Mammotroph Autoregulation: The Differential Roles of the 24K Isoforms of Prolactin*

In this study we have attempted to determine which of the secreted 24K isoforms was responsible for autocrine regulation of PRL secretion by comparing the isoforms synthesized and secreted by normal cells, which do autoregulate, with those synthesized and secreted by GH3 cells, which do not normally autoregulate. Comparable numbers of cells were washed free of serum and then extracted into Tris-buffered saline by sonication and detergent treatment. Proteins present in these cell extracts and in samples of culture medium were then precipitated with cold acetone (-20 C; 48 h) and subsequently dissolved in urea-lysis buffer for 2-dimensional (2-D) electrophoresis. The 2-D patterns for normal cells showed four 24K PRL isoforms inside the cells and three 24K PRL isoforms (designated 2, 3, and 3') secreted into the medium. The 2-D patterns for GH3 cells showed very little intracellular storage of PRL, but what was present was identified as 24K PRL isoform 2. The GH3 cells secreted large amounts of only 24K PRL isoform 2. Preparations of PRL containing only isoforms 1,2, and 3 (at a total radioimmunoassayable concentration of 5 micrograms/ml PRL) were capable of inducing autoregulation in GH3 cells, as evidence by decreased secretion of prelabeled intracellular PRL. Initiation of autoregulation in GH3 cells caused granulation and the intracellular production of isoform 3. Since a) a preparation containing isoforms 1, 2, and 3 was found to induce autoregulation in GH3 cells, b) isoform 1 is not a secreted form, and c) isoform 2 does not cause autoregulation (at least in GH3 cells), it is deduced that isoform 3 is an autocrine form of PRL. Since initiation of autoregulation in GH3 cells caused those cells to produce isoform 3, it is further deduced that the autoregulatory defect in GH3 cells lies in the actual lack of production of isoform 3 and not in an inherent inability of these cells to produce isoform 3.