Mechanisms of bombesin-induced arachidonate mobilization in Swiss 3T3 fibroblasts

Regulation of colonic ion transport by GRP. II. GRP modulates the epithelial response to PGE2

The purpose of this study was to examine the potential modulatory effects of gastrin-releasing peptide (GRP) on prostaglandin (PG) E2-stimulated electrolyte transport across the distal colon epithelium. In an earlier study, PGE2 was shown to reduce net Cl absorption without altering the serosal-to-mucosal unidirectional Cl flux in porcine distal colon (19). In the present study, tissues were pretreated with serosal or mucosal GRP and subsequently stimulated with PGE2. The resulting increase in short-circuit current (ISC) was 152% (serosal GRP) and 49% (mucosal GRP) greater than control PGE2 responses alone. Serosal, but not mucosal, GRP also enhanced the ISC response to vasoactive intestinal peptide. On the basis of flux measurements, the combined effects of serosal GRP and PGE2 resulted in the activation of a transcellular pathway for Cl secretion, which was not activated by either mediator alone. The time course of the PGE2 response was also affected by GRP. Serosal GRP shortened the time to maximum ISC by 35%, whereas mucosal peptide lengthened the time to maximum ISC by 68% These results suggest that GRP acts as a modulator of PG action on electrolyte transport in the distal colon.

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.

Stimulation of arachidonic-acid release from Swiss 3T3 cells by recombinant basic fibroblast growth factor: independence from phosphoinositide turnover

In this study we have attempted to characterize the mechanism of recombinant bovine basic fibroblast growth factor (rbFGF)-induced release of arachidonic acid from prelabelled Swiss 3T3 fibroblasts. Recombinant bFGF caused the release of [3H]arachidonic acid from metabolically labelled cells in a dose- and time-dependent manner. This effect was maximal with 10 ng rbFGF/ml and became significant after a 30-min incubation. Although rbFGF was able to cause a modest increase in total inositol phosphate accumulation, an examination of the time-course of the latter effect revealed that enhanced [3H]arachidonic-acid release could not have been derived from phosphoinositide metabolism. Evidence suggesting that rbFGF-induced release of [3H]arachidonic acid was being mediated via a PLA2 pathway was obtained by pharmacological antagonism using mepacrine, a putative PLA2 inhibitor. Moreover, treatment of cells with neomycin failed to attenuate rbFGF-mediated release of [3H]arachidonic acid. Chelation of extracellular calcium by EGTA was found to abrogate rbFGF-induced liberation of [3H]arachidonic add. Down-regulation of protein kinase C (PKC) by prolonged treatment of cells with the phorbol ester, PMA, was observed to have no effect on the action of rbFGF on [3H]arachidonic add release from Swiss 3T3 fibroblasts. While rbFGF was found to cause the indomethacin-sensitive production of prostaglandin E2 (PGE2) in a dose-dependent manner, this effect was independent of rbFGF-induced reinitiation of DNA synthesis. Clearly, the effect of rbFGF on cellular DNA synthesis was being mediated independently of PGE2 biosynthesis. We discuss the potential importance of the PLA2-signalling pathway in the mechanism of action of fibroblast growth factors.

Mechanisms of platelet-derived growth factor-induced arachidonic acid release in Swiss 3T3 fibroblasts: the role of a localized increase in free Ca2+ concentration beneath the plasma membrane and the activation of protein kinase C

Stimulation of Swiss 3T3 fibroblasts with platelet-derived growth factor (PDGF) results in a transient increase in intracellular free Ca2+ concentration ([Ca2+]i) and a phospholipase A2 (PLA2)-dependent release of arachidonic acid (AA) of 500% over control values. In the absence of extracellular Ca2+, both the PDGF-induced transient increase in [Ca2+]i and AA release were markedly reduced. Buffering the increase in [Ca2+]i with EGTA, introduced into the cells in the form of EGTA acetoxymethylester (AM), abolished the PDGF-induced transient increase in [Ca2+]i, but potentiated the AA release by at least 2-fold compared to cells without EGTA. The EGTA potentiated PDGF-induced AA release was sensitive to extracellular Ca2+ and inhibited to various degrees by both receptor-mediated as well as voltage-operated Ca2+ channel blockers, suggesting that the release of AA may be tightly coupled to the influx of Ca2+. Activation of protein kinase C (PKC) by the phorbol ester, phorbol 12-myristate 13-acetate (TPA) had little effect in promoting AA release by itself. Down-regulation of PKC in Swiss 3T3 fibroblasts by chronic stimulation with 300 nM TPA for 24 h, markedly inhibited the PDGF-stimulated AA release in both the EGTA-loaded and control cells. In conditions where PDGF-induced AA release was inhibited or potentiated, the production of inositol phosphates was unaffected. Thus, PDGF-induced PLA2 dependent AA release in Swiss 3T3 fibroblast is regulated by both PKC-dependent and -independent mechanisms, and is activated by high concentrations of free Ca2+ in the microenvironment beneath the plasma membrane during Ca2+ influx via plasma-membrane Ca2+ channels, despite buffering by EGTA of [Ca2+]i in the bulk cytoplasm of the cell.

1,25-Dihydroxyvitamin D-3 induces arachidonate mobilization in embryonic chick myoblasts

1,25-Dihydroxyvitamin D-3 (1,25(OH)2D3) which activates the phospholipase C (PLC)-protein kinase C (PKC) signalling pathway, induces within 1 min a dose-dependent (10(-11)-10(-7) M) increase in the release of [3H]arachidonic acid ([3H]AA) from prelabeled embryonic chick myoblasts. The response is dependent on extracellular calcium, since it is suppressed by EGTA and nifedipine, a Ca(2+)-channel blocker, and is mimicked by the calcium ionophore A23187. 1,25(OH)2D3-induced release of [3H]AA is not affected by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis. 12-o-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, induces an extracellular Ca(2+)-independent release of [3H]AA and amplifies the release of AA stimulated by 1,25(OH)2D3. 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H7), a PKC inhibitor, markedly suppressed TPA as well as 1,25(OH)2D3-induced [3H]AA release. Down-regulation of cellular PKC abolishes the effect of the phorbol ester, and partially inhibits 1,25(OH)2D3-induced [3H]AA release. Temporally correlated with AA liberation, the hormone increases the formation of lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) and decreases the cellular content of PC and PE. These results indicate that part of AA release by 1,25(OH)2D3 derives from PLA2 activation and that the effects of the hormone are mediated by PKC in a mode independent of phosphoinositide hydrolysis by PLC.

Signal transduction pathways of angiotensin II--induced c-fos gene expression in cardiac myocytes in vitro. Roles of phospholipid-derived second messengers

Angiotensin II (Ang II) causes a rapid induction of immediate-early genes and hypertrophy in the cardiac myocyte. However, the signaling mechanism of Ang II-induced immediate-early gene expression in cardiac myocytes has not been characterized. Therefore, we examined signal transduction of Ang II in neonatal rat cardiac myocytes, using c-fos gene expression as a model system. Transient transfection of c-fos reporter gene constructs indicated that the serum response element is not only required but also sufficient for Ang II-induced activation of the c-fos promoter. Ang II is known to cause an increase in [Ca2+]i. We found that Ang II also causes a small increase in cAMP in cardiac myocytes. However, the Ca2+/cAMP response element of the c-fos gene was not sufficient to confer Ang II responsiveness to the c-fos promoter, and inhibitors of protein kinase A had no effects on Ang II-induced c-fos expression. On the other hand, chelating intracellular Ca2+ with BAPTA-AM inhibited Ang II-induced c-fos expression in a dose-dependent manner, suggesting that Ca2+ is required for Ang II-induced signaling. Measurements of phospholipid-derived second messengers revealed that Ang II increased production of inositol trisphosphate, diacylglycerol, phosphatidic acid, and arachidonic acids, resulting in a sustained increase in protein kinase C activity. This and other evidence suggest that Ang II activates phospholipase C, phospholipase D, and possibly phospholipase A2. All of these second-messenger systems are activated through the AT1 receptor. Pharmacological inhibition of phospholipase C or downregulation of protein kinase C significantly suppressed Ang II-induced c-fos expression. In conclusion, Ang II activates multiple phospholipid-derived second-messenger systems via the AT1 receptor in cardiac myocytes. Among these second-messenger systems, phospholipase C and protein kinase C seem essential for Ang II-induced c-fos gene expression, whereas Ca2+ may play a permissive role. Finally, the "Ang II response element" of the c-fos gene maps to the protein kinase C-dependent portion of the serum response element.

Involvement of G proteins, cytoplasmic calcium, phospholipases, phospholipid-derived second messengers, and protein kinases in signal transduction from mitogenic cell surface receptors

Some putative mitogenic signal transduction mechanisms involving G proteins, calcium, phospholipases, and protein kinases have been discussed. Several elements in this signal transduction scheme are not yet well understood and require further experimental investigation. With regard to the heptahelix receptors, exactly how do they activate PLA2? Is PLA2 activation linked to mitogenic pathways? Is this via stimulation of protein kinase C or perhaps another mechanism? How do heptahelix receptors activate tyrosine phosphorylation, and is it important in their ability to stimulate cell growth? With regard to the various phospholipases that are thought to be regulated by receptor-mediated stimuli, only PI-PLC beta and PI-PLC gamma are well characterized. PLA2, PC-PLD, and PC-PLC require further study in regard to determination of molecular structure and elucidation of mechanisms of phospholipase activation (e.g., what are the molecular mechanisms whereby tyrosine kinases and Ras affect PC-PLC?). The protein kinase C dependent and protein kinase C independent mechanisms that enable mitogenic stimuli to activate the Erk/MAP kinase are enigmatic at this time. How Raf-1 activates SRE-containing gene promoters (such as the fos promoter) is also not known. However, given the current rapid rate of progress in this field, it is likely that a much more complete understanding of the mitogenic signal transduction process will soon be obtained.