| With obesity reaching epidemic proportions worldwide, the need for effective pharmacological interventions has increased. Currently, no drug therapies approach obesity at the level of adipose cell signaling.{09}To study adipogenesis in culture, we used 3T3-L1 preadipocytes, which differentiate synchronously when stimulated with a defined adipogenic cocktail. Because 3T3-L1 cells must proliferate prior to differentiation, we hypothesized that flavonoids, which inhibit proliferation in a number of cell lines, would block 3T3-L1 adipogenesis. We found that the isoflavone genistein, a tyrosine kinase inhibitor, prevented preadipocyte proliferation and differentiation into adipocytes. Genistein exerted its anti-adipogenic effects within the first 72 h of differentiation. The flavonoids naringenin and daidzein, both structurally similar to genistein, had no inhibitory effects on adipogenesis. Our work with daidzein, a tyrosine-kinase inactive analog of genistein, suggested that genistein blocked adipogenesis primarily through its inhibition of tyrosine kinase activity. Tyrosine phosphorylation regulates the activation of certain signaling cascades and transcription factors involved in adipogenesis. Interestingly, genistein had differential effects on the expression of members of the CCAAT/enhancer binding (C/EBP) family of adipogenic transcription factors. Although genistein had no effect on C/EBPβ expression, it blocked the DNA-binding activity of C/EBPβ by up-regulating the dominant-negative C/EBP isoform CHOP. Importantly, we found that genistein impaired the tyrosine phosphorylation of C/EBPβ. Thus far, no published reports have demonstrated a role for phosphotyrosines in regulating the transcriptional activity of C/EBPβ. Our data indicate that impaired C/EBPβ activity prevented the subsequent expression of C/EBPα and peroxisome proliferator-activated receptor gamma (PPARγ) in genistein-treated cells. In mature adipocytes, genistein promoted basal and epinephrine-induced lipolysis.; The anti-adipogenic and lipolytic effects of the soy isoflavone genistein suggest that genistein, as either a dietary component or a pharmacological agent, may aid in the treatment of obesity by reducing adipose tissue mass. In addition, our studies of the mechanisms of genistein's anti-adipogenic effects revealed new targets for the pharmacological treatment of obesity: drugs designed to promote the expression of CHOP or to inhibit the tyrosine phosphorylation of C/EBPβ may block adipogenesis by preventing the expression of C/EBPα and PPARγ. |
