ChREBP和Mlx在调控葡萄糖反应基因表达中的作用

近年的研究结果显示,葡萄糖能在转录水平调控糖酵解和生脂酶基因的表达,对肝糖类和脂类动态平衡起协同调控作用.其重要转录因子是糖反应元件结合蛋白（ChREBP）和Max样蛋白X(Mlx),葡萄糖通过ChREBP.Mlx异二聚体调控葡萄糖反应基因的转录.本文主要综述转录因子ChREBP和Mlx的结构与功能,调控葡萄糖反应基因表达的机制,以及影响转录因子表达的因素.     

Glucose induces FGF21 mRNA expression through ChREBP activation in rat hepatocytes

Fibroblast growth factor 21 (FGF21) has beneficial effects of improving the plasma glucose and lipid profiles in diabetic rodents. Here, we investigated carbohydrate response element binding protein (ChREBP) involvement in the regulation of FGF21 mRNA expression in liver. Glucose stimulation and adenoviral overexpression of dominant active ChREBP increased FGF21 mRNA. Consistently, adenoviral expression of dominant negative Mlx inhibited glucose induction of FGF21 mRNA. Furthermore, deletion studies of mouse FGF21 gene promoter (−2000 to +65 bp) revealed a glucose responsive region between −74 and −52 bp. These findings suggest that FGF21 expression is regulated by ChREBP.     

Single-stranded DNA binding of the cold-shock protein CspB from Bacillus subtilis: NMR mapping and mutational characterization

Cold-shock proteins (CSPs) bind to single-stranded nucleic acids, thereby acting as a "RNA chaperone." To gain deeper insights into the rather unspecific nature of ssDNA/RNA binding, we characterized the binding interface of CspB from Bacillus subtilis to a 25-mer of ssDNA (Y-Box25) using heteronuclear 2D NMR spectroscopy. Seventeen residues, including eight out of nine aromatic amino acids, are directly involved in the Y-Box25 interaction and were identified by extreme line broadening of their cross-peaks. Eight residues belong to the earlier proposed RNP binding motifs. A second set of seven backbone amides becomes evident by major chemical shift perturbations reporting remote conformational rearrangements upon binding. These residues are located in loop beta3-beta4 and loopbeta4-beta5, and include Ile18. The individual contributions of the so-identified residues were examined by fluorescence titration experiments of 15 CspB variants. Phenylalanine substitutions in- and outside the RNP motifs significantly reduce the binding affinity. Unrestricted possible backbone conformations of loop beta3-beta4 also markedly contribute to binding. Stopped-flow fluorescence kinetics revealed that the different binding affinities of CspB variants are determined by the dissociation rate, whereas the association rate remains unchanged. This might be of importance for the "RNA chaperone" activity of CspB.     

Selective formation of ErbB-2/ErbB-3 heterodimers depends on the ErbB-3 affinity of epidermal growth factor-like ligands

EGF-like growth factors activate their ErbB receptors by promoting receptor-mediated homodimerization or, alternatively, by the formation of heterodimers with the orphan ErbB-2 through an as yet unknown mechanism. To investigate the selectivity in dimer formation by ligands, we have applied the phage display approach to obtain ligands with modified C-terminal residues that discriminate between ErbB-2 and ErbB-3 as dimerization partners. We used the epidermal growth factor/transforming growth factor alpha chimera T1E as the template molecule because it binds to ErbB-3 homodimers with low affinity and to ErbB-2/ErbB-3 heterodimers with high affinity. Many phage variants were selected with enhanced binding affinity for ErbB-3 homodimers, indicating that C-terminal residues contribute to the interaction with ErbB-3. These variants were also potent ligands for ErbB-2/ErbB-3 heterodimers despite negative selection for such heterodimers. In contrast, phage variants positively selected for binding to ErbB-2/ErbB-3 heterodimers but negatively selected for binding to ErbB-3 homodimers can be considered as "second best" ErbB-3 binders, which require ErbB-2 heterodimerization for stable complex formation. Our findings imply that epidermal growth factor-like ligands bind ErbB-3 through a multi-domain interaction involving at least both linear endings of the ligand. Apparently the ErbB-3 affinity of a ligand determines whether it can form only ErbB-2/ErbB-3 complexes or also ErbB-3 homodimers. Because no separate binding domain for ErbB-2 could be identified, our data support a model in which ErbB heterodimerization occurs through a receptor-mediated mechanism and not through bivalent ligands.