Reciprocal Interactions Between The P63/Hedgehog Pathways Regulate Bcl-2 Expression In Keratinocytes

Maintenance of epidermal homeostasis involves an intricate balance between proliferation and differentiation. This necessitates the coordinated regulation of a number of developmental pathways including the sonic hedgehog (SHH) and p63 networks. Deregulation of the SHH pathway is an initiating event in many epidermal tumors including Basal Cell Carcinomas (BCC), the most frequently diagnosed cancer. Bcl-2, a critical mediator of survival is overexpressed in BCCs and we, and others have shown that GLI1/2, effector molecules of the SHH pathway upregulate bcl-2 in in vitro and in vivo keratinocyte models. To examine potential regulation of the SHH pathway during epidermal homeostasis, we measured hedgehog (HH) pathway activity in response to increasing calcium concentrations that induce keratinocyte differentiation and, found that higher calcium concentrations are associated with lowered HH pathway activity and an upregulation of suppressor of fused (SUFU), a negative regulator of the SHH pathway. Using an epidermal-specific inducible knockdown mouse model, we demonstrated that \#916;Np63\#945;, a crucial regulator of epidermal development, binds and activates the SUFU promoter in differentiating keratinocytes. In vitro models show that increasing SUFU levels down regulate the bcl-2 promoter and prevent GLI mediated bcl-2 upregulation. This combined with siRNA knockdown studies suggest that SUFU upregulation is essential for bcl-2 downregulation in suprabasalar keratinocytes and the initiation of differentiation. Additionally we were able to show that skin biopsies from SUFU+/- mice demonstrate a significant upregulation of proliferation and bcl-2 expression. Interestingly SUFU+/- mice skin biopsies also showed enhanced p63 protein expression compared to their wild type controls. Additionally, i n vitro studies in keratinocytes suggest that the HH pathway may potentially regulate p63 transcript expression. In conclusion, these observations suggest that regulation of bcl-2 during keratinocyte differentiation in the epidermis is a complex process that involves the interacting nodes of specific signaling cascades and transcription factors acting in concert to determine the normal spatial distribution of proteins. Further, deregulation of these nodes that coordinate keratinocyte differentiation are important contributors to multistep carcinogenesis. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1497.