Tumour necrosis factor-a (TNF-a), a proinflammatory cytokine, is a potent negative regulator of adipocyte differentiation. However, the mechanism of TNF-a-mediated antiadipogenesis remains incompletely understood. In this study, we first confirm that TNF-a inhibits adipogenesis of 3T3-L1 preadipocytes by preventing the early induction of the adipogenic transcription factors peroxisome proliferator-activated receptor-c (PPARc) and CCAAT/enhancer binding protein-a (C/EBPa). This suppression coincides with enhanced expression of several reported mediators of antiadipogenesis that are also targets of the Wnt/b-catenin/T-cell factor 4 (TCF4) pathway. Indeed, we found that TNF-a enhanced TCF4-dependent transcriptional activity during early antiadipogenesis, and promoted the stabilisation of b-catenin throughout antiadipogenesis. We analysed the effect of TNF-a on adipogenesis in 3T3-L1 cells in which b-catenin/TCF signalling was impaired, either via stable knockdown of b-catenin, or by overexpression of dominant-negative TCF4 (dnTCF4). The knockdown of b-catenin enhanced the adipogenic potential of 3T3-L1 preadipocytes and attenuated TNF-a-induced antiadipogenesis. However, b-catenin knockdown also promoted TNF-a-induced apoptosis in these cells. In contrast, overexpression of dnTCF4 prevented TNF-a-induced antiadipogenesis but showed no apparent effect on cell survival. Finally, we show that TNF-a-induced antiadipogenesis and stabilisation of b-catenin requires a functional death domain of TNF-a receptor 1 (TNFR1). Taken together these data suggest that TNFR1-mediated death domain signals can inhibit adipogenesis via a b-catenin/TCF4-dependent pathway.

No abstract

Signal-induced alternative splicing of the CD45 gene in human T cells is essential for proper immune function. Skipping of the CD45 variable exons is controlled, in large part, by the recruitment of PSF to the pre-mRNA substrate upon T cell activation; however, the signaling cascade leading to exon exclusion has remained elusive. Here we demonstrate that in resting T cells PSF is directly phosphorylated by GSK3 thus promoting interaction of PSF with TRAP150 which prevents PSF from binding CD45 pre-mRNA. Upon T cell activation, reduced GSK3 activity leads to reduced PSF phosphorylation, releasing PSF from TRAP150 and allowing it to bind CD45 splicing regulatory elements and repress exon inclusion. Our data place two new players, GSK3 and TRAP150, in the complex network that regulates CD45 alternative splicing and demonstrate a new paradigm for signal transduction from the cell surface to the RNA processing machinery through the multi-functional protein PSF.