Green tea catechins, especially (-)-epigallocatechin gallate (EGCG), have been proposed as a chemopreventative for obesity, diabetes, cancer, and cardiovascular diseases. However, relatively little is known about the mechanism of the action of EGCG on fat cell function. This study was designed to investigate the pathways of EGCG's modulation of the mitogenesis of 3T3-L1 preadipocytes. Preadipocyte proliferation as indicated by an increased number of cells and greater incorporation of bromodeoxyuridine (BrdU) was inhibited by EGCG in dose-, time-, and growth phase-dependent manners. Also, EGCG dose and time dependently decreased levels of phospho-ERK1/2, Cdk2, and cyclin D(1) proteins, reduced Cdk2 activity, and increased levels of G(0)/G(1) growth arrest, p21(waf/cip), and p27(kip1), but not p18(ink), proteins and their associations to Cdk2. However, neither MEK1, ERK1/2, p38 MAPK, phospho-p38, JNK, nor phospho-JNK was changed. Increased phospho-ERK1/2 content and Cdk2 activity, respectively, via the transfection of MEK1 and Cdk2 cDNA into preadipocytes prevented EGCG from reducing cell numbers. These data demonstrate the ERK- and Cdk2-dependent antimitogenic effects of EGCG. Moreover, EGCG was more effective than epicatechin, epicatechin gallate, and epigallocatechin in changing the mitogenic signals. The signal of EGCG in reducing growth of 3T3-L1 preadipocytes differed from that of 3T3 fibroblasts. Results of this study may relate to the mechanism by which EGCG modulates body weight.

Resistin is known as an adipocyte-specific secretory hormone that can cause insulin resistance and decrease adipocyte differentiation. It can be regulated by sexual hormones, but the mechanism of estrogen's actions is still not clear. Using 3T3-L1 adipocytes, we found that 17 beta-estradiol (E2) up-regulated resistin mRNA expression in a dose- and time-dependent manner. The concentration of E2 that increased resistin mRNA levels by 100-250% was approximately 1 nM for a range of 1-24 h of treatment. Treatment with either actinomycin D or cycloheximide prevented E2-stimulated resistin mRNA expression, suggesting that the effect of E2 requires new mRNA and protein synthesis. Although E2 was shown to increase activities of the estrogen receptor (ER) and MAPK kinase 1 and the association of nuclear ER alpha and CCAAT/enhancer binding protein-alpha with the resistin gene promoter, signaling was demonstrated to be blocked by pretreatment with either ICI182780 or PD98059. Neither SB203580 nor LY294002 changed the E2-increased levels of resistin mRNA, but they respectively inhibited E2-stimulated phosphorylation of p38 MAPK and Akt. These results imply the ER alpha, ERK, and CCAAT/enhancer binding protein- are necessary for the E2 stimulation of transcription from the resistin promoter. Moreover, PD98059, but not SB203580 or LY294002, antagonized E2-increased resistin protein release. These data suggest that E2 likely modifies the distribution of the resistin protein between the intracellular and extracellular compartments via an ERK-dependent pathway.

Chen, Yen-Hang, Pei-Fang Hung, and Yung-Hsi Kao. IGF-I downregulates resistin gene expression and protein secretion.

is known as an adipocyte-specific secretory hormone that can cause insulin resistance and decrease adipocyte differentiation. By contrast, green tea catechins, especially (Ϫ)-epigallocatechin gallate (EGCG), have been reported as body weight and diabetes chemopreventatives. Whether EGCG regulates production of Rstn is unknown. Using 3T3-L1 adipocytes, we found that EGCG at 20 and 100 M suppressed Rstn mRNA levels by ϳ35 and 50%, respectively, after 3 h. The basal half-life of Rstn mRNA induced by actinomycin D was Ͼ12 h but shifted to 3 h in the presence of EGCG. This suggests that EGCG regulates the stability of Rstn mRNA. Treatment with cycloheximide did not prevent EGCGsuppressed Rstn mRNA levels, which suggests that the effect of EGCG does not require new protein synthesis. Intracellular Rstn protein significantly decreased in the presence of 100 M EGCG 3 h after treatment, whereas the release of the Rstn protein did not significantly change. This suggests that EGCG may modulate the distribution of Rstn protein between the intracellular and extracellular compartments. EGCG did not affect the amounts of extracellular signal-related kinase-1/2 (ERK1/2), phospho-JNK, phospho-p38, and phospho-Akt proteins but reduced the amounts of phospho-ERK1/2 proteins. Overexpression with MEK1 blocked EGCG-inhibited Rstn mRNA expression. These data suggest that EGCG downregulates Rstn expression via a pathway that is dependent on the ERK pathway.genistein; mitogen-activated protein kinase; extracellular signal-related kinase RESISTIN (RSTN) IS A CYSTEINE-RICH POLYPEPTIDE HORMONE (20,38) that, depending on the species, contains 4 -5 exons, 3-4 introns, 575-1217 bp of mRNA, and 108 -114 amino acids of protein (10 -12.5 kDa) (16,23,44,45). Rstn is first isolated from adipose tissues and found to link obesity to type 2 diabetes (44). In particular, Rstn mRNA expression in adipose tissues of obese humans is higher than that in normal subjects (12). In addition, a single nucleotide polymorphism in the Rstn gene promoter is associated with obesity (14) and diabetes (35), and the plasma Rstn levels are elevated in patients with obesity (12) and type 2 diabetes (59). Moreover, the dominant negative form of Rstn enhances adipogenesis and improves insulin sensitivity (24). Ever since its discovery, Rstn also has been found to possess numerous other actions. For example, Rstn regulates fasted blood glucose (6), causes dyslipidemia (39), suppresses insulin-stimulated glucose uptake in adipocytes (44) and muscle cells (30), inhibits dopamine and norepinephrine release in the hypothalamus (8), and promotes endothelial cell activation (50) and smooth muscle proliferation (9). To our knowledge, the expression of Rstn gene can be regulated by nutritional, endocrine, genetic, pharmacological, and developmental factors (5), and the mechanisms of action of Rstn are emerging. For example, Rstn promotes smooth muscle cell proliferation through the activation of extracellular signalregulated kinase (ERK1/2) and phosphatidylinositol 3-kinase (PI3K) ...