Comparison of the regulation of carboxypeptidase e and prolactin in GH₄C₁ cells, a rat pituitary cell line

The treatment of GH₄C₁ ceils, a prolactin-producing rat anterior pituitary cell line, with estradiol (1 nAf), insulin (300 nM) and epidermal growth factor (10 nM) has been previously shown to substantially increase both the intracellular level of prolactin, as well as the number of secretory granules. In this study, we examined the effect of this treatment on levels of carboxypeptidase E (CPE), a prohormone-processing enzyme. GH₄C₁ cells contain CPE mRNA and enzymatic activity. The secretion of both prolactin and CPE activity from GH₄C₁ cells is stimulated 10-fold by 50 mM KC1 and 2- to 3-fold by 100 nM thyrotropin-releasing hormone, suggesting that these two proteins are contained in secretory granules. Treatment of GH₄C₁ cells with estradiol, insulin, and epidermal growth factor causes an increase in the intracellular level of CPE to approximately 2-fold of control values. This change is much smaller than the change in the level of prolactin: intracellular prolactin is increased 140-fold by the treatment. Kinetic analysis of the CPE activity indicates that the treatment does not alter the K_m of substrate hydrolysis, with the change in activity the result of an increase in apparent V_max. Northern blot analysis indicates that the level of CPE mRNA is not influenced (< 10 %) by the treatment, whereas the level of prolactin mRNA is increased 9-fold. These results indicate that CPE is not coordi- nately regulated with prolactin in the GH₄C₁ cell line, although some regulation of CPE activity does occur.