Coexpression of the mRNAs encoding retinol dehydrogenase isozymes and cellular retinol-binding protein.

We used in situ hybridization of adult rat tissue to show that mRNAs encoding cellular retinol-binding protein (CRBP) and retinol dehydrogenase (RoDH) isozymes I/III and II were expressed in hepatocytes uniformly throughout the liver lobule, but were absent from Kupffer cells and endothelial cells of blood vessels and bile ducts. In kidney, CRBP, RoDH(I), and RoDH(II) were found in the proximal tubules of the cortex. Distal tubules, Henle's loops, collecting ducts, and glomeruli showed little, if any, expression. In testis, CRBP, RoDH(I), and RoDH(II) were found in Sertoli cells. Expression, albeit weaker, also occurred in spermatogonia and primary spermatocytes. Peritubular cells and other germ cells had even weaker expression. Only CRBP and RoDH(II) mRNA were detected in interstitial cells. In lung CRBP, RoDH(I) and RoDH(II) were expressed most intensely in the epithelium of the bronchi and bronchioli, but also occurred in the simple columnar epithelial cells of the alveolar duct and in alveolar type II cells. These data are consistent with the hypothesis that holo-CRBP serves as substrate for retinoic acid biosynthesis because they show that the substrate and the enzyme occur in the same cellular loci in vivo. These data also indicate that multiple cellular sites of retinoic acid biosynthesis occur throughout tissues. Also, the general concordance between mRNA localization and CRBP expression patterns, revealed by previous immunocytochemistry studies, supports and extends the conclusion that CRBP mRNA expression correlates with CRBP expression, based earlier on comparing RNA assays with radioimmunoassays.