The sequence of the 5'-end of the rat CYP17 gene, the transcription initiation site and a comparison with the homologous genes of other species

Rat cytochrome P450c17 gene transcription is initiated at different start sites in extraglandular and glandular tissues

In the male rat, the mRNA of the steroidogenic cytochrome P450c17 is expressed extraglandularly in the stomach, duodenum, kidney and liver, throughout the animal's lifespan, as demonstrated by reverse transcriptase and polymerase chain reaction (RT-PCR) analysis with specific primers. Northern analysis indicated that all tissues, except the kidney, contain high levels of such mRNA, but the relative mobility of liver mRNA is slightly less than that of the testis and other tissues. Thus, we analysed their 5'- and 3'-untranslated terminal regions (UTRs) by means of 5'- and 3'-rapid amplification of cDNA ends (RACE) techniques. All tissues utilised the same polyadenylation site as the testis. In the 5'-UTR of liver mRNA, however, we found a distal transcription start site (TSS) located 252b upstream of that used in testicular P450c17 mRNA, which is placed 41b upstream of the first ATG. The 5'-UTR sequence of liver P450c17 cDNA exactly matched the contiguous upstream untranslated region of the gene, suggesting that alternative splicing was not involved in the synthesis of liver P450c17 transcript. The other tissues used the same TSS present in the testis. Nevertheless, a second TSS located 125b upstream of the first ATG was found in the stomach and duodenum. These results show that the transcriptional regulation of the CYP17 gene in the rat is complex and differs between tissues in the use of TSSs.

Comparison of the hamster and human adrenal P450c17 (17 alpha-hydroxylase/17,20-lyase) using site-directed mutagenesis and molecular modeling

In order to understand the activity specificity of the hamster cytochrome P450 17 alpha-hydroxylase/17,20-lyase (P450c17), we have studied its structure/activity using three hamster P450c17 recombinant mutants (T202N/D240N/D407H). In transiently transfected COS-1 cells, the mutation T202N reduced 17 alpha-hydroxylation of pregnenolone and progesterone to 24 and 44% of wild type (WT), respectively, followed by reduced 17,20-cleavage to 71 and 67%, respectively. On the other hand, the mutation D240N decreased specifically 17,20-lyase activity to 61% of WT when incubated with pregnenolone while the mutation D407H only decreased 17 alpha-hydroxylation to 46% when incubated with progesterone.To comprehend the altered activity profiles of these hamster P450c17 mutants, we have elaborated a 3D model of the hamster P450c17 and compared it to our preceding model of the human P450c17. Analysis of the mutants with this model showed that, without direct contact to the substrates, these mutations transmit structural changes to the active site. By analogy, these results support the concept that any cellular changes modifying the external structure of P450c17, such as phosphorylation, could have influence on its active site and enzymatic activities.

Molecular modeling of the hamster adrenal P450C17

The cytochrome P450C17 (C17) is the steroidogenic enzyme responsible for the conversion of pregnenolone and progesterone to dehydroepiandrosterone (DHEA) and delta4-androstenedione (AD) respectively. This conversion is achieved by two enzymatic activities, 17alpha-hydroxylase and 17,20-lyase, located at the same active site. In man, the adrenal C17 basically only produces DHEA. We have shown that the hamster adrenal C17 produces DHEA as well as AD. Moreover, the hamster like man produces cortisol as its major glucocorticoid. We can thus compare the hamster and human adrenal C17, and use their differences in order to elaborate a strategy for structure-function studies. We have thus engineered hamster adrenal C17 mutants which possess modified enzymatic activities. We also proceeded to elaborate a three-dimensional model of the hamster C17 to visualise the structural impact of these mutations. This model demonstrates that the mutations created are not localised at the active site, but rather in surrounding regions. These could affect the conformation of the active site, in turn, modulating the 17alpha-hydroxylase and 17,20-lyase activities. For example, the mutation T202N is located next to Val 482 and Val 483 which compose the roof of the active site. This mutation decreased both 17alpha-hydroxylase and 17,20-lyase activities, indicating the importance of the roof of the active site for general functionality of the C17.

Regulation of primary response and specific genes in adrenal cells by peptide hormones and growth factors

Using cultured bovine adrenal fasciculata cells (BAC), we investigated the effects of two hormones, corticotropin (ACTH) and angiotensin II (Ang-II) and two growth factors, insulin-like growth factors I (IGF-I) and transforming growth factor beta 1 (TGF beta 1), on the mRNA levels of nuclear proto-oncogenes of the Fos and Jun families and on the mRNA levels of genes expressed in BAC coding for ACTH and AT1 receptors, cytochrome P450scc and P450 17 alpha and 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD). ACTH and IGF-1 increased c-fos and jun-B mRNA levels early with later increases in the levels of mRNA for the ACTH receptor and the three steroidogenic enzymes, and enhanced steroidogenic responses to both ACTH and Ang-II. In contrast, Ang-II increased mRNA coding for the three proto-oncogenes (cfos, c-jun, and jun-B), decreased those for P450 17 alpha and 3 beta-HSD, and caused marked homologous and heterologous steroidogenic desensitization. TGF beta 1 increased only jun-B mRNA and markedly reduced BAC-differentiated functions and steroidogenic responsiveness to both ACTH and Ang-II. The long-term effects of ACTH on human adrenal fasciculata cells were comparable with those observed in BAC, whereas the long term effects of Ang-II and TGF beta 1 were different from those observed in BAC. Whether these species-specific differences are related to a different effect of these factors on proto-oncogene expression is not yet known.

Occurrence of cytochrome P450c17 mRNA and dehydroepiandrosterone biosynthesis in the rat gastrointestinal tract

We have investigated 17 alpha-hydroxylase and C17,20-lyase activities and the presence of cytochrome P450c17 mRNA in the esophagus, stomach, duodenum, and colon of adult rats of both sexes. All tissues converted [4-14C]pregnenolone mainly to dehydroepiandrosterone (DHEA) through the 5-ene-3 beta-hydroxysteroid route as opposed to the 4-ene-3-ketosteroid pathway in a control testicular incubate. Synthesis of dehydroepiandrosterone 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 primarily by the esophagus and colon, respectively. P450c17 mRNA was demonstrated by ribonuclease protection assay in the stomach and duodenum, but not in esophagus and colon. However, a 335 bp-long cDNA fragment, whose sequence corresponded to that of rat P450c17 cDNA, was amplified by reverse transcription (RT) and polymerase chain reaction (PCR) from the poly(A)+ RNAs of all four tissues. This result was further confirmed 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 positive signal in Southern blotting were observed with the total RNAs from adult male adrenal and spleen, which were taken as negative controls 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 protection assay and semiquantitative RT-PCR assay, P450c17 enzyme activity was much higher in the testis, pointing at a possible dissimilarity in the respective rates of mRNA translation. Thus, P450c17 is differentially expressed in the rat gastrointestinal tract, where it leads to the synthesis of the sex steroid precursor DHEA, especially in the duodenum and stomach.