Cyclic nucleotide control of protein kinases

Effects of a water-soluble extract of Cordyceps sinensis on steroidogenesis and capsular morphology of lipid droplets in cultured rat adrenocortical cells

Cordyceps sinensis contains a factor that stimulates corticosteroid production in the animal model. However, it is not known whether this drug acts directly on the adrenal glands or indirectly via the hypothalamus-pituitary axis. In the present study, we used primary rat adrenal cell cultures to investigate the pharmacological function of a water-soluble extract of Cordyceps sinensis (CS) and the signaling pathway involved. Radioimmunoassay of corticosterone indicated that the amount of corticosterone produced by adrenal cells is increased in a positively dose-dependent manner by CS, reaching a maximum at 25 microg/ml. This stimulating effect was seen 1 h after CS treatment and was maintained for up to 24 h. Concomitantly, the lipid droplets in these cells became small and fewer in number. Immunostaining with a monoclonal antibody, A2, a specific marker for the lipid droplet capsule, demonstrated that detachment of the capsule from the lipid droplet occurs in response to CS application and that the period required for decapsulation is inversely related to the concentration of CS applied. The mechanism of CS-induced steroidogenesis is apparently different from that for ACTH, since intracellular cAMP levels were not increased in CS-treated cells. However, combined application with calphostin C, a PKC inhibitor, completely blocked the effect of CS on steroidogenesis, suggesting that activation of PKC may be responsible for the CS-induced steroidogenesis.

Effect of isoproterenol and 3-isobutyl-1-methylxanthine on junctional conductance in heart cell pairs

The influence of isoproterenol (10(-6) M) and 3-isobutyl-1-methylxanthine (IBMX) (10(-6) M) on the junctional conductance (gj) of isolated rat ventricular cell pairs was investigated. It was found that both drugs increased gj within 25-30 s. To test the phosphorylation hypothesis, cAMP-dependent protein kinase inhibitor (20 micrograms/ml) was dialyzed into both cells of the pair. The protein kinase inhibitor suppressed the effect of both drugs on gj. Moreover, the protein kinase inhibitor by itself, reduced gj by 18% (S.E. +/- 9) (n = 10), suggesting that basal levels of cAMP in the cells contributes to modulation of gj. Dialysis of cAMP-protein kinase inhibitor (20 micrograms/ml) in just one cell of the pair induced rectification of the junctional membrane. These results indicate that the activation of cAMP-dependent protein kinase is necessary for the effect of cAMP on junctional conductance in heart.

Augmentation of lymphocyte responses by monoclonal antibodies to the gangliosides GD3 and GD2: the role of protein kinase C, cyclic nucleotides, and intracellular calcium

Previous studies have shown that MAb's against the gangliosides GD3 and GD2 may augment T cell responses to a variety of stimuli. We present evidence that antiganglioside MAb's, like PHA, increase intracellular cGMP and protein kinase C yet have no effect on intracellular Ca2+. Stimulation of T cells with MAb's to GD3 was associated with increased cGMP levels, particularly in the CD8+ T cell subset which showed the highest degree of potentiation by the MAb's. Augmentation of T cell responses by the MAb's to GD3 and GD2 was also mimicked by activation of PKC with phorbol esters but both agents together produced marked synergistic effects on cell division, suggesting they had different but complementary modes of action. Furthermore, use of neomycin to inhibit PKC activation only partially reversed the augmentation of proliferative responses by the antiganglioside MAb's. It did however inhibit the MAb-induced increase in IL2 production and IL2 receptor (Tac) expression. These studies suggest therefore that the potentiation of IL2 production by the MAb's against GD2 and GD3 was due to enhanced activation of PKC whereas their augmentation of proliferative responses appeared to be due to effects on late events in T cell activation and was associated with both increased cGMP levels and activation of PKC.

Covalent labelling of ligand binding sites of human placental S-adenosylhomocysteine hydrolase with 8-azido derivatives of adenosine and cyclic AMP

S-Adenosylhomocysteine hydrolase (AdoHcyase) has previously been identified as a cytoplasmic adenosine and cyclic AMP binding protein. In order to examine the relationship between the adenosine and cyclic AMP binding sites on this enzyme we have explored the use of 8-azido analogues of adenosine and cyclic AMP as photoaffinity reagents for covalently labelling AdoHcyase purified from human placenta. 8-Azidoadenosine (8-N3-Ado), like adenosine, inactivated AdoHcyase, and the rate of inactivation was greatly increased by periodate oxidation. In addition, 8-N3-Ado was found to participate in the first step in the catalytic mechanism for AdoHcyase, resulting in conversion of enzyme-bound NAD+ to NADH, although it was not a substrate for the full enzyme-catalysed reaction. Radioactively labelled 8-N3-Ado, its periodate-oxidized derivative and 8-azidoadenosine 3', 5'-phosphate (8-N3-cAMP) bound specifically to adenosine binding sites on AdoHcyase and, after irradiation, became covalently linked to the enzyme. Photoaffinity-labelled enzyme could be precipitated by monoclonal antibody to human AdoHcyase. Two observations suggested that cyclic AMP and adenosine bind to the same sites on AdoHcyase. First cyclic AMP and adenosine each blocked binding of both radioactively labelled 8-N3-Ado and 8-N3-cAMP, and second, digestion with V8 proteinase generated identical patterns of peptides from AdoHcyase that had been photolabelled with [32P]8-N3-cAMP and [3H]8-N3-Ado. Binding sites for cyclic AMP on AdoHcyase were found to differ functionally and structurally from cyclic AMP binding sites on the R1 regulatory subunit of cyclic AMP-dependent protein kinase.

Phosphorylative neuromodulation of the regulatory subunit of cyclic AMP-dependent protein kinase type II in skeletal muscle

The phosphorylative neuromodulation of the regulatory subunit of protein kinase type II (R-II) in cytosolic fractions from denervated and sham-operated, contralateral soleus muscles of the rat was evaluated. The denervation-induced increase in the 32P-phosphorylation of R-II is not related to an increased dephosphorylation by cation-dependent or cation-independent protein phosphatases in the cytosolic fractions. The level of 32P-phosphorylation of an exogenous heptapeptide substrate (Kemptide) by dissociated catalytic subunits of cyclic AMP-dependent protein kinase in cytosolic fractions from denervated and sham-operated solei did not differ. Also, no change in the concentration of cytosolic R-II assessed by competitive enzyme-linked immunosorbent assays (ELISA) was found after denervation. However, the in vitro 32P-phosphorylation of R-II in these samples was increased. Taken together, our results suggest that the increased availability of autophosphorylatable sites reflects an in vivo modulation of R-II phosphorylation rather than a significant change in total R-II content.

Increased phosphorylation of nuclear protein in myonuclei isolated from denervated skeletal muscle

The slow-twitch soleus muscle of the rat hindlimb was denervated by cutting the sciatic nerve in the midthigh on one side. A sham operation was performed on the contralateral side to provide a control soleus muscle. At 24, 48, 72, and 120 hours after these surgical procedures, the animals were sacrificed and the nuclei which were isolated from these muscles incubated in a phosphorylating medium containing [gamma-32P]ATP. At the 48 hour denervation period only, the in vitro phosphorylation of TCA-precipitable nuclear proteins was significantly increased compared to control values. Resolution of the SDS-solubilized nuclear proteins by polyacrylamide gel electrophoresis revealed that the increased phosphorylation was more marked in some phosphoproteins than others. The significance of these early denervation changes are discussed with respect to increases in the activities of the nuclear RNA polymerases and changes in the phosphorylation of cytosolic protein.