Molecular control of adipogenesis: Regulation by TCF -4N and Frizzleds.

Abstract

Wnts are secreted proteins that play important roles in many aspects of cell fate. Our lab has previously demonstrated that canonical Writ signaling inhibits preadipocyte differentiation. Writs act through Frizzled receptors to activate several signaling pathways. In the canonical pathway, beta-catenin is stabilized and coactivates members of the T-cell factor/lymphoid-enhancing factor (TCF/LEF) family of transcription factors. In addition, Wnt signaling also activates calcium signaling and other pathways independent of beta-catenin. This dissertation describes studies investigating the role of TCF-4N and Frizzleds in regulating preadipocyte and mesenchymal precursor biology. TCF-4N is an alternatively spliced isoform of TCF-4 expressed in preadipocytes that contains an N-terminal interaction domain for beta-catenin but lacks a DNA binding domain. While TCF-4N inhibits coactivation by beta-catenin of a TCF/LEF-dependent promoter, TCF-4N potentiates coactivation by beta-catenin of several non-TCF/LEF-dependent promoters. For example, TCF-4N and beta-catenin synergize with the adipogenic transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) to induce leptin promoter activity. Consistent with TCF-4N's redirecting the actions of beta-catenin in cells, ectopic expression of TCF-4N in 3T3-L1 preadipocytes partially relieves the block of adipogenesis caused by beta-catenin. Thus, we propose that TCF-4N inhibits coactivation by beta-catenin of TCF/LEF transcription factors and potentiates the coactivation by beta-catenin of other transcription factors, such as SF-1 and C/EBPalpha. Effects of Wnt in preadipocytes may be mediated through Frizzled (Fz) 1 and/or Fz2 as these Wnt receptors are expressed in preadipocytes and their expression declines upon induction of differentiation. We ectopically expressed constitutively active chimeras between Wnt8 and Fz1 or Fz2 in preadipocytes and mesenchymal precursor cells. Our results indicate that activated Fz1 increases stability of beta-catenin, inhibits apoptosis, induces osteoblastogenesis, and inhibits adipogenesis. While activated Fz2 does not influence apoptosis or osteoblastogenesis, it inhibits adipogenesis through a mechanism independent of beta-catenin. An important mediator of the beta-catenin independent pathway appears to be calcineurin, because inhibitors of this serine/threonine phosphatase rescue the block to adipogenesis caused by Wnt3a or activated Fz2. These data support a model in which Wnt signaling inhibits adipogenesis through both beta-catenin dependent and independent mechanisms.