Abstract 1026: A non-apoptotic function for Mcl-1 as an activator of canonical Wnt signaling in keratinocytes

Mcl-1 is an anti-apoptotic Bcl-2 family member whose rapid down-regulation is required for UV apoptosis. We recently showed that not only is Mcl-1 a major human keratinocyte (KC) survival protein, but it is required for proper KC differentiation in organotypic culture, implicating a non-apoptotic function for Mcl-1 involved in regulating epidermal morphogenesis. To further explore the function of Mcl-1 in vivo, we generated and analyzed transgenic mice expressing human Mcl-1 under control of the keratin 5 (K5) promoter. K5-Mcl-1 transgenic mice had >2-fold increase in epidermal thickness (p<0.001), increased basal KC BrdU incorporation (∼2-fold) and altered expression of KC differentiation markers. Several K5-Mcl-1 mice developed benign follicular cysts. Since both proliferation and hair follicle morphogenesis are features of increased Wnt/β-catenin signaling, we examined components of the Wnt/β-catenin pathway. Epidermises from K5-Mcl-1 mice had elevated steady state β-catenin, P-GSK3β (Ser9, Ser21) and P-Akt (Ser473), consistent with Wnt pathway activation. To assess Wnt pathway in more detail, we over-expressed Mcl-1 in human KCs by retroviral transduction. Over-expression of Mcl-1, but not Bcl-2 or Bcl-xL, also induced KC proliferation (3.3-fold). Mcl-1 increased the levels of total and active β-catenin, c-Myc, P-GSK-3β, and P-Akt, as well as inducing TCF-4-dependent luciferase reporter activity (p<0.05) in keratinocytes. Q-RT-PCR analysis of Wnt pathway genes identified Disheveled-1, Frizzled-1 and Cyclin D3 as the most highly up-regulated genes common to both Mcl-1 transduced human KCs and K5-Mcl-1 mice. Mcl-1 truncation mutants encoding the N-terminal 208 and 103 amino acids, but lacking the BH domains, were generated and over expressed in KCs. Both Mcl-1 mutants induced ³H-thymidine incorporation (3.4- and 2.9-fold respectively), indicating that the mitogenic activity of Mcl-1 resides in its N-terminal region. Since β-catenin signaling is activated in cutaneous SCC and involved in stem cell maintenance, this non-apoptotic function of Mcl-1 represents a novel and potentially oncogenic activity in addition to its well characterized anti-apoptotic function.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1026.