The influence of retinoids on polyamine and DNA synthesis in mouse epidermis

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Biology of cutaneous squamous cell carcinoma

Nonmelanoma skin cancer is the leading cause of cancer in the United States. Cutaneous squamous cell carcinoma is second only to basal cell carcinoma in prevalence and its incidence is increasing. The biology of squamous cell carcinoma is reviewed under the broad areas of etiology, immunobiology, biochemistry, metastatic potential, and therapy, with emphasis on prevention, diagnosis, and management.

Topical tretinoin in actinic keratosis and basal cell carcinoma

In several studies between 1962 and 1978, topical tretinoin was proved capable of producing complete regression of actinic keratosis and basal cell carcinoma. But because its efficacy is not comparable to that of other modalities, topical tretinoin is currently used only as an adjunct to topical 5-fluorouracil in the treatment of actinic keratosis. One recent report found topical tretinoin ineffective in the chemoprevention of actinic keratosis. Although the oral synthetic retinoids isotretinoin and etretinate have been used in the prevention and treatment of cutaneous malignancy, the potential exists for chronic toxicity from the prolonged systemic therapy that appears necessary for maintaining the chemopreventive effect. For this reason, it may be appropriate to study further the preventive as well as therapeutic effects of topical tretinoin and other retinoids for actinic keratosis and skin cancer. If they prove safe and effective, the use of topical retinoids in the prevention and treatment of cutaneous tumors may be the most significant clinical application of these drugs.

Some thoughts on the relationship between vitamin A and cancer

Retinoids clearly show both prophylactic and therapeutic effects against many kinds of neoplasm. Vitamin A deficiency and vitamin A excess, which are very different states clinically and metabolically, should be separately considered relative to carcinogenesis. The anticancer effects of retinoids and related compounds may be more closely related to their chemical structure than to their similarity to the structure and function of vitamin A. Retinoids act on cells and tissues in a number of ways. Although many interesting ideas have been proposed to clarify their mode of action, no single hypothesis adequately explains their many molecular interactions and responses. Investigation of early molecular interactions between retinoids and cells and the subsequent metabolism of retinoids in neoplastic and normal cells may aid in the clarification of their action as antineoplastic agents.

Alterations in polyamine metabolism during embryonal carcinoma cell differentiation in vitro

Murine embryonal carcinoma (EC) cells can be stimulated to differentiate by several chemical inducers. Since the response of EC cells to induction is likely to occur shortly after exposure to the inducer, we report here the changes that occur in polyamine levels in a number of EC cell lines shortly after exposure to two chemical stimuli, alpha-difluoromethylornithine (DFMO) and retinoic acid (RA). Our results suggest that polyamine levels are important in determining the state of EC cell differentiation, but that reduction in these levels alone is not sufficient to induce differentiation in all the EC cell lines tested. Also, it is apparent that RA does influence levels of polyamines. However, this influence does not seem to be mediated through direct interaction with ODCase.

Inhibition of ornithine decarboxylase by retinoic acid and difluoromethylornithine in relation to their effects on differentiation and proliferation

Murine embryonal carcinoma F9 cells can be induced to differentiate by 2-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC). The differentiated phenotype is similar to that of retinoic acid (RA)-treated F9 cells. In contrast to F9 cells the differentiated cells secrete plasminogen activator and express keratin intermediate filaments. Both DFMO and RA reduce ornithine decarboxylase activity, polyamine levels and inhibit cell proliferation of F9 cells. These compounds also reduce ODC, polyamine levels and proliferation of mouse BALB/c 3T6 fibroblasts. RA inhibits the induction of ODC by insulin, serum and to a lesser extent that of epidermal growth factor (EGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA). The action of DFMO and RA can be distinguished by their response to putrescine. The induction of differentiation and the inhibition of cell proliferation by DFMO can be totally abolished upon the addition of putrescine, whereas the actions of RA are not affected at all. These results suggest that the inhibition of ODC and reduction of polyamines are not causal in the induction of differentiation and the inhibition of proliferation by RA.

Ornithine decarboxylase may be a multifunctional protein

Ornithine decarboxylase may undergo posttranslational modifications which alter its function. Both transamidation of glutamine residues in the enzyme catalyzed by TGase and phosphorylation of serine and threonine residues catalyzed by a polyamine-stimulated protein kinase have been demonstrated. Data are presented which suggest that these modifications result in translocation of the modified protein to the nucleolus where it regulates the activity of RNA polymerase I to transcribe rDNA, the only active nucleolar genes. Transamidation of specific proteins with primary amines catalyzed by intracellular TGase may be an important posttranslational modification, capable of altering genetic transcription. The rapid half-life of ODC (10-15 min) may be related to rapid posttranslational modification with loss of enzymatic activity rather than to protein degradation.

Retinoic acid promotion of papilloma formation in mouse skin

Retinoic acid is a weak promotor of skin tumorigenesis in Charles River CD-1 mice. Multiple papillomas were seen in 17% of the mice treated 3 times weekly with 5.1 micrograms retinoic acid for 20 weeks after initiation by a single treatment with 50 micrograms 7,12-dimethylbenz[alpha]anthracene (DMBA). These results suggest the necessity of a more thorough evaluation of retinoids as tumor promoters before their serious consideration as anti-cancer agents in man.

Retinoids and inhibition of ornithine decarboxylase activity

The tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) causes a dose-dependent induction (up to 300-fold) of ornithine decarboxylase activity in the epidermis of intact mice within 5 hours after the application to the skin of from 1 to 10 nmoles of the ester in 0.2 ml of acetone. Enzyme activity regresses with a half-life of 17 minutes, the shortest known for any enzyme. The induction is inhibited in a dose-dependent manner by a single application of from 0.034 to 3.4 nmoles of retinoic acid within an hour before to an hour after phorbol ester treatment. Of over thirty natural and synthetic retinoids that were tested for this property, all-trans-retinoic acid was most effective. The ability of retinoic acid and its congeners to inhibit tumor promotion by the phorbol ester correlated with the effect of dose, structure, and time of treatment on the induction of ornithine decarboxylase. It appears that a permanently increased level of the enzyme and its product, putrescine, may be one of the essential components of the mechanism of tumor promotion.

Chemoprevention of basal cell carcinoma with isotretinoin

Three patients with multiple basal cell carcinomas, due either to excessive sunlight exposure, the nevoid basal cell carcinoma syndrome, or arsenical insecticide exposure, were treated with oral isotretinoin for 2 1/2 to 4 years. Although higher doses were used initially, approximately 1.5 mg/kg/day was used for long-term therapy in all three patients. Therapeutic effects on existing tumors varied between each patient, and only nine of sixty-five lesions underwent complete clinical regression. No tumors enlarged in two patients; a few tumors enlarged slightly in the third patient, particularly during the later courses of therapy when isotretinoin was given at lower dosage. No new lesions have been observed in any of these three patients. With these encouraging preliminary data, it now may be appropriate to perform larger trials for longer periods of time to determine the usefulness of isotretinoin in the chemoprevention of basal cell carcinoma in patients with multiple tumors.