Secreted frizzled-related protein 4 is silenced by hypermethylation and induces apoptosis in beta-catenin-deficient human mesothelioma cells.

The secreted frizzled-related proteins (SFRPs) function as negative regulators of Wnt signaling and have important implications in tumorigenesis. Frequent promoter hypermethylation of SFRPs has been identified in human cancer. Restoration of SFRP function attenuates Wnt signaling and induces apoptosis in a variety of cancer types. Wnt signaling is known to inhibit apoptosis through activation of beta-catenin/Tcf-mediated transcription. Recently, we identified aberrant Wnt activation as a result of Dishevelled overexpression in malignant mesothelioma. Here, we report that silencing of SFRP4 is correlated with promoter hypermethylation in beta-catenin-deficient mesothelioma cell lines. Reexpression of SFRP4 in these beta-catenin-deficient mesothelioma cell lines blocks Wnt signaling, induces apoptosis, and suppresses growth. Conversely, knocking down SFRP4 by small interfering RNA in cell lines expressing both SFRP4 and beta-catenin stimulates Wnt signaling, promotes cell growth, and inhibits chemodrug-induced apoptosis. Our results suggest that methylation silencing of SFRP4 may play an important role in aberrant Wnt activation in mesothelioma even in the absence of beta-catenin. Our data also suggest that beta-catenin-independent noncanonical pathway(s) may be involved in the apoptotic inhibition caused by activation of Wnt signaling.