Expression of gamma-glutamyl transpeptidase by renal epithelial cells occurs on a cell-by-cell basis and is inhibited by chronic TPA treatment

Abstract

Upon attaining a confluent density, populations of the renal epithelial cell line, LLC-PK1, express progressively many properties characteristic of the renal proximal tubule cell, including gamma-glutamyl transpeptidase activity. Expression of transpeptidase activity was inhibited reversibly by chronic treatment with the phorbol ester tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). TPA treatment inhibited expression of transpeptidase activity regardless of whether added prior to or following appearance of the activity. Increased transpeptidase activity in postconfluent cell populations was due to an increased enzyme Vmax with no change in substrate Km. TPA-treated cell populations exhibited a low Vmax similar to subconfluent populations. Detection of transpeptidase activity at the individual cell level by enzyme histochemistry demonstrated that near-confluent cell populations possessed few transpeptidase activity-positive cells. Progressive expression of transpeptidase activity in the cell population was due to an increasing proportion of cells in the population possessing transpeptidase activity. There was a parallel increase in the proportion of cells expressing transpeptidase protein, detected by immunofluorescence. TPA treatment inhibited appearance of both transpeptidase activity and transpeptidase protein in virtually all cells of the population. These results demonstrate that expression of transpeptidase activity in populations of LLC-PK1 cells occurs on a cell-by-cell basis and reflects expression of transpeptidase protein. Chronic treatment with TPA inhibits reversibly expression of transpeptidase activity and protein, suggesting a role for protein kinase C in regulating expression of this proximal tubule-specific property.