Rapid activation of tyrosine hydroxylase in response to nerve growth factor

Nerve growth factor protein (NGF) was found to rapidly promote the activation of tyrosine hydroxylase in cultured rat PC12 pheochromocytoma cells. PC12 cultures were exposed to NGF for periods of less than 1 h and the soluble contents of homogenates prepared from the cells were assayed for tyrosine hydroxylase activity. Under these conditions, the specific enzymatic activity was increased by 60 +/- 10% (n = 13) in comparison with that in untreated sister cultures. The increase was half maximal by 2-5 min of exposure and at NGF concentrations of about 10 ng/ml (0.36 nM). Antiserum against NGF blocked the effect. Tyrosine hydroxylase activity could also be rapidly increased by NGF in cultures of PC12 cells that had been treated with the factor for several weeks in order to produce a neuron-like phenotype. This was achieved by withdrawing NGF for about 4 h and then readding it for 30 min. The NGF-induced increase of tyrosine hydroxylase activity in PC12 cultures was not affected by inhibition of protein synthesis and therefore appeared to be due to activation of the enzyme. Kinetic experiments revealed that NGF brought about no change in the apparent Km of the enzyme for tyrosine or for cofactor (6- methyltetrahydropteridine ), but that it did significantly increase the apparent maximum specific activity of the enzyme. These observations suggest that NGF (perhaps released by target organs) could promote a rapid and local enhancement of noradrenergic transmission in the sympathetic nervous system.