Prolactin receptor turnover in explants of pseudopregnant rabbit mammary gland

Pseudopregnant rabbit mammary glands in organ culture were used to investigate prolactin (PRL) receptor turnover. Chloroquine (100 microM) results in an increase in prolactin receptor levels (15.7 +/- 1.2% to 35.9 +/- 3.5% specific binding), whereas cycloheximide (1 microgram/ml) induces a rapid decline (to 6.4 +/- 1.2%) suggesting a rapid synthesis and degradation of the receptor molecule. Inhibitors of cellular transcription have little effect on receptor levels. Neither actinomycin D nor dichlororibofuranosylbenzimidazole (DRB) diminish PRL receptor levels whereas total protein synthesis is almost completely inhibited, and chloroquine increases the binding even in the presence of transcriptional inhibitors. These results imply that receptor synthesis continues and that the mRNA for the receptor protein is particularly stable. Ouabain (3 micrometers), which blocks the ATP-dependent Na+/K+ pump, provokes a greater than 60% reduction in PRL receptor levels without modifying total protein synthesis. Dinitrophenol (DNP, 1 mM), an oxidative uncoupler, has little effect on receptor levels, possibly due to a blockage of both synthesis and degradation. Prolactin is capable of inducing a 60% down-regulation of its own receptor, and this phenomenon appears to be energy-dependent because it is partially inhibited by DNP. This process seems to involve an increased rate of receptor degradation. These studies suggest that, at any one time, the level of PRL receptors in a target cell is the result of a dynamic equilibrium between receptor synthesis and degradation and that the most frequent modulations occur at the level of translation and lysosomal degradation. In conclusion, in mammary glands of the pseudopregnant rabbit, the prolactin receptor molecule appears to have a short half-life; the mRNA for this protein, however, is relatively stable.