RESUMO
Chemopreventive retinoids may be more effective if delivered to the lung epithelium by inhalation. 13-cis-Retinoic acid (13-cis-RA) was comparable to all-trans-retinoic acid (RA) in inducing transglutaminase II (TGase II) in cultured human cells. Inhaled 13-cis-RA had a significant stimulatory activity on TGase II in rat lung (P < 0.001) but not in liver tissue (P < 0.544). Furthermore, inhaled 13-cis-RA at daily deposited doses of 1.9 mg/kg/day up-regulated the expression of lung retinoic acid receptors (RARs) alpha, beta, and gamma at day 1 (RARalpha by 3.4-fold, RARbeta by 7.2-fold, and RARgamma by 9.7-fold) and at day 17 (RARalpha by 4.2-fold, RARbeta by 10.0-fold, and RARgamma by 12.9-fold). At a lower aerosol concentration, daily deposited doses of 0.6 mg/kg/day were also effective at 28 days. Lung RARalpha was induced by 4.7-fold, RARbeta by 8.0-fold, and RARgamma by 8.1-fold. Adjustment of dose by exposure duration was also effective; thus, inhalation of an aerosol concentration of 62.2 microg/liter, for durations from 5 to 240 min daily for 14 days, induced all RARs from 30.6- to 74-fold at the shortest exposure time. None of the animals exposed to 13-cis-RA aerosols showed RAR induction in livers. By contrast, a diet containing pharmacological RA (30 microg/g of diet) failed to induce RARs in SENCAR mouse lung, although it induced liver RARs (RARalpha, 21.8-fold; RARbeta, 13.5-fold; RARgamma, 12.5-fold); it also failed to induce lung TGase II. A striking increase of RARalpha expression was evident in the nuclei of hepatocytes. Pharmacological dietary RA stimulated RARalpha, RARbeta, and RARgamma as early as day 1 by 2-, 4-, and 2.1-fold, respectively, without effect on lung RARs. Therefore, 13-cis-RA delivered to lung tissue of rats is a potent stimulant of lung but not liver RARs. Conversely, dietary RA stimulates liver but not lung RARs. These data support the concept that epithelial delivery of chemopreventive retinoids to lung tissue is a more efficacious way to attain up-regulation of TGase II and the retinoid receptors and possibly to achieve chemoprevention.
