OKADAIC ACID MIMICS SEVERAL PROXIMAL EFFECTS OF PROLACTIN IN NB2 LYMPHOMA-CELLS

We previously reported that prolactin-mediated macromolecular synthesi s and mitogenesis are coupled to the activation of mitogen-activated p rotein kinase (MAPK) and p70 S6-kinase (p70(S6K)). Full activation of MAPK requires tyrosine and threonine phosphorylation whereas that of p 70(S6K) requires serine phosphorylation, In the present study, okadaic acid, which inhibits serine/threonine protein phosphatase activity, w as used to explore the linkage of MAPK and p70S6K activation to downst ream effects of prolactin in Nb2 cells, The results show that 1 nM oka daic 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 p70S6K 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, oka daic acid did not affect DNA synthesis or mitogenesis, These results i ndicate that the activation of MAPK and p70(S6K) is necessary for stim ulation of protein synthesis but not sufficient for prolactin-driven m itogenesis.