Lipoxygenase inhibitors inhibit heparin-releasable lipoprotein lipase activity in 3T3-L1 adipocytes and enhance body fat reduction in mice by conjugated linoleic acid.

The t10c12 isomer of conjugated linoleic acid (CLA) reduces lipid accumulation in adipocytes in part by inhibiting heparin-releasable lipoprotein lipase (LPL) activity. We now show that inhibitors of lipoxygenase (LOX) activity (2-[12-hydroxydodeca-5,10-diynyl]-3,5,6-trimethyl-p-benzoquinone; 5,8,11,14-eicosatetraynoic acid; salicylhydroxamic acid; indomethacin; nordihydroguaiaretic acid (NDGA)) produce a similar inhibitory effect on LPL activity in cultured 3T3-L1 mouse adipocytes. Additionally the LOX inhibitors had no effect on, or inhibited, lipolysis in this cell system (measured as glycerol release). Growing mice fed diet containing 0.1% NDGA for 4 weeks displayed 21% reduction in body fat, which was similar to 23% reduction in body fat produced by feeding diet containing a suboptimal amount of CLA (0.1%) for 4 weeks. Feeding diet containing both 0.1% NDGA and 0.1% CLA resulted in 51% reduction in body fat which was accompanied by significant increases in whole body water and protein. Aspirin, an inhibitor of cyclooxygenase 1 and 2, had no effect on LPL activity in 3T3-L1 adipocytes, did not affect body composition when fed to growing mice, and failed to influence the effects of CLA on LPL activity in 3T3-L1 cells or body composition in mice. These findings appear to provide new perspectives and insights into the relationships between CLA, eicosanoids, the control of lipid accumulation in adipocytes, and effects of CLA on the immune system.     

Lipoxygenase inhibitors inhibit heparin-releasable lipoprotein lipase activity in 3T3-L1 adipocytes and enhance body fat reduction in mice by conjugated linoleic acid

The t10c12 isomer of conjugated linoleic acid (CLA) reduces lipid accumulation in adipocytes in part by inhibiting heparin-releasable lipoprotein lipase (LPL) activity. We now show that inhibitors of lipoxygenase (LOX) activity (2-[12-hydroxydodeca-5,10-diynyl]-3,5,6-trimethyl-p-benzoquinone; 5,8,11,14-eicosatetraynoic acid; salicylhydroxamic acid; indomethacin; nordihydroguaiaretic acid (NDGA)) produce a similar inhibitory effect on LPL activity in cultured 3T3-L1 mouse adipocytes. Additionally the LOX inhibitors had no effect on, or inhibited, lipolysis in this cell system (measured as glycerol release). Growing mice fed diet containing 0.1% NDGA for 4 weeks displayed 21% reduction in body fat, which was similar to 23% reduction in body fat produced by feeding diet containing a suboptimal amount of CLA (0.1%) for 4 weeks. Feeding diet containing both 0.1% NDGA and 0.1% CLA resulted in 51% reduction in body fat which was accompanied by significant increases in whole body water and protein. Aspirin, an inhibitor of cyclooxygenase 1 and 2, had no effect on LPL activity in 3T3-L1 adipocytes, did not affect body composition when fed to growing mice, and failed to influence the effects of CLA on LPL activity in 3T3-L1 cells or body composition in mice. These findings appear to provide new perspectives and insights into the relationships between CLA, eicosanoids, the control of lipid accumulation in adipocytes, and effects of CLA on the immune system.     

Insulin and TNF alpha induce expression of the forkhead transcription factor gene Foxc2 in 3T3-L1 adipocytes via PI3K and ERK 1/2-dependent pathways

We have recently identified the winged helix/forkhead gene Foxc2 as a key regulator of adipocyte metabolism that counteracts obesity and diet-induced insulin resistance. This study was performed to elucidate the hormonal regulation of Foxc2 in adipocytes. We find that TNF alpha and insulin induce Foxc2 mRNA in differentiated 3T3-L1 cells with the kinetics of an immediate early response (1-2 h with 100 ng/ml insulin or 5 ng/ml TNF alpha). This induction is, in both cases, attenuated by the PI3K inhibitor wortmannin as well as the MAPK kinase inhibitor PD98059. Furthermore, we show that stimulation of 3T3-L1 adipocytes with phorbol-12-myristate-13-acetate or 8-(4-chlorophenyl)thio-cAMP induces the expression of Foxc2. Interestingly, we find that the basal level of Foxc2 mRNA is down-regulated whereas hormonal responsiveness increases during differentiation of 3T3-L1 from preadipocytes to adipocytes. At the protein level, immunoblots with Foxc2 antibody demonstrated an induction of Foxc2 by insulin and TNF alpha in nuclear extracts of 3T3-L1 adipocytes. EMSA of nuclear proteins from phorbol-12-myristate-13-acetate- and TNF alpha-treated 3T3-L1 adipocytes using a forkhead consensus oligonucleotide revealed specific binding of a Foxc2/DNA complex. In conclusion, our data suggest that insulin and TNF alpha regulate the expression of Foxc2 via a PI3K- and ERK 1/2-dependent pathway in 3T3-L1 adipocytes. Also, signaling pathways downstream of PKA and PKC induce the expression of Foxc2 mRNA.     

Recombinant human tumor necrosis factor does not inhibit lipoprotein lipase in primary cultures of isolated human adipocytes

Previous studies have demonstrated that cachectin/tumor necrosis factor (TNF) inhibits lipoprotein lipase (LPL) activity in cultures of 3T3-L1 cells. To determine whether TNF also inhibits LPL in human adipocytes, primary cultures of isolated human adipocytes were exposed to a spectrum of concentrations of recombinant human TNF. TNF concentrations up to 1000 pM had no effect on either LPL activity or LPL immunoreactive mass in the human adipocytes. Specific binding of 125I-labeled TNF was demonstrated in human adipocytes, and a TNF concentration of 100 pM competed for approximately 50% of the 125I-labeled TNF binding sites. In contrast, the same TNF in the same concentrations progressively inhibited LPL activity and immunoreactive mass in 3T3-L1 cells. Thus, human adipocytes respond to TNF in a different manner than 3T3-L1 cells. TNF may not cause the cachexia of cancer or chronic infection by directly inhibiting LPL in adipose tissue.