Target of skin tumorigenesis: cyclin D1-cdk4

Reduced cyclin D1 ubiquitination in UVB-induced murine squamous cell carcinomas

Ubiquitination of cyclin D1 signals for its proteosomal degradation. To assess the possibility that reduced cyclin D1 proteolysis is a putative mechanism for its accumulation during UVB-induced skin tumorigenesis, ubiquitination activity of cyclin D1 was assessed in UVB-induced murine SCCs. Cyclin D1 was rapidly ubiquitinated by control skin extract, whereas ubiquitination of cyclin D1 was significantly reduced in SCCs. Mutant cyclin D1, in which residues important for GSK3beta-mediated degradation of cyclin D1 are altered to non-phosphorylatable alanine, was not ubiquitinated. We also observed phosphorylation-dependent inactivation of GSK3beta in SCCs. Our results indicate reduced ubiquitination of cyclin D1 in UVB-induced murine SCCs and suggest that inactivation of GSK3beta-dependent cyclin D1 degradation pathway contributes to the accumulation of cyclin D1 in UVB-induced murine SCCs.

Stage-specific alterations of cyclin expression during UVB-induced murine skin tumor development

We have evaluated the in vivo correlation between the expression of cell cycle markers and skin tumor development in SKH-1 hairless mice in a complete photocarcinogenesis protocol. Irradiated mice developed an average of 16 tumors per animal by week 23 with the average number of carcinomas per mouse being 2.1. The expression of p53 and cyclins A and D1 was confined initially to sporadic single cells and gradually developed into foci of patchy intense staining in the basal and granular layers of UVB-exposed epidermis. p53 was expressed in all the papilloma sections examined, whereas cyclins D1 and A were expressed in 68 and 71% of these lesions, respectively. In UVB-induced squamous cell carcinomas (SCC), p53 was expressed in >90% of the tumors, whereas cyclin D1 was detected in 55% of the lesions, and cyclin A staining was limited to 27%. These immunohistochemical observations were confirmed by Western blotting and protein kinase assays. We observed an early wave of cyclin A overexpression and cyclin A protein kinase activity preceding the appearance of detectable tumors. Cyclin D1 and p53 overexpression were coupled with the development of tumors, and these changes are likely to be relevant to the pathogenesis of these lesions.

Tumorigenic conversion of immortal human skin keratinocytes (HaCaT) by elevated temperature

UV-radiation is a major risk factor for non-melanoma skin cancer causing specific mutations in the p53 tumor suppressor gene and other genetic aberrations. We here propose that elevated temperature, as found in sunburn areas, may contribute to skin carcinogenesis as well. Continuous exposure of immortal human HaCaT skin keratinocytes (possessing UV-type p53 mutations) to 40 degrees C reproducibly resulted in tumorigenic conversion and tumorigenicity was stably maintained after recultivation of the tumors. Growth at 40 degrees C was correlated with the appearance of PARP, an enzyme activated by DNA strand breaks and the level corresponded to that seen after 5 Gy gamma-radiation. Concomitantly, comparative genomic hybridization (CGH) analyis demonstrated that chromosomal gains and losses were present in cells maintained at 40 degrees C while largely absent at 37 degrees C. Besides individual chromosomal aberrations, all tumor-derived cells showed gain of chromosomal material on 11q with the smallest common region being 11q13.2 to q14.1. Cyclin D1, a candidate gene of that region was overexpressed in all tumor-derived cells but cyclinD1/cdk4/cdk6 kinase activity was not increased. Thus, these data demonstrate that long-term thermal stress is a potential carcinogenic factor in this relevant skin cancer model, mediating its effect through induction of genetic instability which results in selection of tumorigenic cells characterized by gain of 11q.

A review of tumor suppressor genes in cutaneous neoplasms with emphasis on cell cycle regulators

Cells normally have five options. These include renewal or proliferation, terminal differentiation, quiescence, senescence, and apoptosis. Many factors interact with cell cycle regulators to direct the cells toward these different options. Tumor suppressor genes play a pivotal role in this process. Alterations in these genes may limit the options that cells have and thus play a significant role in the multistep process of carcinogenesis. We will focus on tumor suppressor genes and especially tumor suppressor genes that interact directly with the cell cycle proteins.