Lipoprotein lipase : mechanism for adaptation of activity to the nutritional state

Sammanfattning: Lipoprotein lipase (LPL) is an enzyme to hydrolyze triglycerides in lipoproteins and thereby make the fatty acids available for cellular metabolic reactions. Short-term fasting down-regulates LPL activity in adipose tissue. This regulation is through post-translational mechanism. The objective of this work was to investigate (1) The molecular mechansim for regulation of LPL activity in adipose tissue; (2) The basis for the tissue-specific regulation of LPL in adipose tissue, heart and skeletal muscle. LPL in adipose tissue can be found both inside (intracellular) and outside adipocytes (extracellular). Within adipocytes, neither LPL mass nor the distribution of LPL between active and inactive forms changed on fasting. Extracellular LPL mass also did not change significantly, but shifted from predominantly active to predominantly inactive. Activie, extracellular LPL was distributed in a similar way in the two nutritional states. The down-regulation during fasting is due to a decline of extracellular LPL activity. The up-regulation of LPL activity induced by re-feeding did not need new mRNA. The down-regulation of LPL activity induced by fasting did not occur when mRNA synthesis was inhibited. LPL activity in adipose tissue from fasted rats was fully restored by actinomycin. So fasting switches on a gene, whose product suppresses LPL activity. Similar results were also obtained in experiments on mice. When food was removed from young rats in the early morning, adipose tissue TNF-α activity increased and LPL activity decreased within six hours. There was a negative correlation between TNF-α and LPL activities. Pentoxifylline, that inhibits biosynthesis of TNF-α, almost abolished the rise of TNF-α and the decrease of LPL activity. Actinomycin D virtually abolished the response of LPL activity to fasting or exogenous TNF-α. This study suggests that fasting signals via TNF-α to a gene whose product causes a rapid shift of newly-synthesized LPL molecules towards an inactive form.