Activation of ras oncogenes in chemically transformed BALB/MK-2 mouse keratinocytes

Diminished Expression of C/EBPα in Skin Carcinomas Is Linked to Oncogenic Ras and Reexpression of C/EBPα in Carcinoma Cells Inhibits Proliferation

The basic leucine zipper transcription factor, CCAAT/enhancer binding protein α (C/EBPα) is involved in mitotic growth arrest and has been implicated as a human tumor suppressor in acute myeloid leukemia. We have previously shown that C/EBPα is abundantly expressed in mouse epidermal keratinocytes. In the current study, the expression of C/EBPα was evaluated in seven mouse skin squamous cell carcinoma (SCC) cell lines that contain oncogenic Ha-Ras. C/EBPα mRNA and protein levels were greatly diminished in all seven SCC cell lines compared with normal primary keratinocytes, whereas C/EBPβ levels were not dramatically changed. Reexpression of C/EBPα in these SCC cell lines resulted in the inhibition in SCC cell proliferation. To determine whether the decrease in C/EBPα expression observed in the SCC cell lines also occurred in the carcinoma itself, immunohistochemical staining for C/EBPα in mouse skin SCCs was conducted. All 14 SCCs evaluated displayed negligible C/EBPα protein expression and normal C/EBPβ levels compared with the epidermis and all 14 carcinomas contained mutant Ras. To determine whether oncogenic Ras is involved in the down-regulation of C/EBPα, BALB/MK2 keratinocytes were infected with a retrovirus containing Ras12V, and C/EBPα protein, mRNA and DNA binding levels were determined. Keratinocytes infected with the retrovirus containing oncogenic Ras12V displayed greatly diminished C/EBPα protein, mRNA and DNA binding levels. In addition, BALB/MK2 cells containing endogenous mutant Ras displayed diminished C/EBPα expression and the ectopic expression of a dominant-negative RasN17 partially restored C/EBPα levels in these cells. These results indicate that oncogenic Ras negatively regulates C/EBPα expression and the loss of C/EBPα expression may contribute to the development of skin SCCs.

The importance of being K-Ras

The ras genes give rise to a family of related proteins that have strong transforming potential. Typical in vitro studies fail to discriminate between the transforming activity of the Ras proteins. Although activating mutations in ras genes are commonly found in human disease, they are not evenly distributed between the different ras members. Instead, they are concentrated in k-ras. With the absence of evidence to suggest that k-ras DNA is more prone to mutation than h-ras DNA, this imbalance in mutational frequency suggests a special biological role for the K-Ras protein in vivo.

Rapid phosphorylation of 28-kDa heat-shock protein by treatment with okadaic acid and phorbol ester of BALB/MK-2 mouse keratinocytes

Protein phosphorylation by okadaic acid and 12-O-tetradecanoylphorbol-13-acetate (TPA) was examined using quiescent cultures of BALB/MK-2, a cell line derived from mouse epidermal keratinocytes. Treatment with okadaic acid caused rapid phosphorylation of five proteins with molecular masses of 65, 55, 50, 28 and 15 kDa (p65, p55, p50, p28, p15, respectively) while TPA caused rapid phosphorylation of five proteins with molecular masses of 80, 70, 40, 34 and 28 kDa (p80, p70, p40, p34, p28, respectively). In the present study, we examined p28, a common target protein of okadaic acid and TPA. The phosphorylation of p28 increased depending on time of exposure and doses of okadaic acid and TPA. Combined treatment with okadaic acid and TPA resulted in an additive effect. Its position on two-dimensional gel electrophoresis suggested that p28 is the 28-kDa heat-shock protein (HSP28). This possibility was confirmed by migration of p28 with HSP28 and comparative peptide mapping of the two proteins. The phosphoamino-acid residue of phosphorylated HSP28 was serine. In two-dimensional tryptic peptide maps, the same peptides were phosphorylated after treatment with both okadaic acid and TPA.

Transgenic overexpression of transforming growth factor alpha bypasses the need for c-Ha-ras mutations in mouse skin tumorigenesis

The induction of skin papillomas in mice can be divided into two different stages. Chemical initiation frequently elicits mutations in the Ha-ras gene, leading to the constitutive activation of ras. The second step, promotion, involves repetitive topical application of phorbol esters or wounding, leading to epidermal hyperproliferation and papilloma formation. We have found that overexpression of transforming growth factor alpha (TGF-alpha) in the basal epidermal layer of transgenic mice yielded papillomas directly upon wounding or 12-O-tetradecanoylphorbol-13-acetate treatment without the need for an initiator. Moreover, papillomas from TGF-alpha mice did not exhibit mutations in the Ha-ras gene. Interestingly, TGF-alpha acted synergistically with 12-O-tetradecanoylphorbol-13-acetate to enhance epidermal hyperproliferation. Our results demonstrate a central role for TGF-alpha overexpression in tumorigenesis and provide an important animal model for the study of skin tumorigenesis.

Transgenic overexpression of transforming growth factor alpha bypasses the need for c-Ha-ras mutations in mouse skin tumorigenesis

The induction of skin papillomas in mice can be divided into two different stages. Chemical initiation frequently elicits mutations in the Ha-ras gene, leading to the constitutive activation of ras. The second step, promotion, involves repetitive topical application of phorbol esters or wounding, leading to epidermal hyperproliferation and papilloma formation. We have found that overexpression of transforming growth factor alpha (TGF-alpha) in the basal epidermal layer of transgenic mice yielded papillomas directly upon wounding or 12-O-tetradecanoylphorbol-13-acetate treatment without the need for an initiator. Moreover, papillomas from TGF-alpha mice did not exhibit mutations in the Ha-ras gene. Interestingly, TGF-alpha acted synergistically with 12-O-tetradecanoylphorbol-13-acetate to enhance epidermal hyperproliferation. Our results demonstrate a central role for TGF-alpha overexpression in tumorigenesis and provide an important animal model for the study of skin tumorigenesis.

Sequence specificity in the reaction of benzopyrene diol epoxide with DNA

Benzopyrene diol epoxide (BPDE; (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene), the ultimate carcinogen derived from the polycyclic hydrocarbon benzo[a]pyrene, reacts principally with the guanine bases in DNA. Nineteen double stranded, self-complementary oligonucleotides, containing deoxyguanosine in various sequence contexts, were each treated with tritium labelled BPDE. The extent of reaction was determined by releasing the BPDE-guanine adduct with acid, isolating it by chromatography on a reverse-phase column, and estimating it by its radioactivity. Oligonucleotides containing an isolated guanine, such as AAGTACTT, were little affected by BPDE. Reactivity was increased where the guanine was flanked by another guanine on the same strand (e.g. TACCTAGGTA) or on the complementary strand (e.g. TATTCGAATA), and was highest in mixed G-C sequences such as ATCCGGAT. The results should help predict major sites of attack of BPDE on cellular proto-oncogenes.