A Ferro-Heme Protein Senses Oxygen Levels, Which Modulate the Glucagon-Dependent Activation of the Phosphoenolpyruvate Carboxykinase Gene in Rat Hepatocyte Cultures

Regulation of the gluconeogenic phosphoenolpyruvate carboxykinase and the glycolytic aldolase A gene expression by O2 in rat hepatocyte cultures. Involvement of hydrogen peroxide as mediator in the response to O2

“…In 24 h rat hepatocyte cultures glucagon induced PCK mRNA to a maximum within 2 h; thereafter PCK mRNA declined again in line with previous studies [5,6,10] (not shown). Glucagon elevated PCK mRNA by about 5.5-fold (=100% induction) at arterial pO2 and about 3.6-fold (= 65% induction) at venous pO2 (Fig.…”

“…Spectral analyses and the fluorescence microscopic demonstration of H202 production revealed an H202-generating heme protein as a possible Oz sensor [24]. The hypothesis that an NADPH-like oxidase is part of the 02 sensor is in line with results from this and earlier studies on the regulation of EPO and PCK induction, which had shown the participation of a heme protein [6,9,10]: the hypoxia-induced EPO production was modulated by inducers and inhibitors of a b-type cytochrome, the P-450 system [25], and it was sensitive to 02 but not cyanide [11] or dinitrophenol [10], which excludes respiratory chain type-b cytochromes. Moreover, the hypoxia-induced increases in EPO mRNA [12] and ALD A mRNA were inhibited and the glucagondependent induction of PCK mRNA was increased by H202 (Fig.…”

“…Conversely, the genes for the glycolytic enzymes lactate dehydrogenase A (LDH A), phosphoglycerate kinase 1 (PGK1) and aldolase A (ALD A) as well as the erythropoietin (EPO) gene were induced by hypoxia in Hep3B and HepG2 cells [7,8]. Studies with the 02 competitive ligand carbon monoxide showed that this was due to a heine protein which belongs to the b-type cytochromes [9][10][11]. Western blots with antibodies against the small subunit (22 kDa) and the cytosolic activation factor (47 kDa) of NADPH oxidase demonstrated the presence of an NADPH oxidase-like heme protein in HepG2 cells [I 1].…”

Oxygen sensing and signaling: impact on the regulation of physiologically important genes

“…CO has also been shown to offset hypoxia's effect on the expression of VEGF (Goldberg and Schneider, 1994), platelet derived growth factor (Morita and Kourembanas, 1995), endothelin-1 (Morita and Kourembanas, 1995) and phosphoenolpyruvate carboxykinase (Kietzmann et al, 1993). More recently, Liu et al (1998) and our laboratory (Huang et al, 1999) have shown that CO suppresses the hypoxic activation of HIF-1.…”

Differential expression of IGF-I and IGF-binding protein-1 and -2 in periportal and perivenous zones of rat liver

“…As insulin is inhibitory and glucagon stimulatory for both PEPCK and IGFBP-1 expression (Magnuson et al 1987, Lee et al 1993), this does not, however, explain why the gradient of PEPCK mRNA in the liver acinus is in the opposite direction. Part of the reason why PEPCK is expressed at higher levels in the periportal zone may be that an oxygen-sensing pathway in the gene, mediated by a ferro-haeme protein, leads to 100% glucagon-dependent activation at 16% O 2 , but only 60% at 8% O 2 , levels similar to periportal and perivenous oxygen tensions respectively (Kietzmann et al 1993). The possibility exists that this oxygen-sensing pathway is either lacking, or opposing, in the regulation of IGFBP-1.…”