Cell-type specificity of ectonucleotidase expression and upregulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin
Cytochrome P-450 Enzyme System
We report here that induction of ectoATPase by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is cell-type specific and not a generalized response to aryl hydrocarbon (Ah) receptor activation. TCDD increased [14C]-ATP and -ADP metabolism in two mouse hepatoma lines, Hepa1c1c7 and Hepa1-6 cells, but not in human hepatoma HepG2 or HuH-7 cells, human umbilical vein endothelial cells (HUVEC), chick hepatoma (LMH) cells, or chick primary hepatocytes or cardiac myocytes, even though all of those cell types were Ah receptor-responsive, as evidenced by cytochrome P4501A induction. To determine whether the differences in ectonucleotidase responsiveness to TCDD might be related to differences in cell-type ectonucleotidase expression, ATP and ADP metabolite patterns, the products of several classes of ectonucleotidases including ectonucleoside triphosphate diphosphohydrolases (E-NTPDases), ectophosphodiesterase/pyrophosphatases (E-NPP enzymes) and ectoalkaline phosphatase activities were examined. Those patterns, together with results of enzyme assays, Western blotting, or semiquantitative RT-PCR show that NTPDase2 is the main ectonucleotidase for murine and human hepatoma cells, NTPDase3 for chick hepatocytes and LMH cells, and an E-NPP enzyme for chick cardiac myocytes. Evidence for NTPDase2 expression was lacking in all cells except the mouse and human hepatoma cells. TCDD increased expression of the NTPDase2 gene but only in the mouse and not in the human hepatoma cells. TCDD did not increase NTPDase3, NTPDase1, E-NPP, or alkaline phosphatase in any of the cell types examined. The failure of TCDD to increase ATP metabolism in HUVEC, chick LMH cells, hepatocytes, and cardiac myocytes can be attributed to their lack of NTPDase2 expression, while the increase in ATP metabolism by TCDD in the mouse but not the human hepatoma cells can be explained by differences in TCDD effects on mouse and human hepatoma NTPDase2 gene expression. In addition to characterizing effects of TCDD on ectonucleotidases, these studies reveal major differences in the complements of ectonucleotidases present in different cell types. It is likely that such differences are important for cell-specific susceptibility to extracellular nucleotide toxicity and responses to purinergic signaling.