| The pRb/CDK/cyclin/p16 pathway is altered in the majority of human neoplasias. In particular, Cyclin-dependent kinase 4 (CDK4) has been found mutated in familial melanoma, amplified or overexpressed in human gliomas, sporadic breast carcinomas and sarcomas. We have previously demonstrated that CDK4 ablation inhibits chemically-induced mouse skin papillomas, whereas forced expression of CDK4 in mouse epidermis (K5CDK4) accelerates malignant progression to Squamous Cell Carcinomas (SCC). However, the mechanisms by which changes in CDK4 expression levels control skin tumorigenesis have not been established. In the chemical carcinogenesis protocol, topical application of DMBA and TPA results in clonal expansion of a slow cycling stem cell population localized at the bulge area of the hair follicle. Thus, we hypothesize that CDK4 deletion or overexpression affects tumorigenesis by altering the characteristic and/or the number of Bulge Stem Cells (BSCs). To address this hypothesis, we employed the K15EGFP transgenic mouse model, which expresses EGFP under the control of the keratin 15 promoter in the bulge area of the hair follicle, to generate K5CDK4/K15EGFP as well as CDK4-/-/K15EGFP compound mice. We determined that changes in CDK4 expression affect the asymmetric cell division of the BSC population, favoring a decrease of the BSC pool in K5CDK4 transgenic mice. Importantly, ablation of CDK4 results in the opposite effect leading to an increased pool of BSCs. As a result, this model predicts that the number of transit amplifying or progenitor cells correlates with the susceptibility to papilloma development.;A subset of cells, termed side-population (SP), with characteristics of adult stem cells (SC) has been identified in several tissues, cell lines, as well as human and experimental tumors. The main feature of these putative stem cells is their high efflux capability for antimitotic drugs. The role of SPs in tumorigenesis is controversial; though, a role as cancer stem cells has been reported. Here, we investigated whether a functionally equivalent SP exists in cell lines derived from mouse epidermis and skin tumors in order to define the extent of which the presence of SP is associated with tumor progression. We observed positive correlation between number of SP and aggressiveness of tumor. Interestingly, we determined that the SP is a distinct population from bulge stem cells (BSC) and one of hair follicle progenitors, MTS24 +, colocalized with the SP marker BCRP1+/ABCG2 +. Moreover, we demonstrate for the first time that MTS24+ cells have tumorigenic capacity when grafted to immunodeficient mice. Importantly, the relevant correlation among the SP size and tumor progression suggests that these cells are positively selected during tumor progression. We found an analogous population in vivo, where the BCRP1/ABCG2 transporter and MTS24 were colocalized in patches of cells on the proliferative basal cell layer of skin papillomas. Furthermore, we observed an elevated number of putative stem cells expressing ABCG2 and MTS24 in tumors from our K5CDK4 mouse which showed increased malignant progression to SCC during chemical carcinogenesis protocol. Collectively, our results suggest that a putative MTS24 stem cell population is critical for malignant progression. |
