The Wnt/beta-catenin signalling pathway appears to operate to maintain the undifferentiated state of preadipocytes by inhibiting adipogenic gene expression. To define the mechanisms regulating suppression of Wnt/beta-catenin signalling, we analysed the beta-catenin expression in response to activation of transcription factors that regulate adipogenesis. The results show an extensive down-regulation of nuclear beta-catenin that occurs during the first few days of differentiation of 3T3-L1 preadipocytes and coincides with the induction of the adipogenic transcription factors, C/EBPbeta (CCAAT-enhancer-binding protein) and PPARgamma (peroxisome-proliferator-activated receptor). To assess the role of each of these factors in this process, we conditionally overexpressed C/EBPbeta in Swiss mouse fibroblasts using the TET-off system. Abundant expression of C/EBPbeta alone had minimal effect on beta-catenin expression, whereas expression of C/EBPbeta, in the presence of dexamethasone, induced PPARgamma expression and caused a measurable decrease in beta-catenin. In addition, exposure of cells expressing both C/EBPbeta and PPARgamma to a potent PPARgamma ligand resulted in an even greater decrease in beta-catenin by mechanisms that involve the proteasome. Our studies also suggest a reciprocal relationship between PPARgamma activity and beta-catenin expression, since ectopic production of Wnt-1 in preadipocytes blocked the induction of PPARgamma gene expression. Moreover, by suppressing beta-catenin expression, ectopic expression of PPARgamma in Wnt-1-expressing preadipocytes rescued the block in adipogenesis after their exposure to the PPARgamma ligand, troglitazone.

Studies have demonstrated cross talk between ␤-catenin and peroxisome proliferator-activated receptor ␥ (PPAR␥) signaling pathways. Specifically, activation of PPAR␥ induces the proteasomal degradation of ␤-catenin in cells that express an adenomatous polyposis coli-containing destruction complex. In contrast, oncogenic ␤-catenin is resistant to such degradation and inhibits the expression of PPAR␥ target genes. In the present studies, we demonstrate a functional interaction between ␤-catenin and PPAR␥ that involves the T-cell factor (TCF)/lymphocyte enhancer factor (LEF) binding domain of ␤-catenin and a catenin binding domain (CBD) within PPAR␥. Mutation of K312 and K435 in the TCF/LEF binding domain of an oncogenic ␤-catenin (S37A) significantly reduces its ability to interact with and inhibit the activity of PPAR␥. Furthermore, these mutations render S37A ␤-catenin susceptible to proteasomal degradation in response to activation of PPAR␥. Mutation of F372 within the CBD (helices 7 and 8) of PPAR␥ disrupts its binding to ␤-catenin and significantly reduces the ability of PPAR␥ to induce the proteasomal degradation of ␤-catenin. We suggest that in normal cells, PPAR␥ can function to suppress tumorigenesis and/or Wnt signaling by targeting phosphorylated ␤-catenin to the proteasome through a process involving its CBD. In contrast, oncogenic ␤-catenin resists proteasomal degradation by inhibiting PPAR␥ activity, which requires its TCF/LEF binding domain.

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Tiller angle is an important determinant of plant architecture in rice (Oryza sativa L.). Auxins play a critical role in determining plant architecture; however, the underlying metabolic and signaling mechanisms are still largely unknown. In this study, we have identified a member of the bZIP family of TGA class transcription factors, OsbZIP49, that participates in the regulation of plant architecture and is specifically expressed in gravity-sensing tissues, including the shoot base, nodes and lamina joints. Transgenic rice plants overexpressing OsbZIP49 displayed a tiller-spreading phenotype with reduced plant height and internode lengths. In contrast, CRISPR/Cas9-mediated knockout of OsbZIP49 resulted in a compact architecture. Follow-up studies indicated that the effects of OsbZIP49 on tiller angles are mediated through changes in shoot gravitropic responses. Additionally, we provide evidence that OsbZIP49 activates the expression of indole-3-acetic acid-amido synthetases OsGH3-2 and OsGH3-13 by directly binding to TGACG motifs located within the promoters of both genes. Increased GH3-catalyzed conjugation of indole-3-acetic acid (IAA) in rice transformants overexpressing OsbZIP49 resulted in the increased accumulation of IAA-Asp and IAA-Glu, and a reduction in local free auxin, tryptamine and IAA-Glc levels. Exogenous IAA or naphthylacetic acid (NAA) partially restored shoot gravitropic responses in OsbZIP49-overexpressing plants. Knockout of OsbZIP49 led to reduced expression of both OsGH3-2 and OsGH3-13 within the shoot base, and increased accumulation of IAA and increased OsIAA20 expression levels were observed in transformants following gravistimulation. Taken together, the present results reveal the role transcription factor OsbZIP49 plays in determining plant architecture, primarily due to its influence on local auxin homeostasis.