The purpose of the research is to study the cellular and molecular mechanisms of adaptation to stress with emphasis on the regulation of the various components of the hypothalamic pituitary adrenal (HPA) axis. This includes the expression of hypothalamic corticotropin releasing hormone (CRH) and vasopressin (VP), pituitary CRH and V1b VP receptors, and adrenal steroidogenesis. At the hypothalamic level, studies during the last year in a rat model of chronic arthritis provided further evidence for differential regulation of CRH and VP in parvocellular neurons of the paraventricular nucleus (PVN). As in other chronic inflammation models, arthritic rats showed blunted CRH expression and HPA axis responses to somatosensory stressors, but when exposed to the novel immune challenge of bacterial polysaccharide injection, these rats showed hyperresponsiveness of the HPA axis and a remarkable potentiation of cytokine responses especially in the brain. Studies on the transcriptional regulation of CRH by cAMP signaling showed that minimal elevations in intracellular cAMP are sufficient to fully activate CRH transcription in a hypothalamic neuronal cell line. In vivo and in vitro studies showed that cAMP not only activates CRH transcription but it can also cause transcriptional inhibition through induction of the CREM isoform, inducible cAMP early repressor (ICER). The inhibitory effect of ICER on cAMP dependent transcription is more marked for CRH compared with VP, a fact that may contribute to the distinct patterns of regulation of the two genes during chronic stress. At the pituitary level, we have shown that the number of CRH and VP receptors depends on transcriptional, translational and posttranslational mechanisms. Studies on the transcriptional regulation of the VP V1b receptor identified a region of CT repeats (inverted GAGA box) about 100 bp upstream of the proximal transcription start point. The GAGA box is required for basal promoter activity and mediates positive regulation of transcription through binding of a 130 kDa nuclear protein complex. GAGA binding activity of nuclear extracts increases following stress paradigms known to upregulate pituitary V1b receptors, and co-transfection of Drosophila GAGA binding protein increases endogenous V1b receptor in hypothalamic cells, suggesting that this element plays an important role controlling V1b receptor transcription. At the translational level, the studies showed that the 5'untranslated region (5'UTR) of the V1b receptor mRNA contains elements, which can inhibit or stimulate translation. While the presence of upstream open reading frames in the 5'UTR play a role maintaining low translational activity in basal conditions, an internal ribosome entry site (IRES) can initiate translation independently of cap. Stress conditions which upregulate V1b receptors stimulate IRES activity through protein kinase C and PI3 kinase dependent pathways, providing a mechanism for rapid stimulation of V1b receptor translation to meet physiological requirements.
Showing the most recent 10 out of 44 publications
