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The tumor promoter 12-O-tetra-decanoylphorbol-13-acetate (TPA) has been defined by its ability to promote tumorigenesis on carcinogen-initiated mouse skin. Activation of Wnt/β-catenin signaling has a decisive role in mouse skin carcinogenesis, but it remains unclear how TPA activates Wnt/β-catenin signaling in mouse skin carcinogenesis. Here, we found that TPA could enhance Wnt/β-catenin signaling in a casein kinase 1 (CK1) ε/δ-dependent manner. TPA stabilized CK1ε and enhanced its kinase activity. TPA further induced the phosphorylation of LRP6 at Thr1479 and Ser1490 and the formation of a CK1ε-LRP6-axin1 complex, leading to an increase in cytosolic β-catenin. Moreover, TPA increased the association of β-catenin with TCF4E in a CK1ε/δ-dependent way, resulting in the activation of Wnt target genes. Consistently, treatment with a selective CK1ε/δ inhibitor SR3029 suppressed TPA-induced skin tumor formation in vivo, probably through blocking Wnt/β-catenin signaling. Taken together, our study has identified a pathway by which TPA activates Wnt/β-catenin signaling.

Prodigiosin, a natural red pigment produced by numerous bacterial species, has exhibited promising anticancer activity; however, the molecular mechanisms of action of prodigiosin on malignant cells remain unclear. Aberrant activation of the Wnt/β-catenin signaling cascade is associated with numerous human cancers. In this study, we identified prodigiosin as a potent inhibitor of the Wnt/β-catenin pathway. Prodigiosin blocked Wnt/β-catenin signaling by targeting multiple sites of this pathway, including the low-density lipoprotein-receptor-related protein (LRP) 6, Dishevelled (DVL), and glycogen synthase kinase-3β (GSK3β). In breast cancer MDA-MB-231 and MDA-MB-468 cells, nanomolar concentrations of prodigiosin decreased phosphorylation of LRP6, DVL2, and GSK3β and suppressed β-catenin-stimulated Wnt target gene expression, including expression of cyclin D1. In MDA-MB-231 breast cancer xenografts and MMTV-Wnt1 transgenic mice, administration of prodigiosin slowed tumor progression and reduced the expression of phosphorylated LRP6, phosphorylated and unphosphorylated DVL2, Ser9 phosphorylated GSK3β, active β-catenin, and cyclin D1. Through its ability to inhibit Wnt/β-catenin signaling and reduce cyclin D1 levels, prodigiosin could have therapeutic activity in advanced breast cancers.

WNT signaling is involved in maintaining stem cells in an undifferentiated state; however, it is often unclear which WNTs and WNT receptors are mediating these activities. Here we examined the role of the WNT receptor FZD7 in maintaining human embryonic stem cells (hESCs) in an undifferentiated and pluripotent state. FZD7 expression is significantly elevated in undifferentiated cells relative to differentiated cell populations, and interfering with its expression or function, either by short hairpin RNA-mediated knockdown or with a fragment antigen binding (Fab) molecule directed against FZD7, disrupts the pluripotent state of hESCs. The FZD7-specific Fab blocks signaling by Wnt3a protein by down-regulating FZD7 protein levels, suggesting that FZD7 transduces Wnt signals to activate Wnt/β-catenin signaling. These results demonstrate that FZD7 encodes a regulator of the pluripotent state and that hESCs require endogenous WNT/β-catenin signaling through FZD7 to maintain an undifferentiated phenotype.

Activation of the Wnt/[beta]-catenin pathway promotes the development of several cancers and is an attractive target for chemopreventive and chemotherapeutic agents. Nonsteroidal antiinflammatory drugs (NSAIDs) have been reported to antagonize [beta]-catenin function, but their mechanism of action is not known. We demonstrate here that interference with [beta]-catenin function by NSAIDs does not correlate with cyclooxygenase (COX) inhibition. Instead, NSAID inhibition of [beta]-catenin requires the high level expression of peroxisome proliferator-activated receptor [gamma] (PPAR-[gamma]) and its coreceptor retinoid-X-receptor [alpha] (RXR-[alpha]). Immunoprecipitation experiments show that [beta]-catenin interacts with RXR-[alpha] and PPAR-[gamma] in some malignant cells. Repression of [beta]-catenin-dependent transcription by NSAIDs is thus indirect and depends on the coexpression of other nuclear receptors. peroxisome proliferator-activated receptor [gamma] | retinoid X receptor [alpha]