions:
- Transient increases in hepatic HMG-CoAR gene expression during NS induction and fasting in chronic NS may contribute to hypercholesterolemia.
- The findings suggest a complex interplay between NS, cholesterol synthesis, and metabolic states.
- This study provides insights into the pathogenesis of hypercholesterolemia in NS using an animal model.