The role of Wnt/beta-catenin signaling in renal carcinogenesis: lessons from cadmium toxicity studies.

Wnt/beta-catenin signaling plays a crucial role during embryogenesis. However, this signaling pathway also plays a role in normal adult tissues and in carcinogenesis, including cadmium (Cd2+) induced nephrocarcinogenesis, which is the topic of this review. Wnt/beta-catenin signaling is tightly regulated in mature epithelia to balance cell proliferation, differentiation and death. This is accomplished by modulating phosphorylation of the multifunctional protein beta-catenin which in turn determines its preference for a particular fate, i.e. cell-cell adhesion by binding to E-cadherin, proteasomal degradation, or co-activation of the transcription factor Tcf/Lef. The pivotal role of beta-catenin is not limited to Wnt signaling, but can be challenged by other transcription factors under stress conditions (e.g. FOXO, HIF-1alpha, NF-kappaB, c-jun), where beta-catenin acts as a molecular switch in response to the cellular redox status. Aberrant Wnt/beta-catenin signaling can contribute to carcinogenesis of intestinal, lung or kidney epithelia, either by mutations of its signaling components and/or disruption of linked signaling networks. The nephrotoxic metal Cd2+ causes renal cancer in humans. Because it is not genotoxic Cd2+ is thought to induce mutations and carcinomas indirectly: Possible mechanisms include oxidative stress, inhibition of DNA repair, aberrant gene expression, deregulation of cell proliferation, resistance to apoptosis, and/or disruption of cell adhesion. Wnt signaling may contribute to Cd2+ carcinogenesis because Cd2+ disrupts the junctional E-cadherin/beta-catenin complex, resulting in excessive nuclear translocation of beta-catenin and activation of Tcf4. Up-regulation of target genes of the beta-catenin/Tcf4 complex, such as c-myc, cyclin D1 and the multidrug transporter P-glycoprotein (MDR1/ABCB1), leads to increased proliferation, evasion of apoptosis, adaptation to Cd2+ toxicity and thereby promotes the selection of mutated and pre-neoplastic cells.