ENHANCED EXPRESSION OF GTP-BINDING PROTEINS IN DIFFERENTIATED U937 MONOCYTIC CELLS - POSSIBLE INVOLVEMENT OF TYROSINE KINASE AND PROTEIN-KINASE-C

Monocytic U937 cells were differentiated into mature macrophages in th e presence of 100 nM phorbol 12-myristate 13-acetate (PMA) for 24 h at 37 degrees C. We investigated the alterations in the expression of GT P-binding proteins that take place during differentiation of these cel ls. A 40 KDa alpha-subunit of the inhibitory G-protein was identified by specific antibodies to G(i alpha)-(1/2) in and G(i alpha-3) on West ern blots and also by ADP-ribosylation catalyzed by pertussis toxin. T he expression of the 40 KDa G. subunit was increased 3.4 fold in diffe rentiated cells. The expression of a 43 KDa G(s alpha) subunit identif ied by Western blotting using specific antibody to G(s alpha) and by A DP-ribosylation in the presence of cholera toxin was increased approxi mately 2 fold in differentiated cells. A faintly recognizable 46 KDa G (s alpha) subunit was also increased but to a lesser extent (1.3 fold) . Small molecular weight GTP-binding proteins identified by [S-35]GTP gamma S binding on nitrocellulose blots were also increased significan tly. The PMA-induced expression of G(i alpha-1/2) and G(s alpha) Subun its was blocked to control level by both genistein and staurosporine, inhibitors of protein tyrosine kinase and protein kinase C, respective ly. However, staurosporine was unable to block the PMA-induced express ion of G(i alpha-3); this was blocked only by genistein. These data su ggest a role for tyrosine kinase and protein kinase C in the expressio n of G-proteins during differentiation of U937 cells.