No abstract

The nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥) is involved in glucose homeostasis and synthetic PPAR␥ ligands, the thiazolidinediones, a new class of antidiabetic agents that reduce insulin resistance and, as a secondary effect, reduce hepatic glucose output. PPAR␥ is highly expressed in normal human pancreatic islet ␣-cells that produce glucagon. This peptide hormone is a functional antagonist of insulin stimulating hepatic glucose output. Therefore, the effect of PPAR␥ and thiazolidinediones on glucagon gene transcription was investigated. After transient transfection of a glucagon-reporter fusion gene into a glucagon-producing pancreatic islet cell line, thiazolidinediones inhibited glucagon gene transcription when PPAR␥ was coexpressed. They also reduced glucagon secretion and glucagon tissue levels in primary pancreatic islets. A 5/3-deletion and internal mutation analysis indicated that a pancreatic islet cellspecific enhancer sequence (PISCES) motif within the proximal glucagon promoter element G1 was required for PPAR␥ responsiveness. This sequence motif binds the paired domain transcription factor Pax6. When the PISCES motif within G1 was mutated into a GAL4 binding site, the expression of GAL4-Pax6 restored glucagon promoter activity and PPAR␥ responsiveness. GAL4-Pax6 transcriptional activity was inhibited by PPAR␥ in response to thiazolidinedione treatment also at a minimal viral promoter. These results suggest that PPAR␥ in a ligand-dependent but DNA binding-independent manner inhibits Pax6 transcriptional activity, resulting in inhibition of glucagon gene transcription. These data thereby define Pax6 as a novel functional target of PPAR␥ and suggest that inhibition of glucagon gene expression may be among the multiple mechanisms through which thiazolidinediones improve glycemic control in diabetic subjects. Peroxisome proliferator-activated receptor ␥ (PPAR␥)1 is a member of the ligand-regulated nuclear hormone receptor superfamily (1). Like other nuclear receptors, PPAR␥ comprises an amino-terminal ligand-independent transactivation domain (AF-1), a central DNA-binding domain, and a carboxyl-terminal ligand-binding domain that contains a second, ligand-dependent transactivation surface (AF-2) (1). PPAR␥ binds as a heterodimer with the 9-cis-retinoic acid receptor, RXR, to response elements in target genes to activate transcription. A typical PPRE consists of a direct repeat of hexamer half-sites, TGACCT, spaced by one nucleotide (DR-1) (1). PPAR and RXR occupy the 5Ј and 3Ј half-sites, respectively, and thus show a polarity in binding that is the opposite of that observed for other nuclear receptor-RXR heterodimers (1). Like other nuclear receptors, there is evidence that PPAR␥-RXR require the ligand-dependent recruitment of coactivator proteins like SRC-1, GRIP-1, pCIP, CBP, p300, DRIP205, and p120 (2-5) to effectively stimulate gene transcription. This recruitment is dependent on allosteric alterations in the AF-2 helical domain. A "mouse trap" model of receptor activa...

No abstract

No abstract

River biota are affected by global reach-scale pressures, but most approaches for predicting biota of rivers focus on river reach or segment scale processes and habitats. Moreover, these approaches do not consider long-term morphological changes that affect habitat conditions. In this study, a modelling framework was further developed and tested to assess the effect of pressures at different spatial scales on reach-scale habitat conditions and biota. Ecohydrological and 1D hydrodynamic models were used to predict discharge and water quality at the catchment scale and the resulting water level at the downstream end of a study reach. Long-term reach morphology was modelled using empirical regime equations, meander migration and 2D morphodynamic models. The respective flow and substrate conditions in the study reach were predicted using a 2D hydrodynamic model, and the suitability of these habitats was assessed with novel habitat models. In addition, dispersal models for fish and macroinvertebrates were developed to assess the re-colonization potential and to finally compare habitat suitability and the availability / ability of species to colonize these habitats. Applicability was tested and model performance was assessed by comparing observed and predicted conditions in the lowland Treene River in northern Germany. Technically, it was possible to link the different models, but future applications would benefit from the development of open source software for all modelling steps to enable fully automated model runs. Future research needs concern the physical modelling of long-term morphodynamics, feedback of biota (e.g., macrophytes) on abiotic habitat conditions, species interactions, and empirical data on the hydraulic habitat suitability and dispersal abilities of macroinvertebrates. The modelling framework is flexible and allows for including additional models and investigating different research and management questions, e.g., in climate impact research as well as river restoration and management.