| Wnts are secreted proteins that appear to activate two cellular signaling pathways that are known to play roles in cancer development: (a) the canonical pathway, which results in the transcriptional activation of beta-catenin/TCF responsive genes, and (b) The Wnt/Ca++ pathway, a less understood pathway, which results in the activation of protein kinase C (PKC) and calmodulin kinase (CamKII).;Mutations in components of the canonical pathway that stabilize beta-catenin result in augmented gene transcription and play major roles in many cancers. We employed microarrays to identify transcriptional targets of deregulated beta-catenin in a human epithelial cell line engineered to produce mutant beta-catenin and in ovarian endometrioid adenocarcinomas (OEAs) characterized with respect to mutations affecting the Wnt/beta-catenin pathway. Two genes strongly induced in both systems---FGF20 and DKK1---were studied in detail. Elevated levels of FGF20 RNA were also observed in adenomas from mice carrying the ApcMin allele. Furthermore, FGF20 and DKK1 appear to be direct targets for beta-catenin/TCF transcriptional regulation via LEF/TCF-binding sites. Finally, by using small inhibitory RNAs specific for FGF20 , we show that continued expression of FGF20 is necessary for maintenance of the anchorage-independent growth state in RK3E cells transformed by beta-catenin, implying that FGF-20 may be a critical element in oncogenesis induced by the Wnt/beta-catenin signaling pathway.;Ectopic expression of Wnt-1 in mouse mammary glands results in mammary tumors, and in morphological transformation in the mouse mammary cell line C57MG. We present evidence that activation of the Wnt/beta-catenin pathway is undetectable in Wnt-1-induced tumors, and that stabilized beta-catenin is not sufficient to transform C57MG cells. Instead, we show that PKCs are activated in Wnt-1-induced mouse mammary tumors, but not in tumors induced by Her2/neu and c-Myc, and that PKCs are implicated in the loss of cell contact growth inhibition in Wnt-1-transformed C57MG cells. We also demonstrate that treatment of Wnt-1-induced tumors with Safingol, a PKC inhibitor, results in apoptosis of the tumor cells. Finally, we present evidence that Wnts are expressed, and PKC-alpha is activated, in about 50% of human medullary carcinomas. |
