A Novel Calcium-dependent Phosphorylation of a Ribosome-associated Protein

The S18 ribosomal protein is a putative substrate for Ca2+/calmodulin-activated protein kinase II

The delta-isoform of Ca(2+)/calmodulin-activated protein kinase II (CaMK II) is abundantly expressed in vascular smooth muscle, but relatively little is known about its regulation or its potential cellular substrates. There are few, if any, known substrates of CaMK II that are physiologically relevant in vascular smooth muscle cells. Studies presented earlier (Mishra-Gorur, K., Singer, H. A., and Castellot, J. J., Jr. (2002) Am. J. Pathol., in press) by our laboratory show an inhibitory effect of heparin on CaMK II phosphorylation and activity. During these studies we observed the specific co-immunoprecipitation of a 20-kDa protein with CaMK II. Purification and sequence analysis indicate that this protein is the S18 protein of the 40 S ribosome. S18 was found to be abundantly phosphorylated in response to serum treatment, and this effect was strongly inhibited by heparin. In addition, KN-93, a specific CaMK II inhibitor, blocks S18 phosphorylation in vascular smooth muscle cells; a concomitant 24% reduction in protein synthesis was observed. Taken together these data support the idea that S18 could be a novel substrate for CaMK II, thus providing a potential link between Ca(2+)-mobilizing agents and protein translation.

Activation of ERK by Ca2+ store depletion in rat liver epithelial cells

In rat liver epithelial (WB) cells, Ca2+ pool depletion induced by two independent methods resulted in activation of extracellular signal-regulated protein kinase (ERK). In the first method, Ca2+ pool depletion by thapsigargin increased the activity of ERK, even when rise in cytosolic Ca2+ was blocked with the Ca2+ chelator BAPTA-AM. For the second method, addition of extracellular EGTA at a concentration shown to deplete intracellular Ca2+ pools also increased ERK activity. In each instance, ERK activation, as measured by an immunocomplex kinase assay, was greatly reduced by the tyrosine kinase inhibitor genistein, suggesting that Ca2+ store depletion increased ERK activity through a tyrosine kinase pathway. The intracellular Ca2+-releasing agent thapsigargin increased Fyn activity, which was unaffected by BAPTA-AM pretreatment, suggesting that Fyn activity was unaffected by increased cytosolic free Ca2+. Furthermore, depletion of intracellular Ca2+ with EGTA caused inactivation of protein phosphatase 2A and protein tyrosine phosphatases. ANG II-induced activations of Fyn, Raf-1, and ERK were augmented in cells pretreated with BAPTA-AM, but ANG II-induced expression of the dual-specificity phosphatase mitogen-activated protein kinase phosphatase-1 was blocked by BAPTA-AM pretreatment. Together these results indicate that ERK activity is regulated by the balance of phosphorylation vs. dephosphorylation reactions in intact cells and that the amount of Ca2+ stored in intracellular pools plays an important role in this regulation.

Regulation of translational initiation during cellular responses to stress

Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.

Intracellular calcium mobilization suppresses the TNF-alpha-stimulated synthesis of PAI-1 in human endothelial cells. Indications that calcium acts at a translational level

We investigated in human umbilical vein endothelial cells (HUVECs) the interaction between the signaling pathways triggered by calcium mobilization and those affected by human recombinant tumor necrosis factor-alpha (TNF) on the expression of type-1 plasminogen activator inhibitor (PAI-1). Calcium ionophore A23187 alone exerted a modest increase (50%) on PAI-1 synthesis. TNF alone increased PAI-1 accumulation in the culture medium in a time- and dose-dependent fashion, but this increase was abolished when A23187 was added simultaneously with TNF. The downregulating effect of A23187 was not the result of impaired protein secretion, proteolysis, cytotoxicity, or an apoptotic process. A23187 did not decrease the TNF-enhanced PAI-1 mRNA level but did provoke a significant shift in the distribution pattern of PAI-1 transcripts by increasing the 2.3-kb relative to the 3.2-kb form. Comparable inhibitory effects on PAI-1 protein synthesis were observed when A23187 was added 7 hours after the onset of TNF stimulation, strongly suggesting a posttranscriptional inhibitory action of calcium signaling on TNF-stimulated PAI-1 synthesis. However, treatment with actinomycin D showed that PAI-1 mRNA stability was not altered by the various treatments. Chelation of extracellular calcium by EGTA did not prevent the A23187-induced inhibition of TNF-stimulated PAI-1 protein synthesis, emphasizing the role of internal calcium stores in the inhibition of PAI-1 synthesis. Sucrose gradient fractionation of cell lysates revealed that regardless of which treatment was used, both PAI-1 mRNA transcripts exhibited similar sedimentation profiles in the actively translating polysomal pool, suggesting that the A23187-induced shift had no functional consequence on translation. However, in TNF-stimulated cells, A23187 induced a higher proportion of PAI-1 mRNAs that sedimented in fractions corresponding to less dense polysomes, a phenomenon that usually reflects a slower initiation rate during mRNA translation. A23187 also abolished the increase in PAI-1 synthesis induced by recombinant human interleukin 1 beta, and thapsigargin exerted effects comparable to those of A23187 on PAI-1 synthesis in TNF-stimulated cells. It is proposed that in HUVECs, the A23187-induced release of calcium from endoplasmic stores suppresses at the translational level the increase in PAI-1 synthesis triggered by proinflammatory cytokines.

Long-term effects of calcium availability on prolactin and protein synthesis in human decidual cells

Ever since decidual cells were recognized as the source of decidual prolactin (dPRL), very few reports have dealt with the role of calcium (Ca2+) on dPRL synthesis and release. In a recent work, we described the presence of T-type Ca2+ channels in these cells, giving Ca(2+)-dependent action potentials. However, we failed to demonstrate any action of decidual cell Ca2+ modulation on acute dPRL release, but observed only long-term effects. We have now investigated these effects on decidual protein and dPRL synthesis after 24 h treatments. When Ca2+ channel blockers or EGTA (2 mM) were added to the culture medium, dPRL release and [3H] leucine incorporation into proteins decreased. Increasing external Ca2+ up to 2 mM instead of 0.8 mM or changing the external K+ concentration (30 mM instead of 5.6) had no consequence on dPRL release, whereas 2 mM of Ca2+ enhanced total protein synthesis. No toxicity was noted with these treatments. Finally a possible effect of Ca2+ modulation on dPRL synthesis was studied using [35S] methionine. The specific activity of [35S] methionine on dPRL was similar in control and treated cells (EGTA, 2 mM Ca2+, cobalt). These results support the idea that Ca2+ controls dPRL synthesis in decidual cells, acting only on general protein synthesis processes.

Calcium and signal transduction in plants

Environmental and hormonal signals control diverse physiological processes in plants. The mechanisms by which plant cells perceive and transduce these signals are poorly understood. Understanding biochemical and molecular events involved in signal transduction pathways has become one of the most active areas of plant research. Research during the last 15 years has established that Ca2+ acts as a messenger in transducing external signals. The evidence in support of Ca2+ as a messenger is unequivocal and fulfills all the requirements of a messenger. The role of Ca2+ becomes even more important because it is the only messenger known so far in plants. Since our last review on the Ca2+ messenger system in 1987, there has been tremendous progress in elucidating various aspects of Ca(2+) -signaling pathways in plants. These include demonstration of signal-induced changes in cytosolic Ca2+, calmodulin and calmodulin-like proteins, identification of different Ca2+ channels, characterization of Ca(2+) -dependent protein kinases (CDPKs) both at the biochemical and molecular levels, evidence for the presence of calmodulin-dependent protein kinases, and increased evidence in support of the role of inositol phospholipids in the Ca(2+) -signaling system. Despite the progress in Ca2+ research in plants, it is still in its infancy and much more needs to be done to understand the precise mechanisms by which Ca2+ regulates a wide variety of physiological processes. The purpose of this review is to summarize some of these recent developments in Ca2+ research as it relates to signal transduction in plants.