Recent advances in Ca2+-dependent Ras regulation and cell proliferation

The intermediate-conductance calcium-activated potassium channel KCa3.1 contributes to atherogenesis in mice and humans

Atherosclerosis remains a major cause of death in the developed world despite the success of therapies that lower cholesterol and BP. The intermediate-conductance calcium-activated potassium channel KCa3.1 is expressed in multiple cell types implicated in atherogenesis, and pharmacological blockade of this channel inhibits VSMC and lymphocyte activation in rats and mice. We found that coronary vessels from patients with coronary artery disease expressed elevated levels of KCa3.1. In Apoe(-/-) mice, a genetic model of atherosclerosis, KCa3.1 expression was elevated in the VSMCs, macrophages, and T lymphocytes that infiltrated atherosclerotic lesions. Selective pharmacological blockade and gene silencing of KCa3.1 suppressed proliferation, migration, and oxidative stress of human VSMCs. Furthermore, VSMC proliferation and macrophage activation were reduced in KCa3.1(-/-) mice. In vivo therapy with 2 KCa3.1 blockers, TRAM-34 and clotrimazole, significantly reduced the development of atherosclerosis in aortas of Apoe(-/-) mice by suppressing VSMC proliferation and migration into plaques, decreasing infiltration of plaques by macrophages and T lymphocytes, and reducing oxidative stress. Therapeutic concentrations of TRAM-34 in mice caused no discernible toxicity after repeated dosing and did not compromise the immune response to influenza virus. These data suggest that KCa3.1 blockers represent a promising therapeutic strategy for atherosclerosis.

Binding of calcium ions to Ras promotes Ras guanine nucleotide exchange under emulated physiological conditions

Both Ras protein and calcium play significant roles in various cellular processes via complex signaling transduction networks. However, it is not well understood whether and how Ca(2+) can directly regulate Ras function. Here we demonstrate by isothermal titration calorimetry that Ca(2+) directly binds to the H-Ras.GDP.Mg(2+) complex with moderate affinity at the first binding site followed by two weak binding events. The results from limited proteinase degradation show that Ca(2+) protects the fragments of H-Ras from being further degraded by trypsin and by proteinase K. HPLC studies together with fluorescence spectroscopic measurements indicate that binding of Ca(2+) to the H-Ras.GDP.Mg(2+) complex remarkably promotes guanine nucleotide exchange on H-Ras under emulated physiological Ca(2+) concentration conditions. Addition of high concentrations of either of two macromolecular crowding agents, Ficoll 70 and dextran 70, dramatically enhances H-Ras guanine nucleotide exchange extent in the presence of Ca(2+) at emulated physiological concentrations, and the nucleotide exchange extent increases significantly with the concentrations of crowding agents. Together, these results indicate that binding of calcium ions to H-Ras remarkably promotes H-Ras guanine nucleotide exchange under emulated physiological conditions. We thus propose that Ca(2+) may activate Ras signaling pathway by interaction with Ras, providing clues to understand the role of calcium in regulating Ras function in physiological environments.

Methylation of the calcium channel-related gene, CACNA2D3, is frequent and a poor prognostic factor in gastric cancer

BACKGROUND & AIMS

The calcium channel voltage-dependent alpha2delta subunit consists of 4 genes, CACNA2D1 to CACNA2D4, of which CACNA2D2 and CACNA2D3 are located on 3p21.3 and 3p21.1, respectively. Here, we examined the relation between alpha2delta subunit gene alterations and gastric carcinogenesis.

METHODS

The expression and methylation status of the alpha2delta subunit genes were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and methylation-specific PCR in gastric cancers (GCs). The effects of CACNA2D3 expression were examined by cell proliferation and adhesion assays, and they predicted target gene alterations.

RESULTS

Aberrant methylation of CACNA2D1 and CACNA2D3 mostly corresponded to their expression status in GC cell lines. CACNA2D1/3 methylation was detected in 10 (12.5%) and 24 (30%) of the 80 GC cases, respectively, but no CACNA2D2 methylation was seen in 32 cases. CACNA2D3 methylation was more frequently found in diffuse type than in intestinal type (16/38 [42.1%] vs 8/42 [19.0%]; P = .025) GCs. Among the 53 patients with advanced GCs, patients with cancers showing CACNA2D3 methylation had a significantly shorter survival time than patients without this methylation (P = .003). Exogenous CACNA2D3 expression strongly inhibited cell growth and adhesion and up-regulated p21 and p27 expression in HEK-293T and NUGC4 cells. Inverse effects were seen by CACNA2D3 small interfering RNA treatment in the CACNA2D3-positive cell lines, indicating that CACNA2D3 may have tumor suppressive functions.

CONCLUSIONS

Loss of CACNA2D3 expression through aberrant promoter hypermethylation may contribute to gastric carcinogenesis, and CACNA2D3 methylation is a useful prognostic marker for patients with advanced GC.

Ca2+ signalling checkpoints in cancer: remodelling Ca2+ for cancer cell proliferation and survival

Increases in cytosolic free Ca2+ ([Ca2+]i) represent a ubiquitous signalling mechanism that controls a variety of cellular processes, including proliferation, metabolism and gene transcription, yet under certain conditions increases in intracellular Ca2+ are cytotoxic. Thus, in using Ca2+ as a messenger, cells walk a tightrope in which [Ca2+]i is strictly maintained within defined boundaries. To adhere to these boundaries and to sustain their modified phenotype, many cancer cells remodel the expression or activity of their Ca2+ signalling apparatus. Here, we review the role of Ca2+ in promoting cell proliferation and cell death, how these processes are remodelled in cancer and the opportunities this might provide for therapeutic intervention.

Essential role of Pyk2 and Src kinase activation in neuropeptide-induced proliferation of small cell lung cancer cells

Neuropeptide hormones like bombesin/gastrin-releasing peptide, galanin or bradykinin, acting via auto and paracrine growth loops, represent the principal mitogens of small cell lung cancer (SCLC). These mitogenic neuropeptides activate G(q/11)-coupled receptors which stimulate phospholipase Cbeta activity, followed by rises of the intracellular calcium concentration ([Ca2+](i)) and activation of protein kinase C (PKC). We report here that proline-rich tyrosine kinase Pyk2 is highly expressed in SCLC cells and provides a functional link between neuropeptide-induced increases in [Ca2+](i) and tumor cell proliferation. Activation of Pyk2 and its association with Src kinases critically depends on the elevation of [Ca2+](i), but is independent of PKC. Src kinase activities are crucial for neuropeptide-mediated GTP-loading of Ras and activation of extracellular signal-regulated kinases in SCLC cells. Pyk2 and Src kinases essentially contribute to anchorage-independent proliferation of SCLC cells. Inhibition of either Pyk2 or Src kinases by lentiviral RNAi or pharmacological inhibition with PP2, respectively, attenuated basal and neuropeptide-elicited survival and proliferation of SCLC cells in liquid culture and in soft agar. Thus, neuropeptides stimulate anchorage-independent survival and proliferation of SCLC cells via pathways involving Pyk2 and Src kinases. Therefore, Ca2+-induced Pyk2/Src complex formation may be a rewarding molecular target for novel therapeutic strategies in SCLC cells.

Epigenetic silencing of a Ca(2+)-regulated Ras GTPase-activating protein RASAL defines a new mechanism of Ras activation in human cancers

Ras has achieved notoriety as an oncogene aberrantly activated in multiple human tumors. Approximately 30% of all human tumors express an oncogenic form of this GTPase that is locked in an active conformation as a result of being insensitive to Ras GTPase-activating proteins (GAPs), proteins that normally regulate the inactivation of Ras by enhancing its intrinsic GTPase activity. Besides oncogenic mutations in Ras, signaling by wild-type Ras is also frequently deregulated in tumors through aberrant coupling to activated cell surface receptors. This indicates that alternative mechanisms of aberrant wild-type Ras activation may be involved in tumorigenesis. Here, we describe another mechanism through which aberrant Ras activation is achieved in human cancers. We have established that Ras GTPase-activating-like protein (RASAL), a Ca(2+)-regulated Ras GAP that decodes the frequency of Ca(2+) oscillations, is silenced through CpG methylation in multiple tumors. With the finding that ectopic expression of catalytically active RASAL leads to growth inhibition of these tumor cells by Ras inactivation, we have provided evidence that epigenetically silencing of this Ras GAP represents a mechanism of aberrant Ras activation in certain cancers. Our demonstration that RASAL constitutes a tumor suppressor gene has therefore further emphasized the importance of Ca(2+) in the regulation of Ras signaling and has established that deregulation of this pathway is an important step in Ras-mediated tumorigenesis.

Calcium and cancer: targeting Ca2+ transport

Ca2+ is a ubiquitous cellular signal. Altered expression of specific Ca2+ channels and pumps are characterizing features of some cancers. The ability of Ca2+ to regulate both cell death and proliferation, combined with the potential for pharmacological modulation, offers the opportunity for a set of new drug targets in cancer. However, the ubiquity of the Ca2+ signal is often mistakenly presumed to thwart the specific therapeutic targeting of proteins that transport Ca2+. This Review presents evidence to the contrary and addresses the question: which Ca2+ channels and pumps should be targeted?

Calcium regulates tertiary structure and enzymatic activity of human endometase/matrilysin-2 and its role in promoting human breast cancer cell invasion

Human MMP-26 (matrix metalloproteinase-26) (also known as endometase or matrilysin-2) is a putative biomarker for human carcinomas of breast, prostate and other cancers of epithelial origin. Calcium modulates protein structure and function and may act as a molecular signal or switch in cells. The relationship between MMPs and calcium has barely been studied and is absent for MMP-26. We have investigated the calcium-binding sites and the role of calcium in MMP-26. MMP-26 has one high-affinity and one low-affinity calcium binding site. High-affinity calcium binding was restored at physiologically low calcium conditions with a calcium-dissociation constant of 63 nM without inducing secondary and tertiary structural changes. High-affinity calcium binding protects MMP-26 against thermal denaturation. Mutants of this site (D165A or E191A) lose enzymatic activity. Low-affinity calcium binding was restored at relatively high calcium concentrations and showed a K(d2) (low-affinity calcium-dissociation constant) value of 120 microM, which was accompanied with the recovery of enzymatic activity reversibly and tertiary structural changes, but without secondary structural rearrangements. Mutations at the low-affinity calcium-binding site (C3 site), K189E or D114A, induced enhanced affinity for the Ca2+ ion or an irreversible loss of enzymatic activity triggered by low-affinity calcium binding respectively. Mutation at non-calcium-binding site (V184D at C2 site) showed that C2 is not a true calcium-binding site. Observations from homology-modelled mutant structures correlated with these experimental results. A human breast cancer cell line, MDA-MB-231, transfected with wild-type MMP-26 cDNA showed a calcium-dependent invasive potential when compared with controls that were transfected with an inactive form of MMP-26 (E209A). Calcium-independent high invasiveness was observed in the K189E mutant MDA-MB-231 cell line.

Decoding complex Ca2+ signals through the modulation of Ras signaling

Calcium (Ca(2+)) is a universal regulator of a wide variety of cellular processes. For such control to be achieved, information is encoded within spatial and temporal components of the underlying Ca(2+) signal. One pathway through which Ca(2+) signals are decoded is the Ras binary switch. Here I describe some recent advances that have shed light on how cells can decode the spatial and temporal aspects of Ca(2+) signals through the regulation of this important signalling switch.

Molecular mechanisms involved in Ras inactivation: the annexin A6–p120GAP complex

In mammalian cells, a complex network of signaling pathways tightly regulates a variety of cellular processes, such as proliferation and differentiation. New insights from one of the most-important signaling cascades involved in oncogenesis, the Ras-Raf-MAPK pathway, suggest that the subcellular localisation and assembly of signaling modules of this pathway is crucial to control the biological response. This commonly requires membrane targeting events that are mediated by adaptor/scaffold proteins. Of particular interest is the translocation and complex formation of GTPase-activating proteins (GAPs), such as p120GAP, at the plasma membrane to inactivate Ras. Recent studies indicate that one member of the annexin family, annexin A6 acts as a targeting protein for p120GAP. This review discusses how annexin A6 modulates the involvement of negative regulators of the Ras-Raf-MAPK pathway contributing to Ras inactivation.