Density‐dependent regulation of cell surface γ‐glutamyl transpeptidase in cultured glial cells

A decline in cell surface γ‐glutamyl transpeptidase specific activity was previously observed to be concomitant with C 6 glial cell proliferation. To elucidate the underlying factor(s) mediating γ‐glutamyl transpeptidase down‐regulation, the effects of C 6 cell density and culture conditions on cell surface transpeptidase activity levels were investigated. After 24 h of culture, the transpeptidase specific activities were inversely related to the initial plating densities. The lower‐density cultures showed an induction within 24 h of plating. As the cultures proliferated, the specific transpeptidase activities declined to a common low level at postconfluency. The γ‐glutamyl transpeptidase down‐regulation was unrelated to cell growth rate and was most pronounced during logarithmic proliferation. Induction and down‐regulation of γ‐glutamyl transpeptidase activity at low cell densities were not a result of trypsinization. Supplementation of low‐density cultures with conditioned medium, use of matrix‐coated wells, or periodic replacement of growth media to prevent conditioning had minor effects on the decline of cell surface activity. Kinetic analysis showed that the Michaelis constants and the reaction mechanism were unaltered by cell density, indicating that downregulation was not due to allosteric factors or an alteration in enzyme character. A reduction in the maximal velocity of cell surface transpeptidation at higher cell densities suggested that γ‐glutamyl transpeptidase down‐regulation is related to the concentration of enzyme at the cell surface. Immunocytochemical localization of γ‐glutamyl transpeptidase demonstrated that γ‐glutamyl transpeptidase antigen levels decrease as C 6 cell density increases. These results led us to propose that cell‐cell contact stimulates the disappearance of γ‐glutamyl transpeptidase from the surface of cultured C 6 glial cells.