The Role of Protein Kinase D in Epidermal Tumorigenesis

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DESCRIPTION (provided by applicant): The non-melanoma skin cancers (NMSCs), basal and squamous cell carcinoma, are the most common cancers, occurring more often than all cancers combined, with approximately 1 million new cases diagnosed per year. Although the NMSCs, in particular basal cell carcinomas (BCCs) are, for the most part, essentially curable, surgical treatment often results in scarring and disfigurement, and the risk of recurrence is high (the chance of a second occurrence is 35-50%). The major risk factors for the development of NMSCs are chronic sun exposure (ultraviolet irradiation) and age: thus, these cancers are a major problem in the veteran population. BCCs, which make up approximately 80% of the NMSCs, arise from basal keratinocytes in the epidermis of the skin. Our previous studies have demonstrated that protein kinase D (PKD) is up-regulated in keratinocytes in human BCCs. In addition, we recently demonstrated that PKD1 is activated upon exposure of mouse keratinocytes to ultraviolet B irradiation and protects these cells from UVB-induced apoptosis in vitro. These results suggest that UVB might select for cells with higher levels of pro-proliferative PKD1. Alternatively, active PKD1 may allow survival of UV-damaged cells. This ability of PKD1 to promote survival would be beneficial in preventing excessive apoptosis with low levels of UVB exposure, causing minimal DNA damage that can be repaired. However, if PKD1 allows survival of cells that have suffered irreparable UV-induced DNA damage, these keratinocytes with DNA mutations could continue to proliferate and form skin tumors. Thus, either a pro-proliferative or pro-survival mechanism could provide a means by which PKD1 could contribute to epidermal tumorigenesis. Moreover, these results suggest that small-molecule PKD inhibitors might be a viable therapy for the treatment of non-melanoma skin cancers. Because these inhibitors can be applied topically, with minimal systemic exposure, they could potentially be used with few side effects even if PKD1 has important roles in other cell types. In addition, PKD inhibitors have been proposed as possible novel therapies for treatment of pancreatic cancer. Therefore, an understanding of the role of PKD1 in the epidermis may be important in determining the possibility of epidermal side effects of systemic treatment with these agents. For example, inhibition of PKD1 in the skin could result in increased sun sensitivity, with ultraviolet light triggering massive apoptosis of the keratinocytes comprising the epidermis, in patients on PKD inhibitors for the treatment of internal cancers. In the experiments proposed here, we intend to determine the mechanisms by which ultraviolet B light- induced PKD1 activation protects keratinocytes from cell death, thereby promoting their survival. In addition we will extend our results obtained in mouse keratinocytes in vitro, to the mouse in vivo. Further, we will investigate the effect of ultraviolet B light on PKD1 activation in human keratinocytes in vitro, in reconstituted human skin in situ and in human volunteers in vivo. In particular, our aims are to: (1) test the hypothesis that UVB-activated PKD1 localizes to the mitochondria, at which location the enzyme protects keratinocytes from UV-induced apoptosis, (2) test the hypothesis that PKD1 is activated in human keratinocytes in vitro and reconstituted human epidermis in situ and promotes survival upon UVB exposure, (3) test the hypothesis that PKD1 is activated by UVB in mouse and human skin in vivo and (4) test the hypothesis that UVB selects for keratinocytes with higher PKD1 levels in vitro and in vivo. The results of these studies should allow the identification of novel signaling molecules, such as PKD1 or its downstream pathways, which may be targeted to treat and/or prevent NMSCs in veterans and the U.S. population as a whole. In addition, PKD inhibitors have been proposed as potential novel therapies for the treatment of other types of cancers, such as pancreatic cancer. Evidence concerning the role of PKD in the keratinocyte response to UVB may be important for predicting possible sun sensitivity in patients undergoing systemic chemotherapy with PKD inhibitors.