Calcium wave signaling in cancer cells

Abstract

Ca(2+) functions as an important signaling messenger right from beginning of life to the final moments of the end of life. Ca(2+) is needed at several steps of the cell cycle such as early G(1), at the G(1)/S, and G(2)/M transitions. The Ca(2+) signals in the form of time-dependent changes in intracellular Ca(2+) concentrations, [Ca(2+)](i), are presented as brief spikes organized into regenerative Ca(2+) waves. Ca(2+)-mediated signaling pathways have also been shown to play important roles in carcinogenesis such as transformation of normal cells to cancerous cells, tumor formation and growth, invasion, angiogenesis and metastasis. Since the global Ca(2+) oscillations arise from Ca(2+) waves initiated locally, it results in stochastic oscillations because although each cell has many IP(3)Rs and Ca(2+) ions, the law of large numbers does not apply to the initiating event which is restricted to very few IP(3)Rs due to steep Ca(2+) concentration gradients. The specific Ca(2+) signaling information is likely to be encoded in a calcium code as the amplitude, duration, frequency, waveform or timing of Ca(2+) oscillations and decoded again at a later stage. Since Ca(2+) channels or pumps involved in regulating Ca(2+) signaling pathways show altered expression in cancer, one can target these Ca(2+) channels and pumps as therapeutic options to decrease proliferation of cancer cells and to promote their apoptosis. These studies can provide novel insights into alterations in Ca(2+) wave patterns in carcinogenesis and lead to the development of newer technologies based on Ca(2+) waves for the diagnosis and therapy of cancer.

Fig. 1

Ca²⁺ signaling in cells. In resting cells the intracellular Ca²⁺ concentration, [Ca²⁺]_i, is maintained at ~100 nM by Ca²⁺ removal via plasma membrane Ca²⁺- ATPase (PMCA) and Ca²⁺ uptake into ER by SR Ca²⁺- ATPase (SERCA) transporters. The Na/Ca exchanger (NCX), a major secondary regulator of [Ca²⁺]_i, is electrogenic, exchanging three Na ions for one Ca²⁺. Intracellular Ca²⁺ hyperpolarizes many cells by activating K+ channels, and in some cells, Cl⁻ channels. This decreases CaV channel activity but increases the driving force across active Ca²⁺-permeant channels. In excitatory Ca²⁺ signaling, plasma membrane ion channels are triggered to open by changes in voltage, or extra- or intracellular ligand binding. When open, ~1 million Ca²⁺ ions/s/channel flow down the 20000 fold Ca²⁺ gradient (ECa ~ +150 mV). Initial increase in [Ca²⁺]_i triggers more release, primarily from ER via Ca²⁺-sensitive RyR. GPCR or receptor tyrosine kinase-mediated activation of PLC cleaves PIP2 into IP₃ and DAG. IP₃ is a ligand for the intracellular IP₃R channel spanning the membrane of the ER. GPCRs catalyze the exchange of GDP for GTP on Gα subunits, releasing active Gα and Gβγ subunits that in turn activate PLCβ.

FIG. 2

Elementary and global events of calcium signaling by IP3 in a dose-dependent manner. Inositol trisphosphate receptors (IP3R) are shown arranged in clusters that form Ca²⁺ release sites in the ER. A: at low IP₃ concentration, a small number of IP₃R (shown in green) release Ca²⁺ (shown in blue) into the cytoplasm as elementary event called “blip” whereas others IP₃R (shown in dark red) are not bound with IP₃ and therefore are not active. B: at intermediate concentration of IP₃, a group of IP₃R opens to release Ca²⁺ to form “puff” which remains local because the neighboring IP₃Rs are not active. C: at higher concentrations of IP₃, the activation of a large number of IP₃R leads to formation and global propagation of Ca²⁺ waves. Ca²⁺ released at one cluster can trigger Ca²⁺ release at adjacent clusters by calcium-induced calcium release (CICR) that leads to the formation of Ca²⁺ waves which propagate by successive cycles of Ca²⁺ release, diffusion, and CICR.

Fig. 3

Regulation of cell cycle by Ca²⁺. Ca²⁺ signaling at various key stages of the cell cycle plays crucial roles such as the activation and expression of transcription factors FOS and JUN, CREB) and NFAT as the cells enter the G1 phase. These transcription factors in turn regulate the D-type cyclins, required for activation of cyclin D-CDK4 complexes (D-K4). Later in G1 phase of cell cycle, Ca²⁺ plays an important role in the activity of D-K4 and E-K2 complexes required for phosphorylation and consequently inactivation of retinoblastoma (RB) that causes cells to progress to S phase. During the G₁/S and G₂/M transitory phases of cell cycle, the Ca²⁺ oscillations play important roles in centrosome duplication, maturation, and separation in cytokinesis.

Fig. 4

Ca²⁺ waves activate transcription factor NFAT that in turn controls cell cycle. The changes in Ca²⁺ waves oscillation frequency can activate transcription factors such as transcription factor NFAT resulting in regulation of cellular transcription. Calcineurin plays a major role in the cell cycle progression through G₁ and S phases by regulating CREB1 and the nuclear NFAT. In its inactive phosphorylated state, NFATs localizes in the cytoplasm, but following an increase in [Ca²⁺]_i, the activated calcineurin dephosphorylates NFAT which then translocates to the nucleus and regulates expression of its target genes. The influx of Ca²⁺ through plasma membrane channels such as the store-operated channel ORAI1 as well as efflux of Ca²⁺ from the ER can activate calcineurin and NFAT signaling pathway for inducing changes in cell cycle gene expression. Experiments using the “calcium clamp” have shown that the sensitivity of different transcription factors to [Ca²⁺]_i oscillations is highly frequency dependent.