Metabolic disposition and development of new chemopreventive retinoids

Retinoids in the management of head and neck cancer. An update

Despite smoking cessation programs and recent advances in cancer treatment, it is anticipated that head and neck cancer along with lung and esophageal cancers will still be a serious health hazard worldwide. Cancer chemoprevention represents a promising new area of intensive clinical research. Chemoprevention agents which have been considered in clinical trials with the intention of reversing the carcinogenic process include natural agents as well as synthetic retinoids. At the molecular level these agents seem to modify gene expression through the mediation of intracellular-binding proteins and nuclear receptors. In vitro, retinoids induce tumor cell differentiation, inhibit proliferation and also affect other important cell functions, such as adhesion and invasion. Clinical studies conducted in individuals with premalignant lesions, such as oral leukoplakia or laryngeal papillomatosis, have shown that retinoids have the capability to reverse the malignant process in the majority of cases. Furthermore, in patients with head and neck tumors, who became disease-free, after primary treatment, 13-cis retinoic acid (isotretinoin) significantly reduced the incidence of second primaries as compared to placebo. These exciting results urged investigators in Europe and the USA to organize confirmatory trials as well as other chemoprevention trials in order to evaluate the role of new agents. It is hoped that these trials will clarify several important aspects of chemoprevention, such as the safety and activity of new compounds, the significance of integration of retinoids with chemotherapy and/or biotherapy and the establishment of biological markers as intermediate endpoints of carcinogenesis.

Interactions between 4-HPR and diet in NMU-induced mammary tumorigenesis

The present study was designed to determine whether the chemopreventive effect of the synthetic retinoid N(4-hydroxyphenyl)retinamide (4-HPR) on mammary tumorigenesis was influenced by diet. Three diets were used: the closed-formula grain-based Wayne Lab Blox, the open-formula grain-based NIH-07, and the casein-based semipurified AIN-76A. Groups of 25 virgin female F-344 rats were fed the experimental diets beginning one week before a single injection of N-methyl-N-nitrosourea (NMU, 45 mg/kg body wt i.v.) at 50 days of age. The experimental design was as follows: Group 1, unsupplemented AIN-76A; Group 2, AIN-76A supplemented with 4-HPR starting seven days before NMU until termination (-7); Group 3, AIN-76A supplemented with 4-HPR seven days after NMU until termination (+7); Group 4, Wayne (no 4-HPR); Group 5, Wayne (4-HPR, -7); Group 6, Wayne (4-HPR, +7); Group 7, NIH-07; Group 8, NIH-07 (4-HPR, -7). 4-HPR [782 mg/kg diet (2 mM)] was given to all supplemented groups. Termination was 25 weeks post-NMU. Analysis of tumor incidence, multiplicity, and latency indicated that 1) control rats fed the AIN-76A diet exhibited significantly higher mammary tumor yields than rats fed unsupplemented natural-ingredient diets (Wayne and NIH-07) and 2) 4-HPR inhibited mammary tumor development in the two grain-based diets but enhanced tumor development in the AIN-76A diet. Animals fed the AIN-76A diet gained weight to a greater extent than those fed the Wayne or NIH-07 diets and exhibited lower levels of circulating 4-HPR.

Induction of tissue transglutaminase and apoptosis by retinoic acid in the limb bud

Mesenchymal cells in the limb buds of midgestation mouse embryos suffer prominent cell death upon exposure to retinoic acid (RA), an event likely associated with the micromelic and phocomelic anomalies of the resultant fetuses. It has been suggested, but not yet shown, that cells die by an active process termed apoptosis rather than by necrotic cytolysis. In certain cell types, investigators have previously observed a specific and early effect of RA on transcriptional activation of the gene for tissue transglutaminase (tTG), an enzyme suspected to play a role in apoptosis. We report here a distinct but transient increase in tTG activity which accompanied the initiation of cell death in the mesenchymal cells located in the central core of RA-treated limb buds. We also ascertained microscopically that the cytological appearance of the affected cells was consistent with a characterization of the process of cell death as apoptosis.