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Forms of lipoprotein lipase in rat tissues: in adipose tissue the proportion of inactive lipase increases on fasting.

ABSTRACT: Previous studies have shown that the ratio of lipoprotein lipase (LPL) catalytic activity to LPL mass in tissues differs in different conditions, but it is not clear whether this occurs by a change in the catalytic efficiency of the LPL molecules, or because of a shift in the relation between active and inactive forms of the enzyme. To explore this, we have measured LPL activity and mass in detergent extracts of rat tissues. LPL specific activity was high and similar in heart, skeletal muscle, lung and brain. The liver had significantly lower specific activity, which is in accord with previous findings that the liver takes up and catabolizes LPL. The specific activity was also low in adipose tissue from fasted rats. When tissue extracts were applied to columns of heparin-agarose and eluted by a gradient of NaCl, a peak of active LPL was eluted at 1.0 M NaCl, but there was also a peak of inactive LPL protein, which was eluted at 0.6 M NaCl. In adipose tissue, LPL activity decreased by 70-80% during an overnight fast, whereas LPL mass decreased by only 20-40%. The mass ratio between inactive and active LPL, as separated by heparin-agarose chromatography, increased from 0.5 to over 2 during the fast. In hearts there was no significant difference between fed and fasted rats in total LPL activity, LPL mass or in the distribution between inactive and active forms. The results indicate that the relation between inactive (probably monomeric) and active (dimeric) forms of LPL is a target for post-translational regulation in adipose tissue.

SUBMITTER: Bergo M

PROVIDER: S-EPMC1216995 | biostudies-other | 1996 Feb

REPOSITORIES: biostudies-other

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Project description:BackgroundLipoprotein lipase (LPL) hydrolyzes triglycerides in lipoproteins and makes fatty acids available for tissue metabolism. The activity of the enzyme is modulated in a tissue specific manner by interaction with other proteins. We have studied how feeding/fasting and some related perturbations affect the expression, in rat adipose tissue, of three such proteins, LMF1, an ER protein necessary for folding of LPL into its active dimeric form, the endogenous LPL inhibitor ANGPTL4, and GPIHBP1, that transfers LPL across the endothelium.ResultsThe system underwent moderate circadian oscillations, for LPL in phase with food intake, for ANGPTL4 and GPIHBP1 in the opposite direction. Studies with cycloheximide showed that whereas LPL protein turns over rapidly, ANGPTL4 protein turns over more slowly. Studies with the transcription blocker Actinomycin D showed that transcripts for ANGPTL4 and GPIHBP1, but not LMF1 or LPL, turn over rapidly. When food was withdrawn the expression of ANGPTL4 and GPIHBP1 increased rapidly, and LPL activity decreased. On re-feeding and after injection of insulin the expression of ANGPTL4 and GPIHBP1 decreased rapidly, and LPL activity increased. In ANGPTL4(-/-) mice adipose tissue LPL activity did not show these responses. In old, obese rats that showed signs of insulin resistance, the responses of ANGPTL4 and GPIHBP1 mRNA and of LPL activity were severely blunted (at 26 weeks of age) or almost abolished (at 52 weeks of age).ConclusionsThis study demonstrates directly that ANGPTL4 is necessary for rapid modulation of LPL activity in adipose tissue. ANGPTL4 message levels responded very rapidly to changes in the nutritional state. LPL activity always changed in the opposite direction. This did not happen in Angptl4(-/-) mice. GPIHBP1 message levels also changed rapidly and in the same direction as ANGPTL4, i.e. increased on fasting when LPL activity decreased. This was unexpected because GPIHBP1 is known to stabilize LPL. The plasticity of the LPL system is severely blunted or completely lost in insulin resistant rats.

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Forms of lipoprotein lipase in rat tissues: in adipose tissue the proportion of inactive lipase increases on fasting.

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