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Insulin and (—)-epigallocatechin gallate (EGCG) have been reported to regulate fat cell mitogenesis and adipogenesis, respectively. This study investigated the pathways involved in EGCG modulation of insulin-stimulated mitogenesis in 3T3-L1 preadipocytes. EGCG inhibited insulin stimulation of preadipocyte proliferation in a dose- and time-dependent manner. EGCG also suppressed insulin-stimulated phosphorylation of the insulin receptor-β, insulin receptor (IR) substrates 1 and 2 (IRS1 and IRS2), and mitogen-activated protein kinase pathway proteins, RAF1, MEK1/2, and ERK1/2, but not JNK. Furthermore, EGCG inhibited the association of IR with the IRS1 and IRS2 proteins, but not with the IRS4 protein. These data suggest that EGCG selectively affects particular types of IRS and MAPK family members. Generally, EGCG was more effective than epicatechin, epicatechin gallate, and epigallocatechin in modulating insulin-stimulated mitogenic signaling. We identified the EGCG receptor [also known as the 67-kDa laminin receptor (67LR)] in fat cells and found that its expression was sensitive to growth phase, tissue type, and differentiation state. Pre-treatment of preadipocytes with 67LR antiserum prevented the effects of EGCG on insulin-stimulated phosphorylation of IRS2, RAF1, and ERK1/2 and insulin-stimulated preadipocyte proliferation (cell number and bromodeoxyuridine incorporation). Moreover, EGCG tended to increase insulin-stimulated associations between the 67LR and IR, IRS1, IRS2, and IR54 proteins. These data suggest that EGCG mediates anti-insulin signaling in preadipocyte mitogenesis via the 67LR pathway. [ABSTRACT FROM AUTHOR]

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. Whether environmental estrogens regulate the production of resistin is still not clear. Using 3T3-L1 adipocytes, we found that octylphenol upregulated resistin mRNA expression in dose- and time-dependent manners. The concentration of octylphenol that increased resistin mRNA levels by 50% was ∼100 nM within 6 h of treatment. The basal half-life of resistin mRNA induced by actinomycin D was lengthened by octyiphenol treatment, suggesting that octylphenol decreases the rate of resistin mRNA degradation. In addition, octylphenol stimulated resistin protein expression and release. The basal half-life of resistin protein induced by cycloheximide was lengthened by octylphenol treatment, suggesting that octyiphenol decreases the rate of resistin protein degradation. While octylphenol was shown to increase activities of the estrogen receptor (ER) and MEK1, signaling was demonstrated to be blocked by pretreatment with either ICI-182780 (an ERα antagonist) or U-0126 (a MEK1 inhibitor), in which both inhibitors prevented octylphenol-stimulated phosphorylation of ERK. These results imply that ERα and ERK are necessary for the octylphenol stimulation of resistin mRNA expression. Moreover, U-0126 antagonized the octyiphenol-increased resistin protein expression and release. These data suggest that the way octylphenol signaling increases resistin protein levels is similar to that by which it increases resistin mRNA levels; it is likely mediated through an ERK-dependent pathway. In vivo, octylphenol increased adipose resistin mRNA expression and serum resistin and glucose levels, supporting its in vitro effect. [ABSTRACT FROM AUTHOR]