Dissemination and metastasis may occur at an early tumor stage or even precede primary tumor formation [10, 32, 33]. One force that drives cells disseminating from tumors is the repulsive effect, which is also called contact inhibition of locomotion (CIL). CIL suppresses forward locomotion during cell-cell contact and redirects cell motility, which can enhance cancer invasiveness by favoring redirection of cancer cells into the stromal environment [34].

In primary tumor sites, cancer cells recruit myeloid-derived suppressor cells (MDSCs) to induce the acquisition of the EMT/stemness phenotype in cancer cells at the invasive cancer edge [35]. Ephrin type-A receptor 2 (EphA2) promotes membrane-anchored membrane type-1 matrix metalloproteinase (MT1-MMP) expression in invasive breast cancer cells. MT1-MMP in turn cleaves EphA2 and activates the EphA2/Src/RhoA axis, which induces actomyosin contractility and cell junction disassembly. These changes ultimately induce cell transition to rounded morphology, cell-cell repulsion, dissemination, and single-cell invasion [36].

For seeding and growth at the secondary sites, cancer cells usually infiltrate into circulatory systems such as lymphatic and/or vascular vessels. CTCs are essential for distant metastasis of cancer, but only a small subset of CTCs can generate metastatic tumors [37–39]. This is because circulatory systems and distant organs are usually not hospitable to cancer cells, due to oxidative stress, anoikis, and the immunosurveillance system [40–42]. Melanoma cells with high metastatic ability preferentially upregulate monocarboxylate transporter 1 (MCT1) to consume more lactate, and cells with high MCT1 expression are more prominent in blood than in primary tumors. Selective inhibition of MCT1 does not affect the migration or invasion of melanoma cells or primary tumor growth, but it significantly reduces the frequency of circulating melanoma cells in the blood and cancer metastasis. This outcome is the result of MCT1 promoting cell survival during metastasis, partly by decreasing oxidative stress [37, 43]. Internalized β1-integrin localizes to autophagy-related endomembranes and triggers c-Met-sustained prosurvival in an adhesion-independent manner, preventing detached cancer cells from undergoing anoikis. This mechanism gives cancer cells sufficient time to land successfully [44].

Metastatic cancer cells at secondary sites may die, be cleared, grow into macrometastases, or remain dormant only to grow into micrometastases decades after seeding [9, 32]. The reasons for the dormancy of cancer cells may be attributed to unfit proliferation conditions, such as nutrient starvation or inhibition by the immune system, or they may acquire suppressive factors from the primary tumor, or lack the ability to activate angiogenesis [60]. For example, breast cancer cells can directly engulf mesenchymal stem cells to enhance their prosurvival ability, but they then turn dormant [61].

Bone marrow-derived myeloid cells can become central nervous system (CNS) myeloid-like cells once they translocate to the brain. CNS-native myeloid cells display downregulation of CX3CR1, which enhances C-X-C motif (CXC) chemokine ligand (CXCL)10 signaling to foster an immunosuppressive niche and promote brain metastasis of cancer cells [62]. In addition, astrogliosis and neuroinflammation are instigated in the early stage of metastasis to promote the initial growth of metastatic melanoma cells in the brain [54].

Communications between cancer cells and the TME are pivotal in cancer metastasis. The TME, including the ECM, bacteria, fibroblasts, mesenchymal stem cells, and macrophages, is essential for the phenotype of tumor [12, 81, 82]. Several types of cells derived from bone marrow, including macrophages, neutrophils, mast cells, and myeloid progenitors, are involved in pathological angiogenesis. In addition to reducing the effect of drugs targeting vasculature EC signals, they can facilitate local invasion [9, 83]. Cancer cells recruit gMDSCs and monocytic MDSCs (mMDSCs) for infiltration. The mMDSCs promote cancer cells dissemination by inducing EMT/stemness, while gMDSCs promote metastatic growth by reverting EMT/stemness [35]. Bacteria were firstly detected in human tumors more than 100 years ago. Intracellular bacteria are present in both cancer and immune cells [84]. Exosomes from those bacteria deliver factors such as IL-8 to noninfected cancer cells to increase the metastasis rate [1].

Communications between cancer cells depend on direct interactions or EVs, including exosomes, to transfer biomolecules such as proteins and nucleic acids [97]. Cancer cells undergoing EMT can produce the EMT-inducing transcription factor Six1 to promote the metastasis of non-EMT cancer cells [98]. Collective metastasis is also dependent on the EMT. Clusters of cancer cells internalize N-cadherin, which induces the acquisition of only a partial mesenchymal phenotype characterized by an increase in tissue fluidity that similar to a solid-like-to-fluid-like transitional state. This transitional state allows cells to migrate under physical constraints without abolishing the cell cooperation required for collectiveness [99]. This enhanced cell metastatic potential is associated with poorer prognosis in cancer patients [6, 27]. These tumor cell clusters preferentially display high level of the epithelial cytoskeletal protein keratin 14 (K14), desmosome and hemidesmosome adhesion complex genes but are depleted of major histocompatibility complex (MHC) class II genes [100].

Cancer cells within multicellular clusters can form nano alumina to construct their own internal microenvironment, from which they exchange and concentrate growth factors such as epigen to drive metastatic outgrowth [6]. Although E-cadherin has been widely demonstrated to inhibit cancer metastasis, E-cadherin is intriguingly required for the metastasis of invasive ductal carcinoma (IDC). This requirement is based on that E-cadherin enable cancer cells to form collective metastasis by reducing reactive oxygen species (ROS) and preventing cancer cells from apoptosis, which is especially critical during systemic dissemination and early seeding [39]. Clusters, usually containing as many as 20 cells, that are too large to pass through narrow vessels can reversibly reorganize into single-file chain-like geometries that reduce their hydrodynamic resistances, allowing them to successfully traverse 5~10 μm constrictions, even in whole blood [101]. Correspondingly, PEP06 polypeptide 30, a cluster-dissociating agent, inhibits cancer metastasis by manipulating the EMT, the α_v integrin/FAK/Src axis, and E-cadherin-based intercellular junctions [102, 103].

Antioxidants N-acetylcysteine and vitamin E reduce ROS, free heme level, and stabilize the transcription factor BACH1, resulting in glycolysis-dependent metastasis of lung cancer cells [104]. This finding suggests that oxidative stress may have an inhibitory effect on metastasis at the initial stage. Metastatic melanoma cells but not primary melanoma cells are capable of engulfing and digesting live autologous melanoma-specific CD8⁺ T cells to increase their survival, particularly under nutrient-stress conditions [105].

Some cancer stem cells (CSCs) do not display high levels of mesenchymal genes but express the high level of the fatty acid receptor CD36. These CSCs are critical for metastasis initiation and rely particularly on dietary lipids to promote metastasis [106]. Although the primary and lymph node metastases of melanoma cells display a similar level of cholesterol, metastatic cells in lymph nodes can probably produce or recruit more bile acids to activate vitamin D receptor/YAP, which promote metastatic cells adaptation to their new microenvironment [107].

Aerobic glycolysis (the Warburg effect) has been widely used as a marker of cancer [9, 108]. FABP7 is increased in patients with breast cancer metastases in the brain. It inhibits fatty acid oxidative phosphorylation (OXPHOS) and enhances the formation of lipid droplets, which facilitates HER2⁺ breast cancer metastasis to the brain [109]. Moreover, DTCs may reprogram their metabolism to survive and colonize at metastatic sites. Micrometastatic triple-negative breast cancer (TNBC) cells in the lungs and lymph nodes upregulate mitochondrial OXPHOS, the pharmacological inhibition of which profoundly reduces lung metastasis [38]. CTCs display high expression of peroxisome proliferator-activated receptor-gamma 1α (PGC-1α), which is regulated by Ca²⁺ and is necessary for intravasation of cancer cells into the circulatory systems. PGC-1α enhances mitochondrial biogenesis and respiration coupled with the remodeling of actin cytoskeleton signals. Silencing of PGC-1α ultimately decreases the aggressive properties of cancer cells, such as metastatic lung colonization and nodule formation [38, 110].

Leukocyte immunoglobulin-like receptor B4 (LILRB4) is exclusively expressed on monocytic cells. It inhibits T cell proliferation and T cell-mediated antitumor immunity meanwhile promoting acute myeloid leukemia (AML) cell migration and tumor infiltration [111]. Metastatic castration-resistant prostate cancer (mCRPC) cells induce an increase in the population of osteoclasts in bone marrow. Osteoclasts produce TGFβ, which inhibits Th-1 polarization in the metastatic TME and is the cause of the poor response of metastatic cancer to immune checkpoint therapy [71]. Cancer cells that have metastasized to the liver can recruit macrophages and siphon-activated antigen-specific T cells. These macrophages display high expression of FasL, which diminishes CD4⁺ T cells through their direct contact, which induces FasL/Fas-dependent apoptosis, resulting in systemic immunosuppression [7]. PHD restrains CD8⁺ T cell effector function, also leading to an immunosuppressive microenvironment that favors cancer cell colonization at the lung [64].

Numerous Ca²⁺-dependent proteins containing Ca²⁺-binding sites, such as calmodulin (CaM) [123, 124], Ca²⁺/CaM-dependent kinase (CaMK) [125], calreticulin [126], Ca²⁺ sensor [127], Ca²⁺-sensing receptor (CaSR) [128], calpain [129], calcineurin [130], myosin light-chain kinase (MLCK) [131] and phospholipase S100 family proteins [132], have been widely proven to be vital to cancer metastasis. CaM, with four EF-hands, is one of the most important Ca²⁺ signal-decoding proteins. It can rapidly redistribute to subcellular compartments in response to various signals. Migration signals trigger a spatiotemporal redistribution of CaM to the leading edge of the migrating cell; for example, EGF induced CaM redistribution from the nucleus to the cytoplasm to activate invadopodia-associated proteins [123, 133]. Calreticulin can inhibit ER stress to inhibit EMT, which promotes cancer cells liver metastasis [126]. CaSR located on the plasma membrane senses extracellular Ca²⁺. CaSR induces activation of AKT/β-catenin, leading to cancer metastasis [128, 134]. Calpain cleaves talin, resulting in more rapid adhesion disassembly rates [129]. Neuronal Ca²⁺ sensor 1 (NCS1) promotes the motility of cancer cells by localizing to the cell leading edge [135].

S100A4 reduces the expression of E-cadherin and β-catenin in cancer cells, while S100A4⁺ stromal cells enhance the stem cell-like phenotype of tumor cells [136, 137]. S100A9 can be secreted by TAMs, which is increased in HCC-related TAMs and HCC cells. It enhances the stem cell-like and metastatic properties of HCC cells by activating NF-κB and recruiting more macrophages to infiltrate tumors [138, 139]. Ca²⁺-activated potassium channels promote the proliferation and migration of endometrial and breast cancer cells, while they also mediate blood-brain tumor barrier opening to promote brain metastasis [140–142].

Limited by the scope of this brief review to cover all Ca²⁺-dependent proteins and protein-mediated Ca²⁺ currents, we mainly focus on the proteins critical for Ca²⁺ currents that have also been reported to be involved in cancer metastasis (Table 1).

VGCCs comprise five subtypes and ten members. Endostatin, a commercial agent targeting angiogenesis, reduces glioblastoma cells migration by directly inhibiting T-type VGCCs [147]. Cav1.3 expression is increased in breast cancer, atypical hyperplasia, and endometrial carcinoma tissues. It partially mediates estrogen-induced Ca²⁺ influx and endometrial carcinoma cells migration [144, 145]. Ca²⁺ is increased at filopodia tips and correlated with filopodia stabilization. Filopodia stabilization leads to focal adhesion formation, which is critical to the migration of cells. L-type Ca²⁺ channel blockers, such as amlodipine, besylate, felodipine, manidipine, and cilnidipine, potently inhibit L-type Ca²⁺ channel/calpain-1-mediated filopodia formation, which efficiently reduces the cancer cells migration and invasion rates [145]. The Cav1.2 level is increased in CRCs. Nifedipine, another L-type Ca²⁺ channel blocker, inhibits CRC migration and PD-1 via Cav1.2/Ca²⁺-mediated activation of nuclear factor of activated T cell 2 (NFAT2). The combination of nifedipine with an anti-PD-1 antibody significantly reduces cancer cell liver metastasis and the PD-1⁺ CD8⁺ T cells level among tumor-infiltrating lymphocytes [143].

Orai proteins located on the cell membrane are the core components of Ca²⁺ release-activated Ca²⁺ channels (CRACs) that mediate SOCE. Upon ER Ca²⁺ depletion, STIM activates Orai to mediate Ca²⁺ influx. CRACs play critical roles not only in immune cells but also in cancer cells [13, 14, 201]. Chemoresistant ovarian cancer cells acquire enhanced migratory ability by increasing SOCE-mediated focal adhesion turnover [11]. ACE2 promotes metastasis of breast cancer by activating the PAK1/NF-κB/Snail1 axis via SOCE [202]. MFAP5 secreted by CAFs activates FAK/cAMP response element-binding protein (CREB)/troponin C type 1 (TNNC1) via SOCE to enhance cancer metastasis [149].

CRAC-mediated Ca²⁺ regulates invadopodium formation and ECM degradation [155, 203]. Orai1-medated SOCE promotes phosphorylation of proline-rich tyrosine kinase 2 (Pyk2), which regulates focal adhesion turnover and the EMT of glioma cells [150]. Elevation of Orai2 is positively correlated with lymph node metastasis of gastric cancer (GC). Orai2 induces FAK-mediated mitogen-activated protein kinases(MAPK)/extracellular signal-regulated kinase (ERK) activation and enhances focal adhesion disassembly at the rear edge of metastatic cancer cells [154]. The hEag1 K⁺ channels are essential for breast cancer cell migration because they promote Orai1-mediated Ca²⁺ current [204]. Inhibition of Orai or STIM decreases GTPase Ras- and Rac-mediated formation of new focal complexes at cell protrusions and disassembly of focal adhesions. This action ultimately inhibits breast cancer metastasis to the lung [13]. Orai3 and STIM2 also promote gastroenteropancreatic neuroendocrine and breast cancer metastasis, respectively [8, 151].

The TRP channel superfamily comprises seven subtypes with approximately thirty members, most of which can mediate Ca²⁺ currents. TRP channels are activated by diverse stimuli, including intra- and extra-cellular messengers, temperature, pH, chemical, mechanical and osmotic stress, ROS, and intracellular Ca²⁺ stores [17]. The expression of phospholipid phosphatase 4 (PLPP4) protein is increased, and it promotes cancer cells metastasis probably by activating TRPC channels-mediated Ca²⁺ influx [205]. TRPC1 inhibits the PI3K/AKT pathway and EMT, leading to reduced migration and invasion rates of cancer cells [157]. TRPC5 is elevated in colon cancer, which promotes cancer cell EMT and metastasis via the HIF-1α/Twist axis. In addition to promoting the migration and invasiveness of renal cancer cells, the upregulated expression of TRPC6 is also required for translocation of Orai1 and Orai3 to the plasma membrane of metastatic breast cancer cells [161, 162]. Helicobacter pylori, a well-known major risk factor for GC, increases TRPC6 transcription and Ca²⁺ influx via the Wnt/β-catenin pathway to enhance GC cells migration and invasion [163].

Cancer cells with high level of TRPM2 channels are more susceptible to neutrophil cytotoxicity. A reduction in TRPM2 expression leads to decreased tumor growth but increased DTC seeding potential in the premetastatic lung [206]. Upregulation of TRPM7 expression is correlated with the EMT and metastasis of ovarian cancer via the Ca²⁺-related PI3K/AKT axis [171]. The level of TRPV1 channels is reduced in human GC tissues. This reduction leads to decreased Ca²⁺/calmodulin-dependent protein kinase kinase beta (CaMKKβ) activity and contributes to GC peritoneal dissemination [175]. TRPV4 or TRPV6 is increased in endometrial, breast or prostate cancer, which promotes cancer cells metastasis [177, 178, 181]. Deletion of TRPV4 increases p-VEGFR2^Y1175 level and ECs migration [207]. However, deficiency of TRPV4 causes reduced vascular E-cadherin level and destabilizes tumor vessel integrity, leading to cancer cells lung metastasis [179].

Mitochondria, major intracellular Ca²⁺ stores, are primary sources of ROS, which promote tumor metastasis. MCU located on the inner mitochondrial membrane, is a Ca²⁺-selective channel and critical for major Ca²⁺ influx into mitochondria [208]. MCU-mediated ATP production regulates profibrotic macrophage polarization [209]. cl-CD95L enhances the metastatic dissemination of TNBC cells by enhancing MCU-mediated Ca²⁺ current [210]. However, reduction in histidine triad nucleotide-binding 2 (HINT2) enhances pancreatic cancer lymph node metastasis, probably by increasing MICU1/2 and decreasing EMRE levels to inhibit MCU activity [193, 211].

MCU is significantly associated with cancer aggressiveness and is increased in diverse cancers, including TNBC, HCC, and GC [88, 194–196]. Elevation of MCU level enhances metastatic colonization and MVD in breast cancer. MCU also inhibits the selective sorting of miR-4488 to extracellular vesicles by manipulating Ca²⁺-dependent RNA-binding proteins (RBPs). This process leads to relief of C-X3-C motif chemokine ligand 1 (CX3CL1) repression to enhance tube formation capacity and sprouting capacity [195]. MCUb expression is decreased while that of MCU is increased in invasive TNBC and lymph node metastasis. Silencing MCU increases SOCE and the NADPH/NADH ratio while decreases ROS and HIF-1α levels [196]. MICU1 is decreased in metastatic HCC tissues, and MCU-mediated mitochondrial Ca²⁺ influx induces ROS/c-Jun N-terminal kinase (JNK) activation. This chain of events ultimately facilitates HCC intrahepatic and lung metastasis [194]. MCUR1 is significantly upregulated in metastatic HCC cells, which promote EMT and metastasis via the mitochondrial Ca²⁺-dependent ROS/Nrf2/Notch pathway [197].

TPC family proteins are nicotinic acid adenine dinucleotide phosphate (NAADP)-gated Ca²⁺ release channels located on endosomes, lysosomes, and melanosomes. Reduction in TPC2 expression activates the YAP/TAZ axis to inhibit the expression levels of ORAI1 and PKC-βII, which is found in the metastatic tumors but not in the primary melanoma cells in patients [198].

The EGF-induced EMT is coupled with specific alterations in the mRNA level of ER-related Ca²⁺ channels/pumps, including SERCAs, IP₃R, and RyR [212]. Trifluoperazine, an antipsychotic drug, can reduce glioblastoma invasion by binding with CaM to activate IP₃R-mediated Ca²⁺ release from the ER [213]. Furthermore, IP₃R-3 is increased in colon cancer, while RyR2 is decreased in thyroid carcinoma tissues, both of which are related to aggressiveness [199, 200].