Modulation of glucokinase expression by hypoxia-inducible factor 1 and upstream stimulatory factor 2 in primary rat hepatocytes

Glucokinase (GK) is the key enzyme of glucose utilization in liver and is localized in the less aerobic perivenous area. Until now, the O2-responsive elements in the liver-specific GK promoter are unknown, and therefore the aim of this study was to identify the O2-responsive element in this promoter. We found that the GK promoter sequence -87/-80 matched the binding site for hypoxia inducible factor 1 (HIF-1) and upstream stimulatory factor (USF). In primary rat hepatocytes we could show that venous pO2 enhanced HIF-1alpha and USF-2a levels, both of which activated GK expression. Furthermore, transfection experiments revealed that the GK sequence -87/-80 mediated the HIF-1alpha- or USF-2-dependent activation of the GK promoter. The binding of HIF-1 and USF to the GK-HRE was corroborated by electrophoretic mobility shift assay (EMSA). However, the maximal response to HIF-1alpha or USF was only achieved when constructs with the -87/-80 sequence in context with a 3'-36 bp native GK promoter sequence containing a hepatocyte nuclear factor 4 (HNF-4) binding site were used. HIF-1alpha and HNF-4 additively activated the GK promoter, while USF-2 and HNF-4 together did not show this additive activation. Thus, HIF-1 and USF may play differential roles in the modulation of GK expression in response to O2.     

Characterization of adjacent E-box and nuclear factor 1-like DNA binding sequence in the human CYP1A2 promoter

To better understand the molecular mechanisms of cytochrome P450 1A2 (CYP1A2) regulation, we have characterized a region of the promoter (+3 to -176) that contains a single E-box and an adjacent nuclear factor 1 (NF1)-like DNA binding site. The E-box was shown to specifically bind nuclear proteins that were recognized by antibodies against upstream stimulatory factor (USF) 1 and 2. Comparison of NF1 binding proteins in HepG2 cells and primary cultures of rat hepatocytes revealed different patterns of DNA-protein complexes, all of which were recognized by a general NF1 antibody. Mutations of the E-box resulted in substantial reduction of promoter activity in either primary hepatocytes or HepG2 cells regardless of the presence in the reporter constructs of other CYP1A2 regulatory elements, such as the hepatic nuclear factor 1 (HNF-1) binding site. In contrast, reporter gene activity of the promoter construct harboring the mutated NF1-like binding site was affected by upstream sequences when transfected into HepG2 cells, but not in primary hepatocytes. We conclude that both USF proteins and different isoforms of NF1 contribute to the constitutive expression of CYP1A2.