Recombinant human cachectin/tumor necrosis factor but not interleukin-1 alpha downregulates lipoprotein lipase gene expression at the transcriptional level in mouse 3T3-L1 adipocytes.

Abstract

Lipoprotein lipase (LPL) is synthesized primarily in muscle and adipose tissue and by hydrolyzing triglycerides in chylomicrons and very low density lipoprotein allows uptake of the resultant free fatty acids by these tissues. This report describes the cloning of the mouse LPL gene from which probes were derived to study the regulation of LPL synthesis in the 3T3-L1 adipocyte cell culture system. Preconfluent 3T3-L1 preadipocytes had very small amounts of LPL mRNA (<1 pg/,ug of RNA). At confluency, LPL mRNA levels increased to 5 to 15 pgl,Lg of RNA. After insulin and dexamethasone were added, LPL activity and mRNA levels rose in parallel. Peak mRNA levels were reached within 4 to 10 days, achieving LPL mRNA concentrations of 150 to 500 pg/,ug of RNA. This represents a 15-to 50-fold increase over confluent cells. Two cytokines known to diminish adipose tissue LPL activity were studied to see how their effects were regulated. Recombinant human cachectin/tumor necrosis factor diminished both LPL activity and LPL mRNA levels. The effect on LPL activity compared with mRNA levels was quicker, at a lower dose, and more complete (95 versus 75% maximum effect). The effect of recombinant human cachectin tumor necrosis factor on LPL mRNA levels was shown by nuclear run-on experiments to be exerted transcriptionally. It was also independent of new protein synthesis. Recombinant human interleukin-la diminished only LPL activity but not mRNA levels. This study suggests that during times of stress, cytokines secreted by activated macrophages can alter energy balance by affecting transcriptional and posttranscriptional processes in adipocytes.

Monocyte chemoattractant protein 1 in obesity and insulin resistance

“…This observation may begin to explain why lipid accumulation in adipocytes is inhibited by the presence of MCP-1 (19). Similar effects have been reported for other proinflammatory cytokines such as IL-6 and TNF-␣ (26,27). Taken together, these observations indicate that MCP-1 may have important functions in regulating the maturation and differentiation state of adipocytes.…”

Tumor necrosis factor-alpha eliminates binding of NF-Y and an octamer-binding protein to the lipoprotein lipase promoter in 3T3-L1 adipocytes.

“…TNFa was determined to be identical to cachectin, which was first discovered because it decreased adipose tissue lipoprotein lipase (LPL)' activity (2,3). LPL, a key enzyme in lipid metabolism, is responsible for the hydrolysis of triglycerides from circulating triglyceride-rich lipoproteins, providing free fatty acids and monoacylglycerol for cellular uptake.…”

Muscle-specific overexpression of lipoprotein lipase causes a severe myopathy characterized by proliferation of mitochondria and peroxisomes in transgenic mice.

“…DNA corresponding to the human LPL minigene was detected with a radiolabeled 1.2-kb NcoI-EcoRI fragment from the human LPL cDNA. RNA isolation and blotting techniques were performed as described previously (13). To differentiate human LPL mRNA from the endogenous mouse mRNA a species-specific DNA-probe containing a 1-kb EcoRI fragment from exon 10 of the human LPL gene was used.…”