ABSTRACT
Ornithine decarboxylase (ODC), an important enzyme in the polyamine biosynthetic pathway, is aberrantly regulated in many epithelial tumors of rodents and humans. In murine skin, it has been shown that ODC overexpression provides a sufficient condition for tumor promotion. Therefore, we hypothesized that K6/ODC transgenic mice in which ODC overexpression was targeted to hair follicle keratinocytes might provide a sensitive model for identifying genotoxic carcinogens. Ten known carcinogens or noncarcinogens have been tested in the model so far and results are highly concordant with 2-year rodent bioassays (100% concordant). More importantly, each of two chemicals tested that is recognized as a human carcinogen was identified as a carcinogen in K6/ODC transgenic mice. In addition, 7, 12-dimethylbenz(a)anthracene (DMBA) dose response studies indicated that even at a very low dose, 2 nmol, a high percentage of mice (50%) had already developed tumors 8 weeks after treatment. We conclude that the K6/ODC transgenic mouse model is very sensitive to topical application of genotoxic carcinogens and could therefore be a useful mouse model for carcinogen identification and chemical risk assessment.
Subject(s)
Carcinogenicity Tests , Ornithine Decarboxylase/physiology , 9,10-Dimethyl-1,2-benzanthracene , Animals , Dose-Response Relationship, Drug , Humans , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , Ornithine Decarboxylase/genetics , Skin Neoplasms/chemically inducedABSTRACT
In multistage tumorigenesis models, ornithine decarboxylase (ODC) is usually dysregulated at some point during tumor promotion, an early stage of carcinogenesis. To address the question whether constitutive overexpression of ODC would be a sufficient condition for tumor promotion, mice with high levels of ODC expression targeted to epidermal keratinocytes were used in skin tumorigenesis experiments. Transgenic mice with ODC targeted to hair follicle keratinocytes were much more sensitive than littermate controls to initiation with a single low dose of carcinogen; in fact, such mice no longer required treatment with tumor promoters for tumors to develop. Targeting ODC overexpression to both interfollicular and follicular keratinocytes did not further enhance tumor yield. Our results suggest that most, if not all, target cells for chemical carcinogens in the skin reside in hair follicles, and ODC overexpression is sufficient to activate such cells to expand clonally to form epidermal tumors.
Subject(s)
Ornithine Decarboxylase/metabolism , Skin Neoplasms/enzymology , 9,10-Dimethyl-1,2-benzanthracene , Age Factors , Animals , Animals, Newborn , Benzo(a)pyrene , Hair Follicle/metabolism , Hair Follicle/pathology , Immunohistochemistry , Methylnitronitrosoguanidine , Mice , Mice, Inbred C57BL , Mice, Transgenic , Putrescine/metabolism , Skin/metabolism , Skin/pathology , Skin Neoplasms/pathology , Spermidine/metabolism , Tetradecanoylphorbol Acetate , Time FactorsABSTRACT
Mice that overexpress a mutated ornithine decarboxylase (ODC) transgene in outer root sheath keratinocytes of the hair follicle were used to study the role of this enzyme in regulating hair follicle structure and function. These transgenic mice have a normal first hair cycle, but lose their hair completely beginning 2-3 wk after birth. Transgene overexpression in follicular keratinocytes is first detected at day 12 after birth, coincident with the development of follicular cysts in the upper portion of the dermis. The onset of keratin 6 expression also begins around day 12; because the promoter/regulatory region of the bovine keratin 6 gene was used to target ODC transgene expression of hair follicle keratinocytes, these data demonstrate the faithful temporal and cell type-specific expression of the K6 -driven transgene. The ODC inhibitor 2-difluoromethylornithine could prevent hair loss and partially normalize skin histology if administered before the onset of ODC overexpression. 2-Difluoromethylornithine could also reactivate hair growth in animals with complete hair loss. Our results suggest that ODC is an important regulatory gene for the mouse hair follicle.
Subject(s)
Hair Follicle/physiology , Ornithine Decarboxylase/physiology , Animals , Cattle , Eflornithine/pharmacology , Hair Follicle/drug effects , Male , Mice , Mice, Transgenic , Ornithine Decarboxylase/geneticsABSTRACT
Epidermal papillomas were induced in CD-1 mice by a single topical application of 7,12-dimethylbenzanthracene (DMBA) followed by twice weekly applications of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in acetone. Control groups consisted of mice treated singly or chronically with acetone or TPA. TPA induced a rapid, yet transient 500- to 1000-fold increase in ornithine decarboxylase (ODC) activity which resulted in a 2- to 8.4-fold elevation of putrescine in both singly or chronically TPA-treated mouse epidermis 4-6 h after its application. After 24 h, levels of spermidine, but not spermine, were also elevated. The ODC and arginase activities in the 11 individual papillomas studied averaged 400- and 26-fold higher respectively than basal levels in epidermis. The activity of ODC in most papillomas, unlike ODC in epidermis, could be stimulated by guanosine 5'-triphosphate (GTP). Putrescine and spermidine levels in papillomas, especially those exhibiting highly GTP-stimulated ODC, were substantially higher compared to either normal or TPA-treated epidermis. Although epidermis contains a relatively high ornithine content, its level is even further elevated in papillomas, in some cases as much as 70-fold. The consequences of the constitutively elevated polyamine levels in papillomas caused by the loss of control over the normally tightly regulated polyamine biosynthetic pathway are not known, but could be important in regulating the balance between proliferation and differentiation in this self-renewing epithelial tissue.
