Estrogen regulation of in vitro brain tyrosine hydroxylase activity in the catfish Heteropneustes fossilis:: interactions with cAMP-protein kinase A and protein kinase C systems in enzyme activation

In the present in vitro study, interactions of both cAMP-protein kinase A (PKA) and protein kinase C (PKC) systems were investigated in the estradiol-17 beta (E-2) regulation of forebrain (hypothalamus and telencephalon) tyrosine hydroxylase (TH) activity in the female catfish Heteropneustes fossilis in vitellogenic phase. E-2 produced biphasic effects on TH activity: low concentrations (10(-12)-10(-5)M) stimulated, and high concentrations (10(-3)-10(-4) M) inhibited enzyme activity (Tukey's test, P < 0.05). Co-incubations of the enzyme preparations with cAMP (1.0 mM), IBMX (1.5 mM) or theophylline (1.5 mM) and a low concentration of E-2 (10(-9) M) increased TH activity significantly. However, the co-incubations with a high concentration of E-2 (10(-3) M) decreased it significantly. Pre-incubations of the enzyme preparations with cAMP (0.1 mM), followed by different concentrations of E-2 (10(-12), 10-9, 10(-4), and 10(-3) M) produced concentration-dependent biphasic effects. The pre-incubations with a low concentration of E, (10(-9) M), followed by different concentrations of cAMP (0.05-1.0 mM) produced a significant concentration-dependent stimulation of TH activity and that with a high concentration of E-2 (10(-3) M) produced a significant decrease in TH activity. Co-incubations of high and low E-2 with or without cAMP, and PKA inhibitor (H-89) decreased TH activity significantly. The incubations with H-89 abolished the stimulatory effect of low E-2 or low E-2 + cAMP and intensified the inhibitory effect of high E-2 or high E-2 + cAMP combination. Co-incubations with PKC inhibitor (calphostin C) did not influence the stimulatory effect of low E-2 but lowered the stimulatory effect of low E-2 + cAMP treatment. Kinetic studies showed that the stimulatory effect of a low E-2 concentration was due to a decrease in apparent K-m and an increase in apparent V-max for both cofactor and substrate. and the inhibitory effect of a high E-2 concentration was due to reverse changes in the kinetics. The stimulatory effect of cAMP alone or in combination with low E-2 was related to decreased K-m and increased V-max for the cofactor. The inhibitory effect of PKA and PKC blockers, alone or in combination with E-2 and/or cAMP was due to increased K-m and decreased V-max of the enzyme for the cofactor. The present data suggest that E-2 modulates the short-term activation of brain TH activity differentially and may involve mainly the cAMP-PKA system. m (c) 2005 Elsevier Inc. All rights reserved.