Cellular Cyclic AMP

O Neill, C., Riddle, P., Rosengurt, E. (1985). Stimulating the proliferation of quiescent 3T3 fibroblasts by peptide growth factors or by agents which elevate cellular cyclic AMP level has opposite effects on motility. Expt. Cell Res. 156.65-78. 

The TET-induced inhibitory influence on cyclic 3 ,5 -AMP phosphodiesterase (PDE) activities precedes edema formation in the rat brain [74]. lb clarify the mechanism of the protective action of EGb against TET-toxidty in rats, in vitro and ex vivo effects of EGb on PDE activities of cerebral tissue were investigated [75]. Higher concentrations of EGb (5-250 mg/L) inhibited the PDE activity in the brain in normal rats, whereas lower concentrations (0.25-4.0 mg/L) of EGb enhanced the activity of the enzyme. The inhbitory effect of TET on the high affinity PDE activity (measured with 0.25 ftM cyclic AMP) of the brain was diminished in the presence of low EGb concentrations. Furthermore, preventive and curative treatment of 1 El-poisoned rats with EGb (100 mg/kg, p.o., for 7 days) prevented both the formation of edema and the fall of PDE activity induced by TET alone. These results suggested the antiedema action of EGb might be partly associated with its modulating influences on cellular cyclic AMP levels via activation of membrane-bound PDE. 

The addition of phosphodiesterase inhibitors such as isobutylmethylxanthine (IBMX) and cyclic AMP analogs such as the 8-bromo- and dibutyryl-derivatives resulted in a stimulation of synthesis which could not be blocked by the co-addition of opiates. This implies that opiates are exerting their effect at the level of adenylate cyclase activation. The actual effect of these drugs on cellular cyclic AMP levels was determined by radioimmunoassay as shown in Table III. 

It is incorrect to state that desensitization does not occur in the dopaminergic response of bovine parathyroid cells. We did not, in fact, examine this point directly. Desensitization, however, probably actually does occur at more than one locus in this system. First, the secretory response of the parathyroid cell rapidly becomes refractory to agents such as dopamine and isoproterenol which produce large elevations in cyclic AMP (see ref. 2). Secondly, there is a progressive decrease in cellular cyclic AMP despite the continued presence of dopamine (see Figure 5 in our manuscript) possibly due to desensitization of the receptor-adenylate cyclase compex. 

Principal cells in the renal collecting duct have V2 receptors on their basolateral membranes that couple to to stimulate adenylyl cyclase activity (Figure 29—3). The resulting increase in cellular cyclic AMP and PKA activity triggers increased insertion of water channel—containing vesicles (WCVs) into the apical membrane and decreased endocytosis of WCVs from the apical membrane. The distribution of WCVs between the cytosolic compartment and the apical membrane compartment thus is shifted in favor of the apical membrane compartment. Because WCVs contain preformed aquaporin 2 water channels, their net shift into apical membranes in response to V2 receptor stimulation greatly increases water permeability of the apical membrane. 

The cyclic AMP-phosphodiesterase (PDE) inhibitors reduce the degradation of cellular cyclic AMP the consequences are generally those of elevated cyclic AMP, much as would occur in response to a stimulator of adenylyl cyclase activity. In the heart, the result is positive inotropism. 

The expression of many immediate-early genes, such as c-fos and c-jun, can be induced by 12-0-tetradecanoylphorbol-13-acetate. 12-0-Tetradecan-oylphorbol-13-acetate increased c-fos mRNA, cellular cyclic AMP, and protein kinase A activity in the first 30 min with similar inductive time courses (Huang et al. 1999). Treatment of NIH 3T3 cells with N-[2-(p-bromodimamylaniino)ethyl] -5-iso-... 

Elevation of cellular cyclic-AMP levels, inhibition of phosphodiesterase Quercetin io m 26,28... 

Depletion of ATP in the cells prevents maintenance of the membrane potential, inhibits the functioning of ion pumps, and attenuates cellular signal transduction (e.g., formation of second messengers such as inositol phos phates or cyclic AMP). A marked ATP depletion ultimately impairs the activ-itv of the cell and leads to ceil death. 

FIGURE 2.6 Production of cyclic AMP from ATP by the enzyme adenylate cyclase. Cyclic AMP is a ubiquitous second messenger in cells activating numerous cellular pathways. The adenylate cyclase is activated by the a subunit of Gs-protein and inhibited by the a-subunit of Gj-protein. Cyclic AMP is degraded by phosphodiesterases in the cell. 

Group II assays consist of those monitoring cellular second messengers. Thus, activation of receptors to cause Gs-protein activation of adenylate cyclase will lead to elevation of cytosolic or extracellularly secreted cyclic AMP. This second messenger phosphorylates numerous cyclic AMP-dependent protein kinases, which go on to phosphorylate metabolic enzymes and transport and regulatory proteins (see Chapter 2). Cyclic AMP can be detected either radiometrically or with fluorescent probe technology. 

Second messenger, these are molecules produced by cellular effectors that go on to activate other biochemical processes in the cell. Some examples of second messengers are cyclic AMP, inositol triphosphate, arachidonic acid, and calcium ion (see Chapter 2.2). 

Cyclic nucleotides (cAMP and cGMP) are formed enzymatically from the corresponding triphosphates. As ubiquitous second messengers, they mediate many cellular functions which are initiated by first (extracellular) messengers. Their prime targets in eucaryotic cells are protein kinases ( cyclic AMP-dependent protein kinase, cyclic GMP-dependent protein kinase), ion channels and ensymes. 

In view of the known cellular actions of DA, such as increased K+ efflux and reduced Ca + currents associated with Dj receptor activation in cell lines, inhibition would be the expected response to DA, especially as cyclic AMP, which is increased by Dj receptor activation also inhibits striatal neurons. In fact although many DA synaptic effects are blocked by Dj antagonists like haloperidol, the role of Di receptors should not be overlooked. 

The effect of sphingosine on other enzymes may also contribute to its apoptotic effect. These include the inhibition of calcium/calmodulin-requiring enzymes and DNA primase and the stimulation of casein kinase II and several unidentified kinases (Alessenko, 2000). In addition, sphingosine can increase the cellular concentration of cyclic AMP, which is inhibitory for proliferation in many cell types (Pyne and Pyne, 1996). 

---