Dibutyryl cyclic AMP-induced phosphorylation of specific proteins in adenohypophysial cells

The objective of this work was to identify the natural substrates of cyclic AMP-dependent protein kinase in pituitary cells. Studies were performed using 2 systems: intact pituitary cells stimulated with dibutyryl cyclic AMP (DBC) after preincubation with [gamma-32P]. Phosphorylation of proteins was analyzed by two-dimensional gel electrophoresis, followed by autoradiography. In intact cells, the only clear and reproducible effect of DBC stimulation is increased phosphorylation of 3 proteins (termed A, B, and C), each with a molecular weight of about 20 000 dalton. The time-course and dose-dependence of phosphorylation of A, B and C are generally similar to that for DBC-induced hormone secretion, which is consistent with a role for these proteins in the secretory mechanism. When [gamma-32P]ATP is added to cell extracts, proteins A, B, and C are not measurably phosphorylated, either in the absence or presence of cyclic AMP. This observation suggests that proteins A, B and C may not be directly phosphorylated by cyclic AMP-dependent protein kinase, but may be phosphorylated indirectly by a second kinase. On the other hand, growth hormone and prolactin are readily phosphorylated in cell extracts by cyclic AMP-dependent protein kinase (although they are not phosphorylated in vivo). This finding makes clear the need for caution in interpreting results from broke cell systems.