Abstract 5219: The role of MEK and Antizyme in keratinocyte stem cell expansion and differentiation

Transgenic mice provide a valuable system to study the development of skin tumorigenesis. Previous work from our lab has shown that mice overexpressing constitutively active MEK in the basal layer of the epidermis (K14-MEK mice) exhibit epidermal hyperplasia and spontaneous skin tumor development. These tumors express high levels of ornithine decarboxylase (ODC), the critical regulatory enzyme in the polyamine biosynthetic pathway. Although polyamines are required for normal cell growth, elevated levels of polyamines and ODC activity have been associated with neoplastic transformation and tumor promotion in a variety of tissues. Antizyme (AZ) suppresses ODC activity by targeting ODC for degradation. We have observed that mice overexpressing AZ and MEK in the skin (MEK/AZ mice) showed decreased tumor development compared to mice expressing MEK alone. This has led to the hypothesis that the upregulation of ODC caused by overexpressing constitutively active MEK will result in expansion of epidermal stem cells and decreased keratinocyte differentiation, in addition to the epidermal hyperplasia and tumorigenesis already seen. Overexpression of AZ decreases ODC activity and should reverse these phenotypes. Cell division in epidermal stem cells is asymmetrical, providing one replacement stem cell and a second transient amplifying (TA) cell. TA cells reside within the interfolliclar epidermis (IFE) and after several rounds of division, give rise to terminally differentiated cells in the suprabasal layers of the epidermis expressing early differentiation markers K1/K10 and late differentiation markers loricrin and involucrin. To determine the effect of MEK and AZ expression on stem cell expansion, newborn pups and adult mice were injected with BrdU using different wash out periods to label TA cells and epidermal stem cells. Our results showed that skin from K14-MEK mice had a significant increase in the number of TA cells within the IFE and hair follicle, while mice expressing both the K14-MEK and K5-AZ transgenes showed no difference in number of TA cells compared to wild-type littermates. Additionally, K14-MEK mice had a significant increase in the number of stem cells in the bulge region of the follicle, while AZ expression repressed the MEK-mediated increase in this cell population. Examination of primary keratinocyte cultures from the four genotypes showed that K14-MEK keratinocytes had decreased levels of both early and late differentiation markers compared t