Coordinate modulation of glucocorticoid receptor and glutaminase gene expression in LLC-PK1-F+ cells

The effect of glucocorticoid receptor on glutaminase gene expression and related glutamine metabolism was studied in proximal tubule-like LCC-PK1-F+ cells. These cells express functional glucocorticoid receptors as demonstrated by immunoreactivity with antiglucocorticoid receptor antibody, specific ligand binding, and a 14-fold increase in chloramphenicol acetyltransferase (CAT) reporter gene activity after exposure to dexamethasone (10(-6)M). Dexamethasone exposure for 18 h increased glutaminase mRNA and activity by 32 and 42%, respectively (both P< 0.05, paired t-test), associated with a small (9%) but significant increase in glutamine utilization (P<0.05). In an effort to elicit a greater response, endogenous glucocorticoid receptors were supplemented by transfecting cells with a plasmid, pMAMGR, expressing the rat glucocorticoid receptor gene. Transfected cells expressed a 39-fold increase in CAT activity with dexamethasone treatment, confirming a higher level of functional receptors, but glutaminase mRNA and activity were now decreased by 34 and 32%, respectively, associated with a 15% fall in glutamine utilization after 18-h exposure to dexamethasone. This biphasic response in glutaminase gene expression was mirrored by glucocorticoid receptor mRNA that increased 41% after dexamethasone in LLC-PK(1)-F+ cells, but decreased 63% after transfection (both P<0.05). These findings are consonant with glucocorticoid receptor gene modulation of glutaminase gene expression and glutamine utilization.