Nerve Growth Factor‐Induced Down‐Regulation of Calmodulin‐Dependent Protein Kinase III in PC12 Cells Involves Cyclic AMP‐Dependent Protein Kinase

Abstract

Treatment of PC12 cells with nerve growth factor (NGF), epidermal growth factor (EGF), or agents that raise intracellular cyclic AMP (cAMP) levels (e.g., forskolin) reduces the activity of calmodulin-dependent protein kinase III (CaM-PK III) over a period of 8 h. The mechanism of this effect of NGF has now been examined in more detail, making use of a mutant PC12 cell line (A126-1B2) that is deficient in cAMP-dependent protein kinase activity. Control experiments showed that A126-1B2 cells retain other NGF-mediated responses (e.g., the induction o fornithine decarboxylase, a cAMP-independent event) and contain a complement of CaM-PK III and its substrate, elongation factor-2, comparable to that of wild-tpe cells. The ability of NGF or forskolin, but not of EGF, to down-regulate CaM-PK III was markedly attenuated in a126-1B2 compared to wild-type cells. Treatment of wild-type cells with the cAMP phosphodiesterase inhibitor, isobutylmethylxanthine, enhanced the effects of NGF, bu tnot to EGF. The possibility that NGF led to a stimulation of cAMP-dependent protein kinase activity in wild-type cells was assessed by measurement of the "activation ratio" (-cAMP/+cAMP) of this enzyme before and at various times after NGF addition. A small, but significant, increasein the activation ratio from 0.3 to 0.48 was observed, reaching a peak 5 min after NGF treatment. EGF had no effect on the activation ratio in wild-type cells. These experiments support the hypothesis that NGF, but not EGF, achieves its effects on CaM-PK III by activation of a cAMP-dependent protein kinase and suggest that other actions of NGF may be mediated by the same mechanism.