Carcinogenesis by ionizing radiation in rat skin is readily inhibited by dietary retinyl acetate, an agent that induces keratinization of skin keratinocytes. The mechanism behind retinoid’s alteration of differentiation is not well-understood, but gene expression changes seem to be important. Electron radiation produces a distinct pattern of gene expression, largely related to oxidative stress and proliferative responses. The purpose of the present work was to survey gene expression patterns in skin exposed to a beam of heavy charged ions with or without dietary retinyl acetate as a basis for a better understanding of how the retinoid may be inhibiting carcinogenesis. Rats were exposed to high energy 56Fe ions (a surrogate for space radiation) generated by the alternating gradient synchrotron at Brookhaven Laboratory. At either 1 or 7 days after irradiation about 1.0 cm2 of skin was obtained. After extraction and purification RNA was reverse transcribed to cDNA by using a dT24 primer adjacent to the T7 RNA polymerase promoter region and the SuperScript™ Choice System kit (Gibco BRL, NY). The cDNAs were then transcribed to mRNA the were differentially displayed by means of Affymetrix’s (Santa Clara, CA) rat genome microarray (RAT U34-A). A total of 34 microarrays were processed by BRB ArrayTools software. Interestingly the retinyl acetate was relatively inactive; only 10 genes exhibited a parametric p-value of 0.003 or less, including a metalloproteinase and 2 transferases. For 56Fe ions and 56Fe ions + retinyl acetate, 90 genes and 56 genes with parametric p-values of 0.001 or less were altered by a fold of 3.5x or more respectively. Of the 90 56Fe genes, 41 were unique, while 22 of the 56 retinyl acetate + 56Fe genes were unique. The effect of the retinyl acetate was to reduce the number of genes responding significantly to the 56Fe ions. Some notable genes eliminated by the retinoid included interleukin 1 beta, cathepsin S, plus several proteases and phosphatases suggesting the retinoid was acting to suppress radiation-induced stress responses. The latter effect coupled with pro-keratinization activity may explain, in large part, the retinoids capacity to prevent heavy ion carcinogenesis. Work supported by NASA, NCI and NIEHS.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]