OCCURRENCE OF CYTOCHROME P450C17 MESSENGER-RNA AND DEHYDROEPIANDROSTERONE BIOSYNTHESIS IN THE RAT GASTROINTESTINAL-TRACT

We have investigated 17 alpha-hydroxylase and C-17,C-20-lyase activiti es and the presence of cytochrome P450c17 mRNA in the esophagus, stoma ch, duodenum, and colon of adult rats of both sexes. All tissues conve rted [4-C-14]pregnenolone mainly to dehydroepiandrosterone (DHEA) thro ugh the 5-ene-3 beta-hydroxysteroid route as opposed to the 4-ene-3-ke tosteroid pathway in a control testicular incubate. Synthesis of dehyd roepiandrosterone was particularly high in the duodenum and was found to be lower in the stomach, colon and esophagus, in decreasing order. 20 alpha-Hydroxypregnenolone and progesterone were also formed primari ly by the esophagus and colon, respectively. P450c17 mRNA was demonstr ated by ribonuclease protection assay in the stomach and duodenum, but not in esophagus and colon. However, a 335 bp-long cDNA fragment, who se sequence corresponded to that of rat P450c17 cDNA, was amplified by reverse transcription (RT) and polymerase chain reaction (PCR) from t he poly(A)(+) RNAs of all four tissues. This result was further confir med by Southern blotting using a 794-bp testicular probe. The complete sequence of P450c17 cDNA in the stomach and duodenum was identical to that reported for rat testis P450c17 cDNA. No amplification and no po sitive signal in Southern blotting were observed with the total RNAs f rom adult male adrenal and spleen, which were taken as negative contro ls since they had been previously found unable to form androgens from pregnenolone. Although the levels of transcription in gonads, duodenum and stomach were found to be equivalent, as indicated by the RNase pr otection assay and semiquantitative RT-PCR assay, P450c17 enzyme activ ity was much higher in the testis, pointing at a possible dissimilarit y in the respective rates of mRNA translation. Thus, P450c17 is differ entially expressed in the rat gastrointestinal tract, where it leads t o the synthesis of the sex steroid precursor DHEA, especially in the d uodenum and stomach.