Lactogen-induced protein phosphorylation in Nb2 cells

Lactogenic hormones are potent mitogenic agents in the Nb2 rat lymphoma cell line. Because selective phosphorylation/dephosphorylation of specific proteins by many polypeptide hormones appears to be important in regulating cell growth, the effect of lactogenic hormones on protein phosphorylation was studied in Nb2 cells. Human growth hormone (hGH) promoted the phosphorylation of many proteins (2- to 3-fold) and an Mr 29,000 species (pp29) (greater than or equal to 10-fold). In growth-arrested cells phosphorylation of pp29 peaked 4 h after addition of hGH whereas that of other phosphoproteins increased steadily. Ovine prolactin, another potent mitogenic hormone, caused a concentration-dependent increase in pp29 phosphorylation with an EC50 = 10-20 pM. Other agents which are not mitogenic in this cell line, i.e., 12-O-tetradecanoyl-phorbol-13-acetate (200 nM), 8-Br-cAMP (1 mM), and 8-Br-cGMP (1 mM), did not affect pp29 phosphorylation. Dexamethasone (100 nM), which inhibits lactogen-stimulated growth, reduced hGH-stimulated pp29 phosphorylation to control levels. Fractionation of detergent-treated extracts of hGH-treated cells showed that pp29 is associated with the ribosomal fraction. Taken together, these results show that pp29 phosphorylation is selective and is related to cell growth.