1α,25-dihydroxyvitamin D3 enhances -γglutamyl transpeptidase activity in LLC-PK1 porcine kidney epithelial cells

Abstract

Mammalian γ-glutamyl transpeptidase (GGT) is expressed most highly in the kidney and serves to recover the constituent amino acids of glutathione in the proximal tubules. Serum GGT is used as a marker for obstructive jaundice and alcoholic liver disease and it has been reported that urinary GGT is a potential marker for bone resorption. The present study investigated the effect of derivatives of vitamin D3 on GGT activity in LLC-PK1 porcine renal tubular epithelial cells in order to analyze the biochemical basis of bone metabolism-associated alterations in GGT activity in the kidney. In the presence of 1α,25-dihydroxyvitamin D3 [1,25(OH)2VD 3], GGT activity was observed to be significantly increased in LLC-PK1 cells, with an increase in GGT activity also found in the cell medium. While the stimulatory effect of 1-OH-VD3 was similar to that of 1,25(OH)2VD3, vitamin D3 and 25-OH-VD3 had no effect on GGT activity. The increased GGT activity caused by 1,25(OH) 2VD3 in LLC-PK1 cells was the result of long-term stimulation of the cells, in contrast to the GGT-induced increase in alkaline phosphatase, which is more transient. Polymerase chain reaction analysis revealed that the 1,25(OH)2VD3-induced increase in GGT activity was due to prolonged GGT turnover, rather than increased GGT expression, as no increase in GGT mRNA expression was observed. Thus, it is likely that the expression of GGT is not entirely constitutive in the kidney, but is altered under certain conditions, including under hormonal regulation.