Calmodulin in human serum and the specific release of calmodulin from calmodulin-rich platelets

Regulation of the calcium signal by calmodulin

Stimulus-response coupling mediated by calmodulin involves several steps: a transitory increase in calcium concentration from 0.1 to 10 microM, induced by external stimuli; interaction of calcium with calmodulin, accompanied by stepwise structural transitions; the coordinated interaction with and activation of the many calmodulin-regulated enzymes and proteins. The binding of calcium to calmodulin is a cooperative and selective process that is modulated by magnesium. At physiological ionic strength, and only in the presence of magnesium, a large difference is seen between the affinities of sites III and IV (0.09 X 10(6) M-1) and sites I and II (0.0007 X 10(6) M-1) for calcium. This difference, together with the positive cooperativity previously observed, explains the stepwise conformational changes induced by calcium. The interaction of calmodulin with its target proteins requires the integrity of different portions of the calmodulin molecule. Calmodulin-regulated enzymes can be divided into three classes according to their abilities to bind with and to be activated by calmodulin fragments: enzymes which are activated by the C-terminal fragment, such as the Ca2+-ATPase and phosphorylase kinase; enzymes which require both halves of the molecule, such as cyclic AMP phosphodiesterase and myosin light chain kinase; and enzymes whose interaction with calmodulin fragments is too weak to be detected by activation, such as calcineurin and the multiprotein kinase. Thus different enzymes may be activated by different calmodulin conformers and the stepwise changes exhibited by calmodulin at different calcium levels can be used to regulate different metabolic pathways.

Calcium in the action of growth factors

The proliferation of cells in vivo and in culture is regulated by polypeptide growth factors, such as epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Binding of growth factors to their specific cell-surface receptors initiates a cascade of biochemical events in the cell which ultimately leads to DNA synthesis and cell division. Immediate consequences of receptor activation include tyrosine-specific protein phosphorylations, a sustained increase in cytoplasmic pH and a transient rise in cytoplasmic free Ca2+. The PDGF-induced Ca2+ signal is due to Ca2+ release from intracellular stores, whereas EGF seems to activate a voltage-independent Ca2+ channel in the plasma membrane. Monoclonal antibodies to the EGF receptor that stimulate the tyrosine-specific protein kinase fail to raise [Ca2+]i and are not mitogenic for quiescent cells. These results suggest that activation of the EGF receptor tyrosine kinase is not sufficient for the induction of a Ca2+ signal, and that the rise in [Ca2+]i is indispensable for cell proliferation.

A shear-restricted pathway of platelet procoagulant activity is regulated by IQGAP1

Circulating blood platelets regulate the initial phase of the hemostatic response through adhesive and aggregatory events and by providing the necessary procoagulant surface for prothrombinase complex assembly and thrombin generation. The signaling pathway(s) that regulate platelet procoagulant activity are largely unknown, although they are distinct from platelet aggregatory signals linked to fibrinogen ligation to the conformationally active alpha(IIB)beta(3) integrin. We describe a novel intracellular signaling mechanism involving platelet IQGAP1 that specifically regulates the development of platelet procoagulant activity under conditions of mechanical shear stress. Murine platelets that are deficient in IQGAP1 demonstrate increased prothrombinase activity compared with wild-type littermate controls when activated by a physiological shear stress of 16 dynes/cm(2) (shear rates of 1600 s(-1)) (p < 0.0001), corresponding to approximately 2.5 times the normal shear stress, or approximately 40% degree of stenosis in coronary arteries. The exaggerated prothrombinase activity is not associated with enhanced platelet microvesiculation (cytoskeletal proteolysis) and occurs independently of the intracellular calcium release, [Ca(2+)](i), but it is specifically coupled to the alpha-granule exocytic pathway without concomitant effects on aminophospholipid exposure. These observations identify platelet IQGAP1 as an important modulator of normal hemostasis and as an appropriate pharmacological target for control of platelet procoagulant function.

Tritiated thymidine and bromodeoxyuridine double-labelling studies on growth factors and oral epithelial proliferation in the mouse

Mouse tongue epithelium is characterized by a circadian variation in the number of cells undergoing DNA synthesis. Groups of male BDF1 mice were followed over 48 h and a double-labelling method with tritiated thymidine and bromodeoxyuridine used to determine S-phase labelling indices, together with cell influx to and cell efflux from S, at 4-hourly time points. Control animals exhibited diurnal peaks in labelling index at 03:00 with trough activity 12 h later at 15:00. Cell influx peaked at 23:00 with troughs occurring between 11:00 to 15:00. Peak cell efflux occurred at 07:00 with trough activity at 19:00. Animals injected with epidermal growth factor at 05:00 demonstrated a significant fall in both influx and efflux throughout the 48-h period (P < 0.001), but with preservation of labelling indices, suggesting a slower transit of cells through S-phase, whereas epidermal growth factor injected at 15:00 only produced a significant rise in cell-efflux values. Adrenergic stimulation by intravenous phenylephrine/isoprenaline injection at both 05:00 and 15:00 resulted in a significant rise in cell efflux (P < 0.001), although there was also a rise in labelling index in the 15:00 group (P < 0.001). Animals injected with calmodulin at 05:00 demonstrated a significant reduction in labelling index throughout the 48-h period (P < 0.001), but maintained control values for cell influx and efflux, suggesting faster transit of cells through S. Calmodulin injection at 15:00 produced only a significant reduction in cell influx (P < 0.001). Administration of exogenous growth factors significantly alters the normal rhythmical proliferation of oral epithelial cells in a mouse model. These effects appear to be both growth factor- and time-dependent, and may have both physiological and pathological implications.

Additive effects of extra cellular matrix proteins and platelet derived mitogens on human retinal pigment epithelial cell proliferation and contraction

PURPOSE

In proliferative vitreoretinopathy (PVR) retinal pigment epithelial cells (RPE) cells are thought to synthesize and interact with extracellular matrix (ECM) proteins to form fibrocellular membranes attached to the retina, which the cells then progressively contract detaching the retina. Haemorrhage into the eye is an exacerbating factor in the pathology. To investigate some of the possible interactions between ECM proteins, platelet mitogens and RPE cells in this study, we examined the combined effect of platelet derived mitogens and ECM proteins on RPE cell proliferation and contraction.

METHODS

Cells were cultured on a range of individual ECM proteins as well as on the ECM deposited by normal vitreous fluid and exposed to platelet mitogens. Effects on cell proliferation and cell detachment from these substrates and tissue culture plastic were examined.

RESULTS

We report additive/synergistic effects of platelet mitogens (PDGF and TGFb1) as well as bFGF, with ECM proteins (laminin, fibronectin, collagen 1 and vitreous-deposited ECM) on RPE proliferation. Further we report stimulation of RPE cell contraction on vitreous proteins when exposed to serum prepared from platelet-rich plasma. In this context it was noticeable that it was cells grown on vitreous matrix plus pigment rather than cells grown on clear vitreous that exhibited this behaviour.

CONCLUSIONS

This study supports a combined action of platelet mitogens and matrix proteins in inducing RPE cell proliferation and contractility and provides a simple in vitro model of some of the late stages of PVR.

Raised urinary calmodulin levels in idiopathic myelofibrosis: possible implications for the aetiology of fibrosis

Platelet growth factors (e.g. PDGF and TGF-beta) are thought to be pathogenetically important in the stromal reaction characteristic of idiopathic myelofibrosis (IM). We have investigated a possible pathogenetic role for a further platelet mitogen, calmodulin. Platelets are rich in calmodulin, of which 30-40% is releasable with a time course that differs from alpha-granule proteins. In IM urinary calmodulin concentrations were 3-fold those of the normals controls. We suggest that an abnormal release of calmodulin may occur from platelets/megakaryocytes in patients with IM, and that calmodulin should be considered, along with other growth factors, in the pathogenesis of marrow fibrosis.

A calmodulin-like protein as an extracellular mitogen for the keratinocyte

This study investigated the importance of extracellular calmodulin to the proliferation of the keratinocyte. Normal keratinocytes in culture produced a calmodulin-like protein in their culture media, the level of which increased abruptly and transiently during their growth. This protein was calmodulin-like, in that it specifically bound to a calmodulin affinity column, exhibited calmodulin-like immunoreactivity in both an ELISA and on immunoblots when immunostained with a monoclonal antibody against calmodulin, had an apparent M(r) between 18,000 and 20,000, and stimulated activity in a calmodulin-dependent phosphodiesterase enzyme assay. Addition of exogenous pure calmodulin was of no further mitogenic benefit to the keratinocytes, and slightly reduced proliferation under the culture conditions used. However, addition of either a neutralizing antibody to calmodulin, or W7-agarose, to the culture media of proliferating cells markedly inhibited their proliferation. Accordingly, a calmodulin-like protein was found to satisfy all but one of the criteria for its action as an autocrine growth factor for the keratinocyte. We propose that the lack of mitogenic response to calmodulin in vitro is due to the cell meeting its own requirement for extracellular calmodulin.

Mitogenic role for extracellular calmodulin-like activity in normal human umbilical vein endothelial cells

Normal human umbilical vein endothelial cells cultured on gelatin-coated plastic dishes were found to produce a protein in their media which had calmodulin-like immunoreactivity and biological activity. Further identification of the protein was achieved by examining the incorporation of 14C leucine into protein found in the conditioned medium. Cells produced 14C labelled protein in their medium which specifically bound to an affinity column for calmodulin. This latter material stimulated calmodulin dependent phosphodiesterase activity in vitro and this stimulation was inhibited by the addition of the calmodulin antagonist W7. The presence of calmodulin-like activity and immunoreactivity in the media varied as the cells grew from low to high density, a peak of extracellular calmodulin-like activity preceding an increase in cell number. Extracellular calmodulin-like activity did not correlate with the presence of lactate dehydrogenase in the medium. The addition of pure pig brain calmodulin affected the rate of cell proliferation; significant proliferation to pure calmodulin was only seen in cells at low density, at higher density calmodulin either had no effect or inhibited proliferation. Inhibition of extracellular calmodulin activity by a calmodulin antagonist immobilized on agarose beads, or by an antibody to calmodulin significantly decreased proliferation in all dividing cultures. Taken together this data suggests that, in vitro, calmodulin, or a very closely related protein, influences endothelial cell proliferation through an autocrine mechanism.

Elevated platelet-free calcium in uraemia

Bleeding complications in uraemia are not uncommon. The pathogenesis of haemorrhage in uraemia is still a matter of controversy and the pattern of bleeding suggests a defect of primary haemostasis. Platelet aggregation and biochemistry, including calcium levels, have been studied; however, the results are controversial. We have examined platelet aggregation, platelet-free calcium and calmodulin in platelet-rich plasma because of the significant role of calcium and calmodulin in regulating platelet and other cells' functions. Platelet aggregation in uraemic subjects was similar to that of controls. Platelet basal free cytosolic calcium and platelet calcium in response to 10 microM Ca++ ionophore A23187 in eight subjects with uraemia were 117 +/- 33 nM and 2025 +/- 398 nM (mean +/- SEM) respectively. By contrast in seven matched healthy controls basal calcium and ionophore-stimulated calcium values were 47 +/- 14 nM and 1354 +/- 414 nM, significantly less than in the patients with uraemia (P less than 0.05). The sensitivity of uraemic platelets to A23187 was similar to that of controls. Calmodulin activity in platelet-rich plasma of 12 subjects with uraemia showed no significant difference from that of controls [1.86 +/- 0.29 micrograms/ml (mean +/- SEM) and 2.0 +/- 0.37 micrograms/ml (mean +/- SEM) respectively]. We conclude that despite elevation of platelet calcium in uraemia, which may be due to a plasma factor such as parathyroid hormone, platelet aggregation is normal and bleeding in uraemia is more likely to be due to other factors, including the effect of reduced haematocrit on platelet endothelial interaction. Disturbances in platelet calcium cannot explain the bleeding manifestations in uraemia but warrant further investigation in order to identify the pathogenic mechanisms responsible.

Platelet calmodulin correlates with platelet turnover

We measured the calmodulin content in platelets in 13 normal persons and in 62 patients with hematological diseases. The level of platelet calmodulin was higher in patients with idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus, myeloproliferative disorders, acute leukemia in a recovery phase, aplastic anemia, thrombosis and hypersplenism as compared to the controls. Among the patients with ITP, calmodulin was lower in responders than in nonresponders and those at the initial diagnosis. We also measured the volume, life-span and aggregation of the platelets and demonstrated a significant relationship between the calmodulin level and the platelet volume, and a negative relationship between the calmodulin level and platelet life-span, there was no correlation between the calmodulin level and platelet aggregation. We thus conclude that platelet calmodulin is inversely correlated with platelet turnover.

An extracellular role for calmodulin-like activity in cell proliferation

1. Addition of extracellular pure pig brain calmodulin was found to modulate DNA synthesis and cell proliferation in K562 human leukaemic lymphocytes. At lower cell densities calmodulin significantly stimulated [3H]thymidine uptake; at higher densities it decreased it. 2. A protein biochemically indistinguishable from calmodulin was detected in the cell-conditioned media of rapidly dividing K562 cells. The concentration of calmodulin-like activity found in the conditioned media of these and a range of other normal and neoplastic cells (250-1636 ng/ml) was of the same order as would stimulate DNA synthesis in subconfluent cells. 3. Amounts of extracellular calmodulin-like activity and immunoreactivity varied during cell growth from low to high density, a peak of extracellular calmodulin preceding DNA synthesis in synchronized K562 cells. Extracellular calmodulin concentrations did not correlate with the presence of lactate dehydrogenase in the medium. 4. Inhibition of extracellular calmodulin activity by calmodulin antagonist immobilized on agarose beads, or by antibody to calmodulin, significantly decreased DNA synthesis. 5. These data strongly suggest that calmodulin or a very closely related protein can influence mitosis through an extracellular mechanism.

Evidence for the involvement of calmodulin in natural cytotoxicity using a range of calmodulin antagonists of varying potency and improved specificity

The calmodulin antagonist W7 and 4 of its analogues were examined for their ability to inhibit human NK cell mediated cytotoxicity. With the exception of one of these compounds, which is extremely hydrophobic, there was a good correlation between the ability of drugs to inhibit human NK antitumour cytotoxicity and calmodulin-dependent phosphodiesterase activity in vitro. The most potent of the compounds, 5-iodo-1-C8, an analogue of W7, has an IC50 of 3 microM upon biological and biochemical assay. This particular compound is both more potent and specific than the parent compound W7, is non-toxic to cells over the range used and is also capable of inhibiting the biological activity of NK cells upon pre-treatment of the effector cells, inferring the mechanism of NK cytotoxicity to be calmodulin dependent.