Hepatic apolipoprotein A-I synthesis andmRNA abundance, and whole body energy metabolism in copper-deficient rats.

Abstract

The influence of dietary copper deficiency on hepatic apolipoprotein A-I synthesis and mRNA abundance, as well as on whole body energy metabolism was examined in rats. Weanling male Sprague-Dawley rats were divided into two dietary treatments; copper-adequate (6.0 mg Cu/kg diet) and copper-deficient (0.6 mg Cu/kg diet). After 6 weeks of treatment, an increase in intravascular apolipoprotein A-I pool size was observed in copper-deficient rats. In part I, in vivo hepatic apolipoprotein A-I synthesis was determined by the injection of flooding dose of [³H]phenylalanine and measurement of the incorporation of [³H]phenylalanine into newly synthesized immunoprecipitable apolipoprotein A-I in liver homogenates as well as plasma. A pulse-chase study was designed using [³H]phenylalanine, to determine the in vitro hepatic apolipoprotein A-I synthesis and intracellular degradation in freshly isolated hepatocytes. Furthermore, hepatic apolipoprotein A-I mRNA abundance was determined by dot blot analysis. In part II, rats were individually housed in metabolic cages within indirect calorimetry units to study their energy metabolism. Total body composition was determined by total body electrical conductivity. Copper deficiency resulted in a specific 2-fold increase in hepatic apolipoprotein A-I synthesis and secretion. In vitro hepatic intracellular degradation was small and not affected by copper status. The hepatic apolipoprotein A-I mRNA abundance was increased by 28% when corrected for liver-to-body weight ratio in the copper-deficient rats. Copper deficiency resulted in a distinct shift in energy substrate utilization from carbohydrate to fat. Body weight gain and net energy retention were reduced as a result of copper deficiency. Total body composition analysis showed a reduction in percent fat mass in the copper-deficient rats. The present data suggest that copper deficiency results in an increased cellular demand for lipids as energy substrate in order to maintain an adequate energy balance. The observed increase in hepatic apo A-I synthesis may be a result of an increased demand for HDL formation to sustain an increased flux of lipid substrates between the liver and peripheral tissues, resulting in the observed hypercholesterolemia in copper-deficient rats.