Fenoxycarb promotes adipogenesis in 3T3‐L1 preadipocytes

Lipophilic insect hormones and their analogs affect mammalian physiology by regulating the expression of metabolic genes. Therefore, we determined the effect of fenoxycarb, a juvenile hormone analog, on lipid metabolism in adipocytes. Here, we demonstrated that fenoxycarb dose‐dependently promoted lipid accumulation in 3T3‐L1 adipocytes during adipocyte differentiation and that its lipogenic effect was comparable to that of rosiglitazone, a well‐known ligand for peroxisome proliferator‐activated receptor gamma (PPARγ). Furthermore, fenoxycarb stimulated PPARγ activity without affecting other nuclear receptors, such as liver X receptor (LXR), farnesoid X‐activated receptor (FXR) and Nur77. In addition, fenoxycarb treatment increased the expression of PPARγ and fatty acid transporter protein 1 (FATP1) in 3T3‐L1 adipocytes, suggesting that fenoxycarb may facilitate adipocyte differentiation by enhancing PPARγ signaling, the master regulator of adipogenesis. Together, our results suggest that fenoxycarb promoted lipid accumulation in adipocytes, in part, by stimulating PPARγ.     

Regulatory role of Sirt1 on the gene expression of fatty acid‐binding protein 3 in cultured porcine adipocytes

To investigate whether Sirt1 could modulate fatty acid‐binding protein 3 (FABP3), we treated porcine adipocytes either with the Sirt1 inhibitor nicotinamide (NAM), with the Sirt1 activator resveratrol (RES), or by knockdown of Sirt1 by Sirt1‐siRNA. NAM or knockdown with Sirt1‐siRNA significantly inhibited Sirt1 mRNA expression, while increasing FABP3 mRNA levels. RES or RES + Sirt1‐siRNA treatments further proved that Sirt1 negatively regulated FABP3 gene expression in adipocytes. We also found a similar Sirt1 regulation pattern for PPARγ to that of FABP3 in adipocytes. Furthermore, NAM/RES + PPARγ‐siRNA treatments showed that Sirt1 may regulate the FABP3 gene expression partly through the PPARγ‐mediated signals. In summary, Sirt1 regulates the expression of FABP3 gene in adipocytes, and PPARγ apparently plays an important role in this process. J. Cell. Biochem. 107: 984–991, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of phosphodiesterase 3B gene expression in peroxisome proliferator-activated receptor gamma (+/-) mice independent of adipocyte size

Phosphodiesterase 3B (PDE3B) gene expression is generally reduced in large adipocytes of obese, insulin-resistant mice. This reduced gene expression is restored by peroxisome proliferator-activated receptor (PPAR) gamma ligands accompanied by a reduced fat cell size. To determine whether PDE3B gene expression is regulated by PPAR gamma itself, we analyzed lean PPAR gamma (+/-) mice with adipocyte size comparable to control PPAR gamma (+/+) mice. In adipocytes of PPAR gamma (+/-) mice, PDE3B mRNA and protein were both reduced to 63% of wild-type levels. Basal PDE activity tended to be decreased to 70% of wild-type levels, and, similarly, insulin-induced PDE activity was significantly decreased to 70%. Thus, PPAR gamma is required for PDE3B gene expression independent of adipocyte size.     

Potentiation of glucose uptake in 3T3-L1 adipocytes by PPAR gamma agonists is maintained in cells expressing a PPAR gamma dominant-negative mutant: evidence for selectivity in the downstream responses to PPAR gamma activation

Pharmacological agonists for the nuclear receptor PPAR gamma enhance glucose disposal in a variety of insulin-resistant states in humans and animals. The precise mechanisms whereby activation of PPAR gamma leads to increased glucose uptake in metabolically active cells remain to be determined. Notably, certain novel, synthetic PPAR gamma ligands appear to antagonize thiazolidinedione-induced adipogenesis yet stimulate cellular glucose uptake. We have explored the molecular mechanisms underlying the enhancement of glucose uptake produced by PPAR gamma agonists in 3T3-L1 adipocytes. Rosiglitazone treatment for 48 h significantly increased basal and insulin-stimulated glucose uptake and markedly increased the cellular expression of GLUT1 but not GLUT4. Rosiglitazone increased plasma membrane levels of GLUT1, but not GLUT4, both basally and after insulin stimulation. Surprisingly, adenoviral expression of a dominant-negative mutant PPAR gamma, which was demonstrated to strongly inhibit adipogenesis, completely failed to inhibit rosiglitazone-stimulated glucose uptake. Similar findings were obtained with the non-thiazolidinedione PPAR gamma agonists, GW1929 and GW7845. The insensitivity of PPAR gamma agonist-stimulated glucose uptake to expression of a dominant-negative mutant, compared with the latter's marked inhibitory effects on preadipocyte differentiation, suggests that, as is the case for other nuclear receptors, the precise molecular mechanisms linking PPAR gamma activation to downstream events may differ depending on the nature of the biological response. The growing evidence that the effects of PPAR gamma on adipogenesis and glucose uptake can be dissociated may have important implications for the development of improved antidiabetic drug treatments.     

Glyphosate Inhibits PPAR Gamma Induction and Differentiation of Preadipocytes and is able to Induce Oxidative Stress

Glyphosate‐based herbicides (GF) are extensively used for weed control. Thus, it is important to investigate their putative toxic effects. We have reported that GF at subagriculture concentrations inhibits proliferation and differentiation to adipocytes of 3T3‐L1 fibroblasts. In this investigation, we evaluated the effect of GF on genes upregulated during adipogenesis. GF was able to inhibit the induction of PPAR gamma, the master gene in adipogenesis but not C/EBP beta, which precedes PPAR gamma activation. GF also inhibited differentiation and proliferation of another model of preadipocyte: mouse embryonic fibroblasts. In exponentially growing 3T3‐L1 cells, GF increased lipid peroxidation and the activity of the antioxidant enzyme, superoxide dismutase. We also found that proliferation was inhibited with lower concentrations of GF when time of exposure was extended. Thus, GF was able to inhibit proliferation and differentiation of preadipocytes and to induce oxidative stress, which is indicative of its ability to alter cellular physiology.