β-Catenin functions in both cell–cell adhesion and as a transcriptional coactivator in the canonical Wnt pathway. Nuclear accumulation of β-catenin is the hallmark of active Wnt signaling and is frequently observed in human cancers. Although β-catenin shuttles in and out of the nucleus, the molecular mechanisms underlying its translocation remain poorly understood. Chibby (Cby) is an evolutionarily conserved molecule that inhibits β-catenin–mediated transcriptional activation. Here, we identified 14-3-3ε and 14-3-3ζ as Cby-binding partners using affinity purification/mass spectrometry. 14-3-3 proteins specifically recognize serine 20 within the 14-3-3–binding motif of Cby when phosphorylated by Akt kinase. Notably, 14-3-3 binding results in sequestration of Cby into the cytoplasm. Moreover, Cby and 14-3-3 form a stable tripartite complex with β-catenin, causing β-catenin to partition into the cytoplasm. Our results therefore suggest a novel paradigm through which Cby acts in concert with 14-3-3 proteins to facilitate nuclear export of β-catenin, thereby antagonizing β-catenin signaling.

Chibby (Cby) acts with 14-3-3 to regulate β-catenin localization in the canonical Wnt pathway. We show that Cby harbors functional NLS and NES motifs, and shuttles between the nucleus and cytoplasm. Cby distribution at steady state is controlled by an intricate cooperation between 14-3-3, CRM1 and importin-α, which impacts on β-catenin signaling.

The canonical Wnt/␤-catenin signaling pathway plays diverse roles in embryonic development and disease. Activation of this pathway, likely by Wnt-10b, has been shown to inhibit adipogenesis in cultured 3T3-L1 preadipocytes and in mice. Here, we report that the ␤-catenin antagonist Chibby (Cby) is required for adipocyte differentiation. Cby is expressed in adipose tissue in mice, and Cby protein levels increase during adipogenic differentiation of 3T3-L1 cells. Ectopic expression of Cby induces spontaneous differentiation of these cells into mature adipocytes to an extent similar to that of dominant-negative Tcf-4. In contrast, depletion of Cby by RNA interference potently blocks adipogenesis of 3T3-L1 and mouse embryonic stem cells. In support of this, embryonic fibroblasts obtained from Cby-deficient embryos display attenuated differentiation to the adipogenic lineage. Mechanistically, Cby promotes adipocyte differentiation, in part by inhibiting ␤-catenin, since gain or loss of function of Cby influences ␤-catenin signaling in 3T3-L1 cells. Our results therefore establish Cby as a novel proadipogenic factor required for adipocyte differentiation.Adipose tissue plays critical roles in the regulation of energy homeostasis by storing and releasing fuel as a reservoir and by secreting a number of hormones and cytokines as an endocrine organ (37). Excess body fat, or obesity, is a major public health problem, particularly in industrialized countries, increasing the risk of diabetes, cardiovascular diseases and several types of cancers (28, 37). Conversely, lipoatrophy, the lack of adipose tissue, is also associated with diabetes and a number of other metabolic abnormalities (32). Hence, understanding the signaling pathways that govern adipocyte differentiation is necessary to develop comprehensive therapeutic strategies for the prevention and treatment of these disorders.Adipogenesis involves the formation of preadipocytes from mesenchymal progenitor cells and their differentiation into adipocytes (29,33,34). The cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems, such as 3T3-L1 and 3T3-F442A cell lines (10,31,33,34). In response to hormonal stimuli of adipogenesis, two transcription factors, CCAAT/ enhancer-binding protein ␤ (C/EBP␤) and C/EBP␦, are rapidly and transiently induced. These proteins then stimulate expression of the key adipogenic transcription factors, C/EBP␣ and peroxisome proliferator-activated receptor ␥ (PPAR␥), which act synergistically to induce expression of various adipocyte-specific genes.Intracellular signaling by the Wnt family of secreted glycoproteins regulates cell proliferation, differentiation, and polarity throughout vertebrate embryonic development (27,44,46). ␤-Catenin plays a pivotal role as a transcriptional coactivator in the canonical Wnt pathway (39). In the absence of Wnt signaling, cytoplasmic ␤-catenin becomes phosphorylated by glycogen synthase kinase-3␤ (GSK-3␤) in a complex containing Axin and t...