Leucine catabolism during the differentiation of 3T3-L1 cells. Expression of a mitochondrial enzyme system.

F E Frerman,J L Sabran,J L Taylor,S E Grossberg

doi:10.1016/s0021-9258(18)32336-6

Abstract

Leucine can be utilized efficiently as a precursor for lipid biosynthesis by adipose tissue, especially in the presence of glucose or glucose and insulin. During the differentiation of 3T3-L1 fibroblasts to adipocytes, the rate of lipid biosynthesis from L-[U-14C]leucine increases at least 30-fold and lipogenesis, with [U-14C] acetate as the precursor, increases by 10- to 15-fold. The specific activities of two mitochondrial dehydrogenases in the leucine oxidative pathway, the branched chain alpha-ketoacid dehydrogenase and isovaleryl-CoA dehydrogenase, as well as of leucine:alpha-ketoglutarate transaminase, increase at least 20-fold during the adipose conversion. Isovaleryl-CoA dehydrogenase was assayed in crude extracts using a specific fluorimetric method employing electron transfer flavoprotein as the electron acceptor for the flavoprotein dehydrogenase. The specific activity of 3-hydroxy-3-methylglutaryl-CoA lyase, the mitochondrial enzyme catalyzing the terminal reaction in the leucine degradation pathway, increases 4-fold during differentiation. The increases in the specific activities of the mitochondrial enzymes occur without a change in the specific activity of cytochrome oxidase, indicating that the increases do not simply reflect proliferation of mitochondria. The biosynthesis of at least 20 soluble mitochondrial polypeptides is enhanced during the adipose conversion of the fibroblasts as determined by polyacrylamide gel electrophoresis following incubation of the cells with [35S] methionine. The results provide a conservative estimate of the extent of changes in mitochondrial soluble proteins during the adipose conversion. They also establish that differentiated 3T3-L1 adipocytes metabolize leucine like mature adipose tissue and illustrate the roles of the branched chain alpha-ketoacid dehydrogenase and isovaleryl-CoA dehydrogenase in lipogenesis.

Highlights

Leucine can be utilized efficiently as a precursor for triglyceride biosynthesis [4,15]and in theresponses to lipolipid biosynthesis by adipose tissue, especially in the genic and lipolytic hormones [16,17,18,19] have been studied expresence of glucose or glucose and insulin
CoA lyase, the mitochondrial enzyme catalyzing the In the experiments reported here,we show that the acquiterminal reaction in the leucine degradation pathway, sition of the fatty phenotypeby 3T3-Ll cells is accompanied increases 4-fold during differentiation
In the specific activities of the mitochondrial enzymes We further demonstrate in these differentiating adipocytes occur without a change in the specific activity of cy- the roles in lipogenesis of thebranchedchaina-ketoacid tochrome oxidase, indicating that the increases do not dehydrogenase, and isovaleryl-CoA dehydrogenase

Summary

Introduction

Leucine can be utilized efficiently as a precursor for triglyceride biosynthesis [4,15]and in theresponses to lipolipid biosynthesis by adipose tissue, especially in the genic and lipolytic hormones [16,17,18,19] have been studied expresence of glucose or glucose and insulin. The specific activities of two mitochondrial dehydrogenases in theleucine oxidative pathway, thberanched chain a-ketoacid dehydrogenase and isovaleryl-CoA dehydrogenase, as well as of 1eucine:a-ketoglutarate transaminase, increaseat least 20-fold during the adipose conversion. Unlike other tissues, leucine is asignificant precursor for fatty acid andsterol biosynthesis, especially in the presence of glucose withor withoutinsulin [24,25,26]. CoA lyase, the mitochondrial enzyme catalyzing the In the experiments reported here,we show that the acquiterminal reaction in the leucine degradation pathway, sition of the fatty phenotypeby 3T3-Ll cells is accompanied increases 4-fold during differentiation. In the specific activities of the mitochondrial enzymes We further demonstrate in these differentiating adipocytes occur without a change in the specific activity of cy- the roles in lipogenesis of thebranchedchaina-ketoacid tochrome oxidase, indicating that the increases do not dehydrogenase, and isovaleryl-CoA dehydrogenase. The bio- tion, our data correlate with the finding of increased glutasynthesis of at least 20 soluble mitochondrial polypep- mine synthetase levels in differentiated 3T3-Ll cells [28] t o tides isenhanced during theadipose conversion of the support the hypothesis thatadipocytes couple leucine catabfibroblasts as determined by polyacrylamide gel electrophoresis following incubation of the cells with r3“S]

Methods

Results

Conclusion