Glucocorticoid-regulated stability of a constitutively expressed mouse mammary tumor virus glycoprotein

Glucocorticoids regulate the transport and processing of mouse mammary tumor virus (MMTV) glycoproteins in viral-infected HTC rat hepatoma cells. To begin to determine the role of cellular components involved in this steroid-mediated response, a constitutively expressed MMTV glycoprotein gene containing a mutation in the endoproteolytic cleavage site was used to simplify the viral maturation products. Expression of the uncleavable MMTV glycoprotein gene in transfected HTC rat hepatoma cells demonstrated that treatment with the synthetic glucocorticoid dexamethasone resulted in a 5-fold increase in the steady state level of the intracellular and cell surface MMTV glycoproteins. Under these conditions, dexamethasone did not alter the level of MMTV glycoprotein transcripts. Pulse-chase radiolabeling with [35S]methionine demonstrated that dexamethasone did not affect the apparent rate of MMTV glycoprotein translation, and an analysis of oligosaccharide side-chain structure by endoglycosidase-H digestion revealed that glucocorticoids did not alter the 45-min endoplasmic reticulum to Golgi transit time. Pulse-chase kinetic analysis of 4-h pulse-labeled cells revealed that the half-life of the mature glycosylated MMTV polyprotein, gp78, was 105 min in glucocorticoid-treated cells and 45 min in untreated cells. Taken together, our results suggest that glucocorticoids increase the stability of MMTV glycoproteins by a posttranslational mechanism and that this effect may be occurring relatively early in the exocytic pathway.