Arginine vasopressin (AVP) causes the reversible phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein in the ovine anterior pituitary: evidence that MARCKS phosphorylation is associated with adrenocorticotropin (ACTH) secretion

Abstract

We have recently shown that AVP causes a protein kinase C (PKC)-dependent increase in ACTH release and biosynthesis in ovine anterior pituitary cells. In these cells, AVP also causes the translocation of PKC from the cytosol to the cell membrane which is maximal at 5 min, but the intracellular events distal to protein kinase C activation that underlie ACTH secretion have not been well characterized to date. Since the MARCKS protein has been implicated in neurosecretion and is phosphorylated by PKC in synaptosomes, studies were carried out to determine whether AVP might cause MARCKS phosphorylation in the ovine anterior pituitary, and to determine whether this phenomenon might be temporally correlated with PKC translocation and the release of ACTH. When cytosolic fractions of rat brain, ovine anterior pituitary, and cultured ovine anterior pituitary cells were incubated with purified PKC, several proteins were phosphorylated including those in the region of 83–85 kDa. After precipitation of the proteins with 40% acetic acid, the 83–85 kDa phosphoproteins were selectively recovered in the acid soluble phase. Phosphopeptide maps of either the 83 or 85 kDa proteins were generated with Staphylococcus aureus V8 protease and revealed 13 and 9 kDa phosphopeptides, which are characteristic of the authentic MARCKS protein. An identical phosphopeptide map was also obtained when the MARCKS protein was selectively extracted from intact 32P-labeled anterior pituitary cells. MARCKS phosphorylation was markedly increased when ovine anterior pituitary cells were exposed to 1 μM phorbol 12-myristate 13-acetate (PMA). When the cells were exposed to l μM AVP, MARCKS phosphorylation increased at 15 s and reached the maximal plateau value at 30 s. MARCKS phosphorylation then started to diminish at 2 min, and baseline levels were attained by 10 min. In the same cells, AVP stimulated ACTH release in a biphasic manner — during the first 30 s, there resulted a rapid burst of ACTH secretion that was followed by a slower, but sustained rate of secretion. We conclude that: (1) AVP causes a rapid, and reversible, phosphorylation of the MARCKS protein in the ovine anterior pituitary; (2) since the AVP-induced increase in MARCKS phosphorylation occurs much earlier in these cells than does PKC trans-location, MARCKS phosphorylation may provide a more sensitive index of the onset of PKC activation than the translocation assay; (3) the close temporal association between MARCKS phosphorylation and the rapid early release of ACTH suggests that MARCKS phosphorylation may be involved in the initial intracellular events that underly exocytosis of the hormone.