Regulation of adenylyl cyclase activity in human peripheral blood mononuclear cells: effects of protein kinase inhibitors and of a calcium ionophore

Increase of cytosolic calcium induced by trichosanthin suppresses cAMP/PKC levels through the inhibition of adenylyl cyclase activity in HeLa cells

Increase of cytosolic free calcium played a pivotal role in apoptotic cells induced by trichosanthin. However, little is known about the influence of cytosolic calcium increase on adenylyl cyclase activity and intracellular cAMP signaling pathway in HeLa cells. The present study showed that an influx of extracellular Ca2+ initiated by trichosanthin was required for the suppression of adenylyl cyclase activity and decrease of intracellular cAMP level. Furthermore, this inhibition was abolished by activation of PKC rather than PKA. Therefore, our results suggested that increase of cytosolic calcium induced by trichosanthin inhibits cAMP levels via suppression of adenylyl cyclase activity.

Cyclic AMP-dependent activation of rhodopsin gene transcription in cultured retinal precursor cells of chicken embryo

The present study describes a robust 50-fold increase in rhodopsin gene transcription by cAMP in cultured retinal precursor cells of chicken embryo. Retinal cells isolated at embryonic day 8 (E8) and cultured for 3 days in serum-supplemented medium differentiated mostly into red-sensitive cones and to a lesser degree into green-sensitive cones, as indicated by real-time RT-PCR quantification of each specific opsin mRNA. In contrast, both rhodopsin mRNA concentration and rhodopsin gene promoter activity required the presence of cAMP-increasing agents [forskolin and 3-isobutyl-1-methylxanthine (IBMX)] to reach significant levels. This response was rod-specific and was sufficient to activate rhodopsin gene transcription in serum-free medium. The increase in rhodopsin mRNA levels evoked by a series of cAMP analogs suggested the response was mediated by protein kinase A, not by EPAC. Membrane depolarization by high KCl concentration also increased rhodopsin mRNA levels and this response was strongly potentiated by IBMX. The rhodopsin gene response to cAMP-increasing agents was developmentally gated between E6 and E7. Rod-specific transducin alpha subunit mRNA levels also increased up to 50-fold in response to forskolin and IBMX, while rod-specific phosphodiesterase-VI and rod arrestin transcripts increased 3- to 10-fold. These results suggest a cAMP-mediated signaling pathway may play a role in rod differentiation.

The cAMP response to isoprenaline in mononuclear cells is markedly increased in the presence of platelets

We have previously shown that the addition of platelets to mononuclear cells (MNC) increases cAMP in MNC. This response may be of interest because the physical interaction between platelets and MNC plays an important role in the inflammatory process. We have now demonstrated that the addition of both isoprenaline and platelets to MNC resulted in a marked amplification of the cAMP response. Prostaglandins, ATP and adenosine and the P-selectin ligand PSGL-1 could not account for the response. No substance was found in the supernatant that could increase cAMP in MNC. W7, a Ca(2+)-calmodulin inhibitor and the addition of EDTA reduced the response to both platelets and isoprenaline. Furthermore, we have demonstrated that mRNA for type I adenylyl cyclase, which is sensitive to Ca(2+), is present in MNC. No increase in Ca(2+) in the cytoplasma in MNC was recorded, however, by quantitative fluorescence microscopy after addition of platelets to MNC. It is possible that there are small increments in Ca(2+) at the binding sites, which we were unable to detect by our technique. Alternatively the binding of platelets to MNC may induce intermolecular interactions in the cell membrane which may facilitate the synthesis of cAMP.