S-phase arrest in mouse keratinocytes exposed to multiple doses of ultraviolet B/A radiation.

Exposure to solar ultraviolet (UV) radiation is believed to cause most human skin carcinomas. Despite the large body of evidence connecting UV exposure with skin cancer, the frequency and level of human exposure to repetitive doses of UV light will most likely continue for occupational and recreational reasons. By investigating the cellular response of keratinocytes to multiple, physiologically relevant doses of UV, we hope to better understand the processes involved in UV-induced skin cancer. In this study, we used a UV exposure model to investigate the cell-cycle response of keratinocytes exposed to multiple doses of UV-B/A radiation in which the UV-C component (wavelengths below 290 nm) had been filtered out. Our results indicated that exposure of asynchronous mouse keratinocytes to three doses of 200 J/m2 UV-B/A radiation at 30 min intervals produced an inhibition of DNA synthesis and S-phase arrest between 7 and 25 h after the last irradiation. The S-phase arrest was not due to a reduction in the level of cyclin E and A proteins but was accompanied by inhibition of cyclin-dependent kinase 2 (cdk2) activity. We observed a similar pattern of cdk2 inhibition induced by multiple UV-B/A irradiations in mouse embryo fibroblasts from p21WAF null mice, indicating that the inhibition of cdk2 was independent of p21WAF in these cells.

Differential effects of UV-B and UV-C components of solar radiation on MAP kinase signal transduction pathways in epidermal keratinocytes.

Exposure to solar ultraviolet (UV) light is a major cause of skin cancer, the most common human neoplasm. The earth's upper atmosphere absorbs the high energy UV-C wavelengths (100-280 nm), while allowing transmission of UV-B (280-320 nm) and UV-A (320-400 nm). It is therefore UV-B and to some extent UV-A, that contributes to most human skin malignancies. We report that the exposure of cultured keratinocytes or skin to UV-C radiation causes activation of MAP kinases (ERK and JNK). In contrast, the solar radiation associated with skin cancer (UV-B) was an ineffective activator of the ERK and JNK signal transduction pathways. Therefore, while exposure of epidermal cells to UV-C radiation under laboratory conditions causes marked activation of MAP kinase signal transduction pathways, only a low level of MAP kinase signaling is involved in the response of skin to biologically relevant solar radiation.

Transient expression of the cloned mouse c-Ha-ras 5' upstream region in transfected primary SENCAR mouse keratinocytes demonstrates its power as a promoter element.

The mouse Ha-ras oncogene is activated by point mutation and overexpressed in developing papillomas during two-stage skin carcinogenesis in SENCAR mice. One of our research aims is to characterize the factors regulating Ha-ras gene expression at the transcriptional level in SENCAR mouse epidermis. Towards this goal, we sequenced 1400 bp of the 5' upstream region of the mouse Ha-ras gene so as to characterize various cis-regulatory elements present in the gene. We identified seven sites with the proper consensus sequence for binding the SP1 transcription factor and three potential binding sites for the CTF-1 factor. In addition, we located a 13-base sequence with 92% homology to the consensus sequence for an estrogen response element and two hexamers with consensus sequences identical to the core sequence of the glucocorticoid response element. A series of transient gene expression vectors was constructed in which various regions of the mouse Ha-ras 5' upstream region were fused to the chloramphenicol acetyltransferase (CAT) gene. These expression plasmids were transfected into newborn and adult primary SENCAR epidermal cells, the epidermal cell population that presumably contains the stem cells involved in two-stage skin tumorigenesis. Transient gene expression assays carried out after 48-72 h indicated that a 2.3-kb Ha-ras 5' fragment produced CAT activity comparable to that produced by pSV2CAT and pdolCMVCAT, both of which are plasmids with strong viral promoters and enhancers driving CAT gene expression. Maintenance of transfected keratinocytes under both nondifferentiating (0.05 mM calcium) and differentiating (1.2 mM calcium) culture conditions demonstrated that the mouse Ha-ras upstream region was relatively unresponsive to changes in calcium concentration in transient expression assays carried out in either newborn or adult keratinocytes. Our results demonstrated the power of the cloned mouse Ha-ras promoter and upstream region in driving transient gene expression after transfection into primary keratinocytes.

Binding of the 97 kDa glucocorticoid receptor to the 5' upstream flanking region of the mouse c-Ha-ras oncogene.

Glucocorticoids regulate transcription of specific genes through the interaction of glucocorticoid hormone receptor complexes with DNA binding sites called glucocorticoid response elements (GREs). The GRE consensus sequence has been defined to be the imperfect palindromic sequence 5'-GGTACANNNTGTTCT-3', the most highly conserved portion being the 5'-TGTTCT-3' hexamer. We have identified 5 potential GREs in the 5' upstream noncoding region of the mouse c-Ha-ras oncogene, two with the same hexanucleotide sequence and three with a similar sequence. When subcloned fragments of the mouse c-Ha-ras 5' upstream region (containing the 2 hexamer GREs of exact homology) were fused to the chloramphenicol acetyltransferase (CAT) reporter gene and transfected into HeLa cells, CAT expression driven from the ras promoter was induced up to 3-fold in the presence of dexamethasone. To determine whether the 5' upstream region of the mouse Ha-ras gene was capable of specifically interacting with the glucocorticoid receptor complex, we performed Southwestern blot analysis showing that cloned DNA fragments from the 5' upstream region of the mouse c-Ha-ras gene were able to bind a 97 kDa protein in whole cell extracts from both primary SENCAR mouse epidermal cells and HeLa G cells. Immunodepletion of the epidermal cell extract with a monoclonal antibody to the glucocorticoid receptor verified that the 97 kDa protein bound by the Ha-ras 5' region was indeed the glucocorticoid receptor protein. Our results demonstrate that the upstream noncoding region of the mouse c-Ha-ras gene binds the glucocorticoid receptor. Furthermore, the presence of glucocorticoids enhances the transcription of the mouse Ha-ras promoter region in a transient gene expression assay.

Epidermal papillomas and carcinomas induced in uninitiated mouse skin by tumor promoters alone contain a point mutation in the 61st codon of the Ha-ras oncogene.

Previous results in a number of laboratories have demonstrated that epidermal papillomas and carcinomas induced by the two-stage protocol of initiation and promotion contain a point mutation in the 61st codon of the c-Ha-ras oncogene when the initiating agent used is 7,12-dimethylbenz[a]anthracene (DMBA). In the present report, we have analyzed DNA purified from 'spontaneously initiated' papillomas and carcinomas induced in SENCAR mouse epidermis after repetitive treatments with a tumor-promoting agent. Southern blot hybridization studies of tumor DNA digested with diagnostic restriction endonucleases demonstrated that seven of nine papillomas and carcinomas contained a point mutation in the 61st codon of one allele of the c-Ha-ras oncogene. The implications of our findings with respect to the role which a point-mutated Ha-ras proto-oncogene plays in initiation of skin tumorigenesis are discussed.

Elevated expression and point mutation of the Ha-ras proto-oncogene in mouse skin tumors promoted by benzoyl peroxide and other promoting agents.

The two-stage skin carcinogenesis model of initiation and promotion in SENCAR mice has been used to examine the effects of various tumor-promoting agents on the expression of the Ha-ras oncogene in early stages of tumorigenesis in vivo. Papillomas were induced in 7,12-dimethylbenz[a]anthracene (DMBA)-initiated SENCAR mouse epidermis by (i) complete promotion with benzoyl peroxide; (ii) complete promotion with 12-O-tetradecanoyl phorbol-13-acetate (TPA); and (iii) two-stage promotion with TPA for 2 weeks followed by mezerein for 9 weeks. Results of Northern blot hybridization analyses show that early papillomas contain significantly elevated levels of Ha-ras polyadenylated [poly(A)+] RNA, irrespective of the type of tumor promotion regimen used. This pattern holds for promoters of the phorbol ester class as well as for the free radical generating agent benzoyl peroxide. Furthermore, digestion of tumor DNA with diagnostic restriction endonucleases demonstrated that 9-week-old papillomas induced by DMBA contained a point mutation in the 61st codon of one allele of the Ha-ras gene. The results represent the earliest stage in the development of a papilloma at which a Ha-ras point mutation has been reported.