Acute Stimulation by Peptide Hormones of Mitochondrial Steroid Hormone Synthesis: Production of a Similiar Protein in Three Steroidogenic Tissues

Summary

In three steroidogenic cell types, adrenal cortex, corpus luteum and Leydig, addition of tissue specific peptide hormone or cAMP causes a dose dependent increase in steroid hormone synthesis. Since both the initiation and the maintenance of stimulated steroidogenesis depend on continuing protein synthesis, two-dimensional electrophoresis was used to monitor proteins synthesized or modified in control and in stimulated cells from all three tissues. Previously, in adrenal cortex and corpus luteum cells and, here, in Leydig cells, we have detected a protein (i) whose accumulation is induced and a protein (p) whose synthesis is repressed by either stimulant. The peptide hormone or cAMP dose response of the accumulation of protein and repression of synthesis of p parallel the increase in steroid synthesis in these stimulated cells. Proteins i and p from both adrenal cortex and corpus luteum give rise to similar cleavage products when incubated with Staphylococcus aureus V-8 protease: however, i is not produced from p by post-translational modification. In addition, we have now detected in these tissues a more acidic form of p (p_a) and of i (i_a), which are the post-translational products of the more basic proteins p (p_b) and i (i_b) respectively. All four proteins have the same molecular weight, but differ in isoelectric point; the four adrenal proteins give very similar proteolytic cleavage products. Studies on adrenal cells indicate that proteins i_b and i_a are phosphorylated and that p_b and p_a are not. Taken together, these studies indicate that (1) protein i_b may be the protein whose synthesis is necessary for stimulated steroid production, (2) protein i_b is produced by a cAMP-mediated, co-translational phosphorylation of protein p_b, and (3) the continuing synthesis of protein i_b may be necessary, because it is rapidly and post-translationally converted to i_a.