Purpose: Astrocytes cope-up the hypoxia conditions by up regulating the activity of the enzymes catalyzing the irreversible steps of the glycolytic pathway. The phosphofructokinase1 (PFK1), which converts fructose-6-phosphate to fructose-1, 6-bisphosphate, is the major regulatory enzyme of the glycolytic pathway. For this purpose, we investigated the expression regulation of the PFK1 during chemically induced hypoxia.
Principal result: After 48 h of the chemically induced hypoxia induction of the C6 glioma cells, the PFK1 protein depicted strong up regulation, with no appreciable change in its mRNA levels. The di-cistronic assay indicated the presence of a weak internal ribosome entry site (IRES) element in the 5'UTR of the PFK1 mRNA. Interestingly, the weak IRES element of the PFK1 was strongly up regulated after 48 h of the chemically induced hypoxia, indicative of a possible mechanism responsible for the induction of the PFK1 protein. The authenticity of the hypoxia-regulated IRES element of the PFK1, relative to the presence of the cryptic promoter element and/or the cryptic splicing was established using promoterless di-cistronic assay and the RT-PCR analysis. Moreover, the ectopic expression of the polypyrimidine tract binding (PTB) protein resulted in the enhanced activity of the IRES element of the PFK1. Additionally, it was established that the chemically induced hypoxia resulted in the increased shuttling of the PTB from the cell nucleus to the cytosol.
Major conclusion: The presence of a hypoxia responsive IRES element, in the 5'UTR of the PFK1 was established to be the possible mechanism responsible for the up regulation of the PFK1 protein. Our data provides an interesting mechanism that may explain the increased glycolytic capacity of the astrocytes after brain hypoxia.
Keywords: Astrocytes; C6 glioma; Hypoxia; IRES element; Phosphofructokinase1; Polypyrimidine tract binding protein.
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