Okadaic Acid Mimics Several Proximal Effects of Prolactin in Nb2 Lymphoma Cells

Abstract

We previously reported that prolactin-mediated macromolecular synthesis and mitogenesis are coupled to the activation of mitogen-activated protein kinase (MAPK) and p70 S6-kinase (p70^S6K). Full activation of MAPK requires tyrosine and threonine phosphorylation whereas that of p70^S6K requires serine phosphorylation. In the present study, okadaic acid, which inhibits serine/threonine protein phosphatase activity, was used to explore the linkage of MAPK and p70^S6K activation to downstream effects of prolactin in Nb2 cells. The results show that 1 nM okadaic acid augmented prolactin-stimulated mitogenesis and synthesis of protein and DNA 250%, 42%, and 70%, respectively. Addition of okadaic acid alone a) stimulated and sustained p70^S6K activity (5-to 8-fold) and MAPK (3.5-to 5-fold); and b) increased protein synthesis with the maximum effect being about equal to that of prolactin (2.1-fold with 1 nM okadaic acid versus 2.3-fold with 0.2 nM prolactin). However, okadaic acid did not affect DNA synthesis or mitogenesis. These results indicate that the activation of MAPK and p70^S6K is necessary for stimulation of protein synthesis but not sufficient for prolactin-driven mitogenesis.