The mouse model of experimental skin carcinogenesis has long proposed that epidermal stem cells are a major target of carcinogens and form the latent neoplastic population that ultimately expands into tumors. We have provided experimental evidence of this using the CD34 knockout (CD34KO) mouse, based on the hypothesis that CD34, which is uniquely expressed on hair follicle stem cells, plays a dynamic role in mediating stem cell response to carcinogens. CD34KO mice exhibit a striking impairment in skin tumor development and we have linked this phenotype to aberrant tumor promoter-induced activation of hair follicle cycling and stem cell proliferation. Further, we have evidence suggesting that hair follicle stem cells lacking CD34 have a reduced proliferative potential, based upon cell cycle analysis and growth in clonogenic culture, demonstrating that CD34 is critical for stem cell responsiveness as well underscoring the contribution of hair follicle stem cell contribution to tumor development. Our laboratory was the first to demonstrate that CD34 is expressed on mouse hair follicle stem cells, and through this, we developed methodology that facilitated enrichment for living stem cells. Earlier, we provided evidence from gene expression analysis of CD34+ keratinocytes of upregulation in the expression of the Deleted in Split Hand/Split Foot 1 gene (Dss1) following TPA treatment of mouse skin, which has subsequently been shown to have a growth regulatory role in the cell. We have determined that Dss1 mediates its growth regulatory effects through direct binding with the Rpn3/S3 subunit of the 19S regulatory particle of the proteasome assembly, with important implications in binding of the proteasome to ubiquitinylated substrates and subsequent protein degradation. In that regard, we are the first to provide biochemical evidence indicating that the R3IM motif of HsDSS1, in conjunction with 19S RP/20S CP (core particle) complex, regulates proteasome assembly and function through interaction with Rpn3/S3, and utilizes a specific subset of polyubiquitinated p53 as a substrate.
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