The transcriptional repressor Bcl6 is a male-specific rat liver gene product and one of 24 early GH-response genes encoding DNA-binding proteins. Presently, the sex specificity of Bcl6 was shown to emerge at puberty, when hepatic Bcl6 mRNA was induced in males and repressed in females by the female plasma GH profile. Hepatic Bcl6 mRNA was increased to near-normal male levels in hypophysectomized females and was extinguished in intact males given a continuous GH infusion (female-like GH pattern). Bcl6 was also repressed in adult male somatostatin-deficient mice, where plasma GH profiles are female like. Hepatic Bcl6 RNA was rapidly down-regulated by GH pulse treatment, both in hypophysectomized male rats and in primary rat hepatocytes. Bcl6 was substantially induced in female mice deficient in hepatic signal transducer and activator of transcription (STAT)5a/STAT5b, suggesting that these STAT transcriptional mediators of GH signaling repress Bcl6. Indeed, STAT5 was bound to Bcl6 STAT5-binding region-B, previously associated with Bcl6 repression, in both male and female liver chromatin. STAT5 also bound to Bcl6 region-A in male chromatin but only during a plasma GH pulse. Analysis of primary transcripts (heterogeneous nuclear RNA) across the Bcl6 gene revealed a novel mechanism of GH-dependent sex specificity, with two apparent blocks in Bcl6 transcription elongation seen in female liver and in continuous GH-treated male liver, one early in intron 4 and one in exon 5, which together reduced transcription beyond exon 5 more than 300-fold. Finally, Bcl6 was bound to a subset of STAT5-binding sites in male liver chromatin, including a Socs2 STAT5-binding site where Bcl6 binding increased substantially between plasma GH pulses, i.e. when STAT5 binding was low. Bcl6 and STAT5 binding are thus inversely coordinated by the endogenous pulses of pituitary GH release, suggesting this male-specific transcriptional repressor modulates hepatic GH signaling to select STAT5 target genes.

No abstract

Human cytochrome P450 2A13 (CYP2A13), which is highly efficient in the metabolic activation of a major tobacco-specific carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), may play important roles in xenobiotic toxicity and tobacco-related tumorigenesis in the respiratory tract. The aim of this study was to identify any genetic polymorphisms of the CYP2A13 gene, which may alter the metabolic capacities of the enzyme. Polymerase chain reaction (PCR) single-strand conformational polymorphism analysis was used to identify single-nucleotide polymorphisms (SNPs) in all of the exons and at the exon-intron boundaries, and PCR-restriction fragment length polymorphism analysis and DNA sequencing were used to determine the frequencies of the newly identified variant alleles in the four major ethnic groups. Blood spot DNA from more than 100 individuals was used for these analyses. Seven variant alleles were found, but only one SNP was detected in the coding region, in exon 5, leading to an Arg257Cys amino acid change. The frequencies of the Arg257Cys allele in white, black, Hispanic, and Asian individuals are 1.9%, 14.4%, 5.8%, and 7.7%, respectively. Functional analysis of the variant protein was performed following its heterologous expression. The Arg257Cys variant was 37 to 56% less active than the wild-type Arg-257 protein toward all substrates tested. With NNK, Cys-257 had higher K m and lower V max values than did Arg-257, with a Ͼ2-fold decrease in catalytic efficiency. The Arg257Cys mutation could provide some protection against xenobiotic toxicity in the respiratory tract to individuals who are homozygous for the Cys-257 allele.

ABSTRACT:Despite recent progress in the identification and characterization of numerous nasal biotransformation enzymes in laboratory animals, the expression of biotransformation genes in human nasal mucosa remains difficult to study. Given the potential role of nasal biotransformation enzymes in the metabolism of airborne chemicals, including fragrance compounds and therapeutic agents, as well as the potential interspecies differences between laboratory animals and humans, it would be highly desirable to identify those biotransformation genes that are expressed in human nasal mucosa. In this study, a global gene expression analysis was performed to compare biotransformation enzymes expressed in human fetal and adult nasal mucosa to those expressed in liver. The identities of a list of biotransformation genes with apparently nasal mucosa-selective expression were subsequently confirmed by RNA-polymerase chain reaction (PCR) and DNA sequencing of the PCR products. Further quantitative RNA-PCR experiments indicated that, in the fetus, aldehyde dehydrogenase 6 (ALDH6), CYP1B1, CYP2F1, CYP4B1, and UDP glucuronosyltransferase 2A1 are expressed preferentially in the nasal mucosa and that ALDH7, flavin-containing monooxygenase 1, and glutathione S-transferase P1 are at least as abundant in the nasal mucosa as in the liver. The nasal mucosal expression of CYP2E1 was also detected. These findings provide a basis for further explorations of the metabolic capacity of the human nasal mucosa for xenobiotic compounds.

No abstract