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

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Extracellular calmodulin regulates growth and cAMP-mediated chemotaxis in Dictyostelium discoideum

The existence of extracellular calmodulin (CaM) has had a long and controversial history. CaM is a ubiquitous calcium-binding protein that has been found in every eukaryotic cell system. Calcium-free apo-CaM and Ca(2+)/CaM exert their effects by binding to and regulating the activity of CaM-binding proteins (CaMBPs). Most of the research done to date on CaM and its CaMBPs has focused on their intracellular functions. The presence of extracellular CaM is well established in a number of plants where it functions in proliferation, cell wall regeneration, gene regulation and germination. While CaM has been detected extracellularly in several animal species, including frog, rat, rabbit and human, its extracellular localization and functions are less well established. In contrast the study of extracellular CaM in eukaryotic microbes remains to be done. Here we show that CaM is constitutively expressed and secreted throughout asexual development in Dictyostelium where the presence of extracellular CaM dose-dependently inhibits cell proliferation but increases cAMP mediated chemotaxis. During development, extracellular CaM localizes within the slime sheath where it coexists with at least one CaMBP, the matricellular CaM-binding protein CyrA. Coupled with previous research, this work provides direct evidence for the existence of extracellular CaM in the Dictyostelium and provides insight into its functions in this model amoebozoan.

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.

Apoplastic Calmodulin Receptor-like Binding Proteins in Suspension-cultured Cells of Arabidopsis thaliana

Calmodulin, a highly conserved protein family that has long been well known as an intracellular calcium sensor, was identified in the culture medium and cell walls of Arabidopsis thaliana suspension-cultured cells by immunoblotting assay. A promotion effect by applying exogenous purified calmodulin and an inhibition effect by the addition of anti-calmodulin anti-serum or calmodulin antagonist to the medium on proliferation of suspension cells were found by monitoring incorporation of [methyl-3H]thymidine into nuclear DNA. Radioligand binding analysis with 35S-labeled calmodulin indicated the presence of specific, reversible, and saturable calmodulin binding sites on the surface of both A. thaliana suspension-cultured cells and its protoplasts; among them at least one is on the surface of Arabidopsis protoplasts, with the Kd approximately 9.2 nM, and two are on the out-surface of Arabidopsis suspension-cultured cells, with Kd values of approximately 47.5 and 830 nM. Chemical crosslinking of 35S-labeled calmodulin to protoplasts revealed 117- and 41-kDa plasma membrane proteins specifically bound to calmodulin, whereas cross-linking with intact suspension-cultured cells verified more calmodulin binding proteins which might be cell wall-associated in addition to membrane-localized. Taking together, our data provide first evidence for the presence of apoplastic calmodulin receptor-like binding proteins on the cell surface of Arabidopsis suspension-cultured cells, which strongly supports our previous idea that apoplastic calmodulin functions as a peptide signal involved in regulation of cell growth and development.

Conformation and vasoreactivity of VIP in phospholipids: effects of calmodulin

The purpose of this study was to determine the conformation and vasorelaxant effects of vasoactive intestinal peptide (VIP) self-associated with sterically stabilized phospholipid micelles (SSM) and whether calmodulin modulates both of these processes. Circular dichroism spectroscopy revealed that VIP is unordered in aqueous solution at room temperature but assumes appreciable a helix conformation in SSM. This conformational transition was amplified at 37 degrees C and by a low concentration of calmodulin (0.1 nM). Suffusion of VIP in SSM elicited significant time- and concentration-dependent potentiation of vasodilation relative to that elicited by aqueous VIP in the in situ hamster cheek pouch (P < 0.05). This response was significantly potentiated by calmodulin (0.1 nM). Collectively, these data indicate that exogenous calmodulin interacts with VIP in SSM to elicit conformational transition of VIP molecule from a predominantly random coil in aqueous environment to alpha helix in SSM. This process is associated with potentiation and prolongation of VIP-induced vasodilation in the in situ peripheral microcirculation.

Exogenous calmodulin potentiates vasodilation elicited by phospholipid-associated VIP in vivo

The purpose of this study was to determine whether exogenous calmodulin potentiates vasoactive intestinal peptide (VIP)-induced vasodilation in vivo and, if so, whether this response is amplified by association of VIP with sterically stabilized liposomes. Using intravital microscopy, we found that calmodulin suffused together with aqueous and liposomal VIP did not potentiate vasodilation elicited by VIP in the in situ hamster cheek pouch. However, preincubation of calmodulin with liposomal, but not aqueous, VIP for 1 and 2 h and overnight at 4 degrees C before suffusion significantly potentiated vasodilation (P < 0.05). Calmodulin-induced responses were significantly attenuated by calmidazolium, trifluoperazine, and NG-nitro-L-arginine methyl ester (L-NAME) but not D-NAME. The effects of L-NAME were reversed by L- but not D-arginine. Indomethacin had no significant effects on calmodulin-induced responses. Calmodulin had no significant effects on adenosine-, isoproterenol-, acetylcholine-, and calcium ionophore A-23187-induced vasodilation. Collectively, these data indicate that exogenous calmodulin amplifies vasodilation elicited by phospholipid-associated, but not aqueous, VIP in the in situ peripheral microcirculation in a specific, calmodulin active sites-, and nitric oxide-dependent fashion. We suggest that extracellular calmodulin, phospholipids, and VIP form a novel functionally coordinated class of endogenous vasodilators.

Endothelial cells and extracellular calmodulin inhibit monocyte tumor necrosis factor release and augment neutrophil elastase release

Cultured human umbilical vein endothelial cells inhibited tumor necrosis factor-alpha release from whole blood or isolated mononuclear cells exposed to endotoxin. In contrast, the endothelial cells augmented neutrophil elastase release in the same blood. A protein with these functional properties was isolated from endothelial cell-conditioned media and, surprisingly, was identified as calmodulin. Authentic calmodulin mimicked the effect of endothelium. 125I-Calmodulin bound to a high affinity site on monocytic cell lines (Kd approximately 30 nM, in agreement with its functional activity). Cross-linking of 125I-calmodulin to monocytic cells identified a candidate calmodulin receptor. We conclude that calmodulin possesses an extracellular signaling role in addition to its intracellular regulatory functions. Calmodulin released at sites of tissue injury or possibly by specific mechanisms in the endothelium can bind to receptors, modulating the activities of inflammatory cells.

Expression of Ca2+/calmodulin-dependent protein kinase types II and IV, and reduced DNA synthesis due to the Ca2+/calmodulin-dependent protein kinase inhibitor KN-62 (1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenyl piperazine) in small cell lung carcinoma

Because changes in intracellular Ca2+ affect progression through the mitotic cell cycle, we investigated the role of Ca2+-binding proteins in regulating cell cycle progression. Evidence was found demonstrating that the activation of Ca2+/calmodulin-dependent protein kinase (CaM kinase) inhibits cell cycle progression in small cell lung carcinoma (SCLC) cells. We also demonstrated that SCLC cells express both CaM kinase type II (CaMKII) and CaM kinase type IV (CaMKIV). Five independent SCLC cell lines expressed proteins reactive with antibody to the CaMKII beta subunit, but none expressed detectable proteins reactive with antibody to the CaMKII alpha subunit. All SCLC cell lines tested expressed both the alpha and beta isoforms of CaMKIV. Immunoprecipitation of CaMKII from SCLC cells yielded multiple proteins that autophosphorylated in the presence of Ca2+ / calmodulin. Autophosphorylation was inhibited by the CaMKII(281-302) peptide, which corresponds to the CaMKII autoinhibitory domain, and by 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4- phenylpiperazine (KN-62), a specific CaM kinase antagonist. Influx of Ca2+ through voltage-gated Ca2+ channels stimulated phosphorylation of CaMKII in SCLC cells, and this was inhibited by KN-62. Incubation of SCLC cells of KN-62 potently inhibited DNA synthesis, and slowed progression through S phase. Similar anti-proliferative effects of KN-62 occurred in SK-N-SH human neuroblastoma cells, which express both CaMKII and CaMKIV, and in K562 human chronic myelogenous leukemia cells, which express CaMKII but not CaMKIV. The expression of both CaMKII and CaMKIV by SCLC cells, and the sensitivity of these cells to the anti-proliferative effects of KN-62, suggest a role for CaM kinase in regulating SCLC proliferation.

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.