Effect of exogenous calmodulin on lymphocyte proliferation in normal subjects

Extracellular calmodulin: A polypeptide signal in plants?

Traditionally, calmodulin (CaM) was thought to be a multi-functional receptor for intracellular Ca(2+) signals. But in the last ten years, it was found that CaM also exists and acts extracellularly in animal and plant cells to regulate many important physiological functions. Laboratory studies by the authors showed that extracellular CaM in plant cells can stimulate the proliferation of suspension cultured cell and protoplast; regulate pollen germination and pollen tube elongation, and stimulate the light-independent gene expression of Rubisco small subunit (rbcS). Furthermore, we defined the trans-membrane and intracellular signal transduction pathways for extracellular CaM by using a pollen system. The components in this pathway include heterotrimeric G-protein, phospholipase C, IP(3), calcium signal and protein phosphorylation etc. Based on our findings, we suggest that extracellular CaM is a polypeptide signal in plants. This idea strongly argues against the traditional concept that there is no intercellular polypeptide signal in plants.

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.

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.

Inhibition of HIV-1, HIV-2 and SIV envelope glycoprotein-mediated cell fusion by calmodulin

Calmodulin, an EF-hand protein, inhibited the fusion between CD4+ human cells and cells stably expressing HIV-1 envelope proteins. Fusion was also inhibited when HIV-1, HIV-2 or SIV envelope glycoproteins were expressed by vaccinia virus (VV) recombinants, but calmodulin did not inhibit syncytia formation induced by measles virus glycoproteins. Calmodulin also inhibited fusion induced by vPE17, a VV-recombinant expressing a truncated form of HIV-1gp160 which lacks the two known calmodulin-binding sites located in the cytoplasmic domain of gp41. The inhibitory activity was specific to calmodulin among the EF-hand proteins. These observations may be important in understanding the mechanism of retroviral envelope glycoprotein-mediated cell fusion. Several possible mechanisms of action are discussed.

Extracellular calmodulin-binding proteins in plants: purification of a 21-kDa calmodulin-binding protein

A 21-kDa calmodulin (CaM)-binding protein and a 19-kDa calmodulin-binding protein were detected in 0.1 M CaCl₂ extracts of Angelica dahurica L. suspension-cultured cells and carrot (Daucus carota L.) suspension-cultured cells, respectively, using a biotinylated cauliflower CaM gel-overlay technique in the presence of 1 mM Ca²⁺. No bands, or very weak bands, were shown on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels overlayed with biotinylated cauliflower CaM when 1 mM Ca²⁺ was replaced by 5 mM EGTA, indicating that the binding of these two CaM-binding proteins to CaM was dependent on Ca²⁺. Less 21-kDa CaM-binding protein was found in culture medium of Angelica dahurica suspension cells; however, a 21-kDa protein was abundant in the cell wall. We believe that the 21-kDa CaM-binding protein is mainly in the cell wall of Angelica dahurica. Based on its reaction with periodic acid-Schiff (PAS) reagent, this 21-kDa protein would appear to be a glycoprotein. The 21-kDa CaM-binding protein was purified by a procedure including Sephadex G-100 gel filtration and CM-Sepharose cation-exchange column chromatography. The purity reached 91% according to gel scanning. The purified 21-kDa CaM-binding protein inhibited the activity of CaM-dependent NAD kinase and the degree of inhibition increased with augmentation of the 21-kDa protein, which appeared to be the typical characteristic of CaM-binding protein.

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.

Transforming NRK cells with avian sarcoma virus reduces the extracellular Ca2+ requirement without affecting the calcicalmodulin requirement for the G1/S transition

NRK rat cells infected with a transformation-defective, temperature-sensitive (ts) mutant of the avian sarcoma virus could not proliferate in Ca2+-deficient medium at a nonpermissive temperature (40 degrees C) that inactivated the viral pp60v-src-transforming product and rendered the cells phenotypically untransformed. However, these arrested cells were stimulated to initiate DNA replication with little or no delay while still in the Ca2+-deficient medium, either by adding Ca2+ or calmodulin at 40 degrees C or by reducing the temperature to 36 degrees C which restored the transformed phenotype by rapidly reactivating pp60v-src. The G1/S transition triggered by restoring the transformed phenotype was suppressed by three different anticalmodulin drugs (R24571, trifluoperazine, W7). The suppression by one of these drugs, trifluoperazine, was overcome by adding calmodulin. Thus, neoplastic transformation by the avian sarcoma virus sharply reduces the extracellular Ca2+ requirement for the initiation of DNA replication without bypassing a calcicalmodulin-dependent mechanism also needed for the G1/S transition.

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

Calmodulin(CaM)-like activity was detected in human serum and foetal calf serum, with an activity i0 times more than that detectable in plasma. Serum CaM was largely accounted for by release from human platelets as confirmed by both radioimmunoassay and sodium-dodecyl-sulphate/polyacrylamide-gel electrophoresis of CaM partially purified from platelet releasate obtained in response to thrombin. Lactate dehydrogenase release was unaffected by thrombin. Platelet CaM content was very variable (1.3 to 11.3 pg/mg protein; n = 15). It is suggested that intact platelets are rich in CaM and that release of CaM during preparation explains the variation in CaM content reported here and in the literature.

Effects of extracellular calmodulin and calmodulin antagonists on B16 melanoma cell growth

Two drugs known to inhibit the action of calmodulin, prochlorperazine offP) and N-(6-aminohexyl)-5-chloro-1-napthalene sulfonamide (W7), were investigated for their ability to control cell proliferation in murine B16 melanoma cells in culture. PCP and W7 inhibited [3H]thymidine uptake in these cells, 50% inhibition occurring with 13 microM PCP and 40 microM W7. In the presence of relatively high concentrations of fetal calf serum (FCS), cells withstood high concentrations of both drugs (100 microM PCP and 200 microM W7) and showed increased pigment production. Drug-inhibited DNA synthesis could be reversed by the addition of fresh medium containing FCS or by the addition of exogenous pure calmodulin. Extracellular calmodulin itself stimulated DNA synthesis. FCS was found to contain calmodulin-like activity at concentrations that may be relevant to the stimulation of [3H]thymidine uptake by cells in culture.