Human fetal testis endoplasmic reticulum glucose-6-phosphatase enzyme protein

Computational identification of transcriptionally co-regulated genes, validation with the four ANT isoform genes

BACKGROUND

The analysis of gene promoters is essential to understand the mechanisms of transcriptional regulation required under the effects of physiological processes, nutritional intake or pathologies. In higher eukaryotes, transcriptional regulation implies the recruitment of a set of regulatory proteins that bind on combinations of nucleotide motifs. We developed a computational analysis of promoter nucleotide sequences, to identify co-regulated genes by combining several programs that allowed us to build regulatory models and perform a crossed analysis on several databases. This strategy was tested on a set of four human genes encoding isoforms 1 to 4 of the mitochondrial ADP/ATP carrier ANT. Each isoform has a specific tissue expression profile linked to its role in cellular bioenergetics.

RESULTS

From their promoter sequence and from the phylogenetic evolution of these ANT genes in mammals, we constructed combinations of specific regulatory elements. These models were screened using the full human genome and databases of promoter sequences from human and several other mammalian species. For each of transcriptionally regulated ANT1, 2 and 4 genes, a set of co-regulated genes was identified and their over-expression was verified in microarray databases.

CONCLUSIONS

Most of the identified genes encode proteins with a cellular function and specificity in agreement with those of the corresponding ANT isoform. Our in silico study shows that the tissue specific gene expression is mainly driven by promoter regulatory sequences located up to about a thousand base pairs upstream the transcription start site. Moreover, this computational strategy on the study of regulatory pathways should provide, along with transcriptomics and metabolomics, data to construct cellular metabolic networks.

Identification and characterisation of a new human glucose-6-phosphatase isoform

The liver endoplasmic reticulum glucose-6-phosphatase catalytic subunit (G6PC1) catalyses glucose 6-phosphate hydrolysis during gluconeogenesis and glycogenolysis. The highest glucose-6-phosphatase activities are found in the liver and the kidney; there have been many reports of glucose 6-phosphate hydrolysis in other tissues. We cloned a new G6Pase isoform (G6PC3) from human brain encoded by a six-exon gene (chromosome 17q21). G6PC3 protein was able to hydrolyse glucose 6-phosphate in transfected Chinese hamster ovary cells. The optimal pH for glucose 6-phosphate hydrolysis was lower and the K(m) higher relative to G6PC1. G6PC3 preferentially hydrolyzed other substrates including pNPP and 2-deoxy-glucose-6-phosphate compared to the liver enzyme.

Conformational change of the catalytic subunit of glucose-6-phosphatase in rat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver microsomal vesicles. Several observations indicate that the orientation of the catalytic subunit is different in the fetal liver in comparison with the adult form: (i) the phosphohydrolase activity was not latent using glucose-6-phosphate as substrate, and in the case of other phosphoesters it was less latent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosphate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-phosphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enzyme inhibited the phosphohydrolase activity in fetal but not in adult rat liver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could be detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and glucose accessible spaces were proportionally smaller than in adult rat liver microsomes. These data demonstrate that the components of the glucose-6-phosphatase system are already present, although to a lower extent, in fetal liver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.

Ontogeny of the murine glucose-6-phosphatase system

A deficiency in microsomal glucose-6-phosphatase (G6Pase) activity causes glycogen storage disease type 1 (GSD-1), a clinically and biochemically heterogeneous group of diseases. It has been suggested that catalysis by G6Pase involves multiple components, with defects in the G6Pase catalytic unit causing GSD-1a and defects in the putative substrate and product translocases causing GSD-1b, 1c, and 1d. However, this model is open to debate. To elucidate the G6Pase system, we have examined G6Pase mRNA expression, G6Pase activity, and glucose 6-phosphate (G6P) transport activity in the murine liver and kidney during normal development. In the liver, G6Pase mRNA and enzymatic activity were detected at 18 days gestation and increased markedly at parturition, before leveling off to adult levels. In the kidney, G6Pase mRNA and enzymatic activity appeared at 19 days gestation and peaked at weaning, suggesting that kidney G6Pase may have a different metabolic role. In situ hybridization analysis demonstrated that, in addition to the liver and kidney, the intestine expressed G6Pase. Despite the expression of G6Pase in the embryonic liver, microsomal G6P transport activity was not detectable until birth, peaking at about age 4 weeks. Our study strongly supports the multicomponent model for the G6Pase system.