Connexin 43 expression is associated with increased malignancy in prostate cancer cell lines and functions to promote migration

Connexin-Containing Vesicles for Drug Delivery

Connexin is a transmembrane protein present on the cell membrane of most cell types. Connexins assemble into a hexameric hemichannel known as connexon that pairs with another hemichannel present on a neighboring cell to form gap junction that acts as a channel or pore for the transport of ions and small molecules between the cytoplasm of the two cells. Extracellular vesicles released from connexin-expressing cells could carry connexin hemichannels on their surface and couple with another connexin hemichannel on a distant recipient cell to allow the transfer of the intravesicular content directly into the cytoplasm. Connexin-containing vesicles can be potentially utilized for intracellular drug delivery. In this review, we introduced cell-derived, connexin-containing extracellular vesicles and cell-free connexin-containing liposomes, methods of preparing them, procedures to load cargos in them, factors regulating the connexin hemichannel activity, (potential) applications of connexin-containing vesicles in drug delivery, and finally the challenges and future directions in realizing the promises of this platform delivery system for (intracellular) drug delivery.

Mapping the Anti-Cancer Activity of α-Connexin Carboxyl-Terminal (aCT1) Peptide in Resistant HER2+ Breast Cancer

Connexin 43 (Cx43) is a protein encoded by the GJA1 gene and is a component of cell membrane structures called gap junctions, which facilitate intercellular communication. Prior evidence indicates that elevated GJA1 expression in the HER2-positive (HER2+) subtype of breast cancer is associated with poor prognosis. Prior evidence also suggests that HER2+ breast cancers that have become refractory to HER2-targeted agents have a loss of Cx43 gap junction intercellular communication (GJIC). In this study, a Cx43-targeted agent called alpha-connexin carboxyl-terminal peptide (aCT1) is examined to determine whether GJIC can be rescued in refractory HER2+ breast cancer cells. A proposed mechanism of action for aCT1 is binding to the tight junction protein Zonal Occludens-1 (ZO-1). However, the true scope of activity for aCT1 has not been explored. In this study, mass spectrometry proteomic analysis is used to determine the breadth of aCT1-interacting proteins. The NanoString nCounter Breast Cancer 360 panel is also used to examine the effect of aCT1 on cancer signaling in HER2+ breast cancer cells. Findings from this study show a dynamic range of binding partners for aCT1, many of which regulate gene expression and RNA biology. nCounter analysis shows that a number of pathways are significantly impacted by aCT1, including upregulation of apoptotic factors, leading to the prediction and demonstration that aCT1 can boost the cell death effects of cisplatin and lapatinib in HER2+ breast cancer cells that have become resistant to HER2-targeted agents.

Targeting NANOG and FAK via Cx26-derived Cell-penetrating Peptides in Triple-negative Breast Cancer

Triple-negative breast cancer (TNBC) represents the most lethal and treatment-resistant breast cancer subtype with limited treatment options. We previously identified a protein complex unique to TNBC composed of the gap junction protein connexin 26 (Cx26), the pluripotency transcription factor NANOG, and focal adhesion kinase (FAK). We sought to determine whether a peptide mimetic of the interaction region of Cx26 attenuated tumor growth in preclinical models. We designed peptides based on Cx26 juxtamembrane domains and performed binding experiments with NANOG and FAK using surface plasmon resonance. Binding studies revealed that the Cx26 C-terminal tail and intracellular loop bound to NANOG and FAK with submicromolar-to-micromolar affinity and that a 5-amino acid sequence in the C-terminal tail of Cx26 (RYCSG) was sufficient for binding. Peptides with high affinity were engineered with a cell-penetrating antennapedia sequence and assessed in functional assays including cell proliferation, tumorsphere formation, and in vivo tumor growth, and downstream signaling changes were measured. The cell-penetrating Cx26 peptide (aCx26-pep) disrupted self-renewal while reducing nuclear FAK and NANOG and inhibiting NANOG target gene expression in TNBC cells but not luminal mammary epithelial cells. In vivo, aCx26-pep reduced tumor growth and proliferation and induced cell death. Here, we provide proof-of-concept that a Cx26 peptide-based strategy inhibits growth and alters NANOG activity specifically in TNBC, indicating the therapeutic potential of this targeting approach.

Expression of the methylcytosine dioxygenase ten-eleven translocation-2 and connexin 43 in inflammatory bowel disease and colorectal cancer

BACKGROUND

Inflammatory bowel disease (IBD) constitutes a substantial risk factor for colorectal cancer. Connexin 43 (Cx43) is a protein that forms gap junction (GJ) complexes involved in intercellular communication, and its expression is altered under pathological conditions, such as IBD and cancer. Recent studies have implicated epigenetic processes modulating DNA methylation in the pathogenesis of diverse inflammatory and malignant diseases. The ten-eleven translocation-2 (TET-2) enzyme catalyzes the demethylation, hence, regulating the activity of various cancer-promoting and tumor-suppressor genes.

AIM

To investigate Cx43 and TET-2 expression levels and presence of 5-hydroxymethylcytosine (5-hmC) marks under inflammatory conditions both in vitro and in vivo.

METHODS

TET-2 expression was evaluated in parental HT-29 cells and in HT-29 cells expressing low or high levels of Cx43, a putative tumor-suppressor gene whose expression varies in IBD and colorectal cancer, and which has been implicated in the inflammatory process and in tumor onset. The dextran sulfate sodium-induced colitis model was reproduced in BALB/c mice to evaluate the expression of TET-2 and Cx43 under inflammatory conditions in vivo. In addition, archived colon tissue sections from normal, IBD (ulcerative colitis), and sporadic colon adenocarcinoma patients were obtained and evaluated for the expression of TET-2 and Cx43. Expression levels were reported at the transcriptional level by quantitative real-time polymerase chain reaction, and at the translational level by Western blotting and immunofluorescence.

RESULTS

Under inflammatory conditions, Cx43 and TET-2 expression levels increased compared to non-inflammatory conditions. TET-2 upregulation was more pronounced in Cx43-deficient cells. Moreover, colon tissue sections from normal, ulcerative colitis, and sporadic colon adenocarcinoma patients corroborated that Cx43 expression increased in IBD and decreased in adenocarcinoma, compared to tissues from non-IBD subjects. However, TET-2 expression and 5-hmC mark levels decreased in samples from patients with ulcerative colitis or cancer. Cx43 and TET-2 expression levels were also investigated in an experimental colitis mouse model. Interestingly, mice exposed to carbenoxolone (CBX), a GJ inhibitor, had upregulated TET-2 levels. Collectively, these results show that TET-2 levels and activity increased under inflammatory conditions, in cells downregulating gap junctional protein Cx43, and in colon tissues from mice exposed to CBX.

CONCLUSION

These results suggest that TET-2 expression levels, as well as Cx43 expression levels, are modulated in models of intestinal inflammation. We hypothesize that TET-2 may demethylate genes involved in inflammation and tumorigenesis, such as Cx43, potentially contributing to intestinal inflammation and associated carcinogenesis.

Connexin 43 Contributes to the Sensitization of Colorectal Cancer Cells to Photodynamic Therapy through Akt Inhibition

BACKGROUND

Photodynamic therapy could be one approach to treat colorectal cancer, but resistance leads to failure of therapy. Akt activation is a cellular survival response to photodynamic therapy and is also a reason for resistance. Thus, inhibition of Akt is a strategy to decrease resistance. Akt interacts with connexin 43, another protein involved in photodynamic therapy resistance. Connexin 43 is widely expressed in different human tissues and has a complex role in tumor development. However, the mechanism of inhibition of Akt by connexin 43 that sensitizes colorectal cancer cells to photodynamic therapy needs further investigation.

METHODS

In this study, two colorectal cancer cells with low phosphorylated connexin 43 level were used to explore this mechanism. LY294002 was used as an Akt inhibitor, and connexin 43-pCMV3 was transfected into cells to increase connexin 43 expression.

RESULTS

Akt and connexin 43 inhibit each other in both colorectal cancer cell lines. In vitro and in vivo experiments showed that LY294002 and connexin 43 transfection sensitized cells to hematoporphyrin-Photodynamic therapy. LY294002 increased the sensitivity of cells to photodynamic therapy with a pronounced effect in cells with high expression levels of connexin 43.

CONCLUSIONS

Connexin 43 should be considered an important factor in increasing the phototoxicity of photodynamic therapy in colorectal cancer through Akt inhibition.

Lipocalin-2 (LCN2) Deficiency Leads to Cellular Changes in Highly Metastatic Human Prostate Cancer Cell Line PC-3

The transporter protein lipocalin-2 (LCN2) also termed neutrophil-gelatinase-associated lipocalin (NGAL) has pleiotropic effects in tumorigenesis in various cancers. Since the precise role of LCN2 in prostate cancer (PCa) is poorly understood, we aimed to elucidate its functions in PCa in vitro. For this purpose, LCN2 was transiently suppressed or permanently depleted in human PC-3 cells using siRNA or CRISPR/Cas9-mediated knockout. Effects of LCN2 suppression on expression of different tumorigenic markers were investigated by Western blot analysis and RT-qPCR. LCN2 knockout cells were analyzed for cellular changes and their ability to cope endoplasmic stress compared to parenteral PC-3 cells. Reduced LCN2 was accompanied by decreased expression of IL-1β and Cx43. In PC-3 cells, LCN2 deficiency leads to reduced proliferation, diminished expression of pro-inflammatory cytokines, lower adhesion, and disrupted F-actin distribution. In addition, IL-1β expression strongly correlated with LCN2 levels. LCN2 knockout cells showed enhanced and sustained activation of unfolded protein response proteins when treated with tunicamycin or cultured under glucose deprivation. Interestingly, an inverse correlation between phosphorylation of eukaryotic initiation factor 2 α subunit (p-eIF2α) and LCN2 expression was observed suggesting that LCN2 triggers protein synthesis under stress conditions. The finding that LCN2 depletion leads to significant phenotypic and cellular changes in PC-3 cells adds LCN2 as a valuable target for the treatment of PCa.

Collective metastasis: coordinating the multicellular voyage

The metastatic process is arduous. Cancer cells must escape the confines of the primary tumor, make their way into and travel through the circulation, then survive and proliferate in unfavorable microenvironments. A key question is how cancer cells overcome these multiple barriers to orchestrate distant organ colonization. Accumulating evidence in human patients and animal models supports the hypothesis that clusters of tumor cells can complete the entire metastatic journey in a process referred to as collective metastasis. Here we highlight recent studies unraveling how multicellular coordination, via both physical and biochemical coupling of cells, induces cooperative properties advantageous for the completion of metastasis. We discuss conceptual challenges and unique mechanisms arising from collective dissemination that are distinct from single cell-based metastasis. Finally, we consider how the dissection of molecular transitions regulating collective metastasis could offer potential insight into cancer therapy.

Metastatic Spread in Prostate Cancer Patients Influencing Radiotherapy Response

Radiotherapy and surgery are curative treatment options for localized prostate cancer (PCa) with a 5-year survival rate of nearly 100%. Once PCa cells spread into distant organs, such as bone, the overall survival rate of patients drops dramatically. The metastatic cascade and organotropism of PCa cells are regulated by different cellular subtypes, organ microenvironment, and their interactions. This cross-talk leads to pre-metastatic niche formation that releases chemo-attractive factors enforcing the formation of distant metastasis. Biological characteristics of PCa metastasis impacting on metastatic sites, burden, and latency is of clinical relevance. Therefore, the implementation of modern hybrid imaging technologies into clinical routine increased the sensitivity to detect metastases at earlier stages. This enlarged the number of PCa patients diagnosed with a limited number of metastases, summarized as oligometastatic disease. These patients can be treated with androgen deprivation in combination with local-ablative radiotherapy or radiopharmaceuticals directed to metastatic sites. Unfortunately, the number of patients with disease recurrence is high due to the enormous heterogeneity within the oligometastatic patient population and the lack of available biomarkers with predictive potential for metastasis-directed radiotherapy. Another, so far unmet clinical need is the diagnosis of minimal residual disease before onset of clinical manifestation and/or early relapse after initial therapy. Here, monitoring of circulating and disseminating tumor cells in PCa patients during the course of radiotherapy may give us novel insight into how metastatic spread is influenced by radiotherapy and vice versa. In summary, this review critically compares current clinical concepts for metastatic PCa patients and discuss the implementation of recent preclinical findings improving our understanding of metastatic dissemination and radiotherapy resistance into standard of care.

Connexins in Cancer: Jekyll or Hyde?

The expression, localization, and function of connexins, the protein subunits that comprise gap junctions, are often altered in cancer. In addition to cell-cell coupling through gap junction channels, connexins also form hemichannels that allow communication between the cell and the extracellular space and perform non-junctional intracellular activities. Historically, connexins have been considered tumor suppressors; however, they can also serve tumor-promoting functions in some contexts. Here, we review the literature surrounding connexins in cancer cells in terms of specific connexin functions and propose that connexins function upstream of most, if not all, of the hallmarks of cancer. The development of advanced connexin targeting approaches remains an opportunity for the field to further interrogate the role of connexins in cancer phenotypes, particularly through the use of in vivo models. More specific modulators of connexin function will both help elucidate the functions of connexins in cancer and advance connexin-specific therapies in the clinic.

Gap junctions amplify spatial variations in cell volume in proliferating tumor spheroids

Sustained proliferation is a significant driver of cancer progression. Cell-cycle advancement is coupled with cell size, but it remains unclear how multiple cells interact to control their volume in 3D clusters. In this study, we propose a mechano-osmotic model to investigate the evolution of volume dynamics within multicellular systems. Volume control depends on an interplay between multiple cellular constituents, including gap junctions, mechanosensitive ion channels, energy-consuming ion pumps, and the actomyosin cortex, that coordinate to manipulate cellular osmolarity. In connected cells, we show that mechanical loading leads to the emergence of osmotic pressure gradients between cells with consequent increases in cellular ion concentrations driving swelling. We identify how gap junctions can amplify spatial variations in cell volume within multicellular spheroids and, further, describe how the process depends on proliferation-induced solid stress. Our model may provide new insight into the role of gap junctions in breast cancer progression.

Lactate in the Tumor Microenvironment: An Essential Molecule in Cancer Progression and Treatment

Simple Summary

The role of lactate in cancer described by Otto Warburg in 1927 states that cancer cells uptake high amount of glucose with a marked increase in lactate production, this is known as the “Warburg effect”. Since then lactate turn out to be a major signaling molecule in cancer progression. Its release from tumor cells is accompanied by acidification ranging from 6.3 to 6.9 in the tumor microenvironment (TME) which favors processes such as tumor promotion, angiogenesis, metastasis, tumor resistance and more importantly, immunosuppression which has been associated with a poor outcome. The goal of this review is to examine and discuss in deep detail the recent studies that address the role of lactate in all these cancerous processes. Lastly, we explore the efforts to target the lactate production and its transport as a promising approach for cancer therapeutics.

Abstract

Cancer is a complex disease that includes the reprogramming of metabolic pathways by malignant proliferating cells, including those affecting the tumor microenvironment (TME). The “TME concept” was introduced in recognition of the roles played by factors other than tumor cells in cancer progression. In response to the hypoxic or semi-hypoxic characteristic of the TME, cancer cells generate a large amount of lactate via the metabolism of glucose and glutamine. Export of this newly generated lactate by the tumor cells together with H+ prevents intracellular acidification but acidifies the TME. In recent years, the importance of lactate and acidosis in carcinogenesis has gained increasing attention, including the role of lactate as a tumor-promoting metabolite. Here we review the existing literature on lactate metabolism in tumor cells and the ability of extracellular lactate to direct the metabolic reprogramming of those cells. Studies demonstrating the roles of lactate in biological processes that drive or sustain carcinogenesis (tumor promotion, angiogenesis, metastasis and tumor resistance) and lactate’s role as an immunosuppressor that contributes to tumor evasion are also considered. Finally, we consider recent therapeutic efforts using available drugs directed at and interfering with lactate production and transport in cancer treatment.

Emerging Players in Prostate Cancer-Bone Niche Communication

Patients with advanced prostate cancer (PCa) frequently develop skeletal metastases that are associated with fractures, disability, and increased mortality. Within the bone metastatic niche, mutual interactions between tumor cells and osteoblasts have been proposed as major contributors of osteotropism by PCa. Here, we highlight the emerging role of PCa-derived extracellular vesicles (EVs) in reprogramming osteoblasts and support of premetastatic niche formation. We also develop the concept of cancer-associated osteoblasts (CAOs) and outline the potential of PCa cells to acquire an osteoblastic phenotype, termed osteomimicry, as two strategies that PCa utilizes to create a favorable protected niche. Finally, we delineate future research that may help to deconstruct the complexity of PCa osteotropism.

Cx43 Present at the Leading Edge Membrane Governs Promigratory Effects of Osteoblast-Conditioned Medium on Human Prostate Cancer Cells in the Context of Bone Metastasis

Simple Summary

In its late stages, prostate cancer (PCa) is characterized by a high propensity to form osteoblastic bone metastases, mainly treated by palliative approaches. In a previous work, we demonstrated that a gap junctional protein, connexin43 (Cx43) is implicated both in the increase of aggressiveness of PCa cells and in their impact on bone. To analyze the reciprocal part of the dialogue, the current study addresses the role of Cx43 in the impact of bone microenvironment on PCa cells abilities. Using Cx43-overexpressing PCa cell lines, we determined that Cx43 is necessary for promigratory effect induced by osteoblastic conditioned media (ObCM) on individual cells. Next, we demonstrated the requirement of Cx43 membrane localization at the leading edge and the involvement of the cytoplasmic part in this ObCM-induced migration. Overall, our findings precise the role of Cx43 during PCa progression and its putative use as aggressiveness marker and as potential therapeutic targets.

Abstract

Among the different interacting molecules implicated in bone metastases, connexin43 (Cx43) may increase sensitivity of prostate cancer (PCa) cells to bone microenvironment, as suggested by our in silico and human tissue samples analyses that revealed increased level of Cx43 expression with PCa progression and a Cx43 specific expression in bone secondary sites. The goal of the present study was to understand how Cx43 influences PCa cells sensitivity and aggressiveness to bone microenvironment. By means of Cx43-overexpressing PCa cell lines, we revealed a Cx43-dependent promigratory effect of osteoblastic conditioned media (ObCM). This effect on directional migration relied on the presence of Cx43 at the plasma membrane and not on gap junctional intercellular communication and hemichannel functions. ObCM stimulation induced Rac1 activation and Cx43 interaction with cortactin in protrusions of migrating PCa cells. Finally, by transfecting two different truncated forms of Cx43 in LNCaP cells, we determined that the carboxy terminal (CT) part of Cx43 is crucial for the responsiveness of PCa cells to ObCM. Our study demonstrates that Cx43 level and its membrane localization modulate the phenotypic response of PCa cells to osteoblastic microenvironment and that its CT domain plays a pivotal role.

Cx43 promotes SHF-DPCs proliferation in the hair follicle of Albas cashmere goats from anagen to telogen

Connexin 43 (Cx43), known to form gap junction transmembrane channels between the cytoplasm of two adjacent cells, plays a key role in physiological functions, such as regulating cell growth, differentiation, and maintaining tissue homeostasis. Cashmere goat is an important farm animal that provides cashmere, which was produced by secondary hair follicles (SHF), for human consumption; however, there is no report about the role of Cx43 on the growth and development of SHF in cashmere goat. In this study, we investigated the effect of Cx43 on proliferation secondary hair follicle dermal papilla cells (SHF-DPCs) in Albas cashmere goat. In SHF-DPCs, Cx43 overexpression promoted cell proliferation and upregulated the expression of IGF-1, whereas Cx43 knockdown was associated with the opposite effects. These results suggested that Cx43 may promote cell proliferation by inducing IGF-1. Overall, our research not only contributes to a better understanding of the mechanism of the growth and development of SHF in cashmere goat, but also shed light on cashmere quality control in the future.

Antagonistic Functions of Connexin 43 during the Development of Primary or Secondary Bone Tumors

Despite research and clinical advances during recent decades, bone cancers remain a leading cause of death worldwide. There is a low survival rate for patients with primary bone tumors such as osteosarcoma and Ewing’s sarcoma or secondary bone tumors such as bone metastases from prostate carcinoma. Gap junctions are specialized plasma membrane structures consisting of transmembrane channels that directly link the cytoplasm of adjacent cells, thereby enabling the direct exchange of small signaling molecules between cells. Discoveries of human genetic disorders due to genetic mutations in gap junction proteins (connexins) and experimental data using connexin knockout mice have provided significant evidence that gap-junctional intercellular communication (Gj) is crucial for tissue function. Thus, the dysfunction of Gj may be responsible for the development of some diseases. Gj is thus a main mechanism for tumor cells to communicate with other tumor cells and their surrounding microenvironment to survive and proliferate. If it is well accepted that a low level of connexin expression favors cancer cell proliferation and therefore primary tumor development, more evidence is suggesting that a high level of connexin expression stimulates various cellular process such as intravasation, extravasation, or migration of metastatic cells. If so, connexin expression would facilitate secondary tumor dissemination. This paper discusses evidence that suggests that connexin 43 plays an antagonistic role in the development of primary bone tumors as a tumor suppressor and secondary bone tumors as a tumor promoter.

Connexin 43 plays a role in proliferation and migration of pulmonary arterial fibroblasts in response to hypoxia

Pulmonary hypertension (PH) is a disease associated with vasoconstriction and remodelling of the pulmonary vasculature. Pulmonary artery fibroblasts (PAFs) play an important role in hypoxic-induced remodelling. Connexin 43 (Cx43) is involved in cellular communication and regulation of the pulmonary vasculature. Using both in vitro and in vivo models of PH, the aims of this study were to (i) investigate the role of Cx43 in hypoxic-induced proliferation and migration of rat PAFs (rPAFs) and rat pulmonary artery smooth muscle cells (rPASMCs) and (ii) determine whether Cx43 expression is dysregulated in the rat sugen5416/hypoxic model of PH. The role of Cx43 in hypoxic-induced proliferation and migration was investigated using Gap27 (a pharmacological inhibitor of Cx43) or genetic knockdown of Cx43 using siRNA. Cx43 protein expression was increased by hypoxia in rPAFs but not rPASMCs. Hypoxic exposure, in the presence of serum, resulted in an increase in proliferation of rPAFs but not rPASMCs. Hypoxic exposure caused migration of rPAFs but not rPASMCs. Phosphorylation of p38 mitogen-activated protein kinase (MAPK) and ERK1/2 were increased by hypoxia in rPAFs. The effects of hypoxia on proliferation, migration and MAPK phosphorylation in rPAFs were attenuated in the presence of Gap27 or Cx43 siRNA. Cx43 protein expression was increased in sugen5416/hypoxic rat lung; this increased expression was not observed in sugen5416/hypoxic rats treated with the MAPK pathway inhibitor GS-444217. In conclusion, Cx43 is involved in the proliferation and migration of rPAFs in response to hypoxia via the MAPK signalling pathway.

Development of a Cx46 Targeting Strategy for Cancer Stem Cells

Gap-junction-mediated cell-cell communication enables tumor cells to synchronize complex processes. We previously found that glioblastoma cancer stem cells (CSCs) express higher levels of the gap junction protein Cx46 compared to non-stem tumor cells (non-CSCs) and that this was necessary and sufficient for CSC maintenance. To understand the mechanism underlying this requirement, we use point mutants to disrupt specific functions of Cx46 and find that Cx46-mediated gap-junction coupling is critical for CSCs. To develop a Cx46 targeting strategy, we screen a clinically relevant small molecule library and identify clofazimine as an inhibitor of Cx46-specific cell-cell communication. Clofazimine attenuates proliferation, self-renewal, and tumor growth and synergizes with temozolomide to induce apoptosis. Although clofazimine does not cross the blood-brain barrier, the combination of clofazimine derivatives optimized for brain penetrance with standard-of-care therapies may target glioblastoma CSCs. Furthermore, these results demonstrate the importance of targeting cell-cell communication as an anti-cancer therapy.

Squid Ink Polysaccharides Protect Human Fibroblast Against Oxidative Stress by Regulating NADPH Oxidase and Connexin43

Oxidation injury to skin is one of the main reasons for skin aging. The aim of the present study was to explore the protective effect of squid ink polysaccharides and its mechanism of action against H₂O₂-induced dermal fibroblast damage. Our results show that squid ink polysaccharides effectively reduce the fibroblast oxidative damage mediated by the up-regulation of NADPH oxidase and Connexin43. Concurrently, squid ink polysaccharides decrease the ROS induced up-regulation of MMP1 and MMP9 to decrease MMP-mediated skin aging. Therefore, we hypothesize that squid ink polysaccharides play an antioxidant role by inhibiting the expression of NADPH oxidase and connexin43. This provides a new target for the effective clinical prevention and treatment of oxidative skin damage.

Connexins in Astrocyte Migration

Astrocytes have long been considered the supportive cells of the central nervous system, but during the last decades, they have gained much more attention because of their active participation in the modulation of neuronal function. For example, after brain damage, astrocytes become reactive and undergo characteristic morphological and molecular changes, such as hypertrophy and increase in the expression of glial fibrillary acidic protein (GFAP), in a process known as astrogliosis. After severe damage, astrocytes migrate to the lesion site and proliferate, which leads to the formation of a glial scar. At this scar-forming stage, astrocytes secrete many factors, such as extracellular matrix proteins, cytokines, growth factors and chondroitin sulfate proteoglycans, stop migrating, and the process is irreversible. Although reactive gliosis is a normal physiological response that can protect brain cells from further damage, it also has detrimental effects on neuronal survival, by creating a hostile and non-permissive environment for axonal repair. The transformation of astrocytes from reactive to scar-forming astrocytes highlights migration as a relevant regulator of glial scar formation, and further emphasizes the importance of efficient communication between astrocytes in order to orchestrate cell migration. The coordination between astrocytes occurs mainly through Connexin (Cx) channels, in the form of direct cell-cell contact (gap junctions, GJs) or contact between the extracellular matrix and the astrocytes (hemichannels, HCs). Reactive astrocytes increase the expression levels of several proteins involved in astrocyte migration, such as α_vβ₃ Integrin, Syndecan-4 proteoglycan, the purinergic receptor P2X7, Pannexin1, and Cx43 HCs. Evidence has indicated that Cx43 HCs play a role in regulating astrocyte migration through the release of small molecules to the extracellular space, which then activate receptors in the same or adjacent cells to continue the signaling cascades required for astrocyte migration. In this review, we describe the communication of astrocytes through Cxs, the role of Cxs in inflammation and astrocyte migration, and discuss the molecular mechanisms that regulate Cx43 HCs, which may provide a therapeutic window of opportunity to control astrogliosis and the progression of neurodegenerative diseases.

MicroRNA-1298-5p inhibits cell proliferation and the invasiveness of bladder cancer cells via down-regulation of connexin 43

MicroRNA (miR)-1298 is widely down-regulated in a variety of malignant tumors, which facilitates cell proliferation, invasiveness, and migration. However, the specific biological function of miR-1298 in bladder cancer (BC) is still unknown. Connexin 43 (Cx43) is often up-regulated in tumors. Identifying miRNAs that target Cx43 in the setting of BC will help to develop Cx43-based therapies for BC. In this study, the results demonstrated that the expression levels of miR-1298 and Cx43 were significantly down-regulated and up-regulated, respectively, in BC tissues. Overexpression of miR-1298 inhibited cell proliferation, migration, and invasiveness in two BC cell lines as determined using MTT assays, cell cycle assays, colony formation assays, Transwell assays, gelatin zymography, and Western blot. In addition, we found that miR-1298 decreased Cx43 expression by directly targeting the 3'-UTR. Further, we observed that the promotion of BC cell proliferation, migration, and invasiveness from Cx43 on could be partially attenuated by overexpressing miR-1298. Moreover, the protein expression of p-ERK was ameliorated after transfection with overexpressed-miR-1298. Knockdown of Cx43 reversed the promotion of cell migration and invasiveness due to decreased expression of miR-1298. All of the data from our study indicate that miR-1298 could be a diagnostic marker of BC and a potential therapeutic agent via inhibiting Cx43.

Direct Intercellular Communications and Cancer: A Snapshot of the Biological Roles of Connexins in Prostate Cancer

Tissue homeostasis is the result of a complex intercellular network controlling the behavior of every cell for the survival of the whole organism. In mammalian tissues, cells do communicate via diverse long- and short-range communication mechanisms. While long-range communication involves hormones through blood circulation and neural transmission, short-range communication mechanisms include either paracrine diffusible factors or direct interactions (e.g., gap junctions, intercellular bridges and tunneling nanotubes) or a mixture of both (e.g., exosomes). Tumor growth represents an alteration of tissue homeostasis and could be the consequence of intercellular network disruption. In this network, direct short-range intercellular communication seems to be particularly involved. The first type of these intercellular communications thought to be involved in cancer progression were gap junctions and their protein subunits, the connexins. From these studies came the general assumption that global decreased connexin expression is correlated to tumor progression and increased cell proliferation. However, this assumption appeared more complicated by the fact that connexins may act also as pro-tumorigenic. Then, the concept that direct intercellular communication could be involved in cancer has been expanded to include new forms of intercellular communication such as tunneling nanotubes (TNTs) and exosomes. TNTs are intercellular bridges that allow free exchange of small molecules or even mitochondria depending on the presence of gap junctions. The majority of current research shows that such exchanges promote cancer progression by increasing resistance to hypoxia and chemotherapy. If exosomes are also involved in these mechanisms, more studies are needed to understand their precise role. Prostate cancer (PCa) represents a type of malignancy with one of the highest incidence rates worldwide. The precise role of these types of direct short-range intercellular communication has been considered in the progression of PCa. However, even though data are in favor of connexins playing a key role in PCa progression, a clear understanding of the role of TNTs and exosomes is needed to define their precise role in this malignancy. This review article summarizes the current view of the main mechanisms involved in short-range intercellular communication and their implications in cancer and delves into the biological, predictive and therapeutic role of connexins in PCa.

What makes cells move: Requirements and obstacles for leader cells in collective invasion

Most recently, mounting evidence has shown that cancer cells can invade as a cohesive and multicellular group with coordinated movement, which is called collective invasion. In this cohesive cancer cell group, cancer cells at the front of collective invasion are defined as leader cell that are responsible for many aspects of collective invasion, including sensing the microenvironment, determining the invasion direction, modifying the path of invasion and transmitting information to other cells. To fulfill their dispensable roles, leader cells are required to embark on some specific phenotypes with unusual expression of some proteins and it's very important to investigate into these proteins as they may serve as potential therapeutic targets. Here, in this review we will summarize current knowledge on four emerging proteins highly expressed in leader cells including K14, ΔNp63α, Dll4 and cysteine protease cathepsin B (CTSB), with a focus on their important roles in collective invasion and special mechanisms by which they promote collective invasion.

Cx43 Expression Correlates with Breast Cancer Metastasis in MDA-MB-231 Cells In Vitro, In a Mouse Xenograft Model and in Human Breast Cancer Tissues

Connexins regulate multiple cellular functions and are considered tumor suppressors. Connexin43 (Cx43) is frequently down-regulated in breast tumors. However, Cx43 regulation during cancer onset and metastasis is complex and context-dependent. We investigated the effect of Cx43 over-expression or knock-down on the metastatic potential of MDA-MB-231 breast cancer cells in vitro and in vivo and in human breast cancer tissues. MDA-MB-231 cells over-expressing (Cx43D) or down-regulating Cx43 (shCx43) were generated and used in proliferation, migration, and invasion assays. The regulation of genes/proteins implicated in progression, invasion and metastasis was assessed in vitro and in immune-compromized mice injected with MDA-MB-231, Cx43D or shCx43 cells. Primary tumor onset/growth, metastasis and overall survival of these animals was monitored and evaluated. In addition, Cx43 expression in human breast carcinoma samples was assessed by qPCR. Cx43 over-expression increased protein levels of epithelial markers E-cadherin and zonula occludens 1 expression and resulted in the sequestration of β-catenin at the cell membrane, while Cx43 knock-down induced protein expression of the mesenchymal marker N-cadherin and an increased invasive potential of shCx43 cells. In vivo, in mice xenografted with breast cancer cells, Cx43 over-expression decreased tumor volume, attenuated cell metastasis to lungs and liver and increased overall mice survival. Importantly, the expression of Cx43 in triple negative human breast cancer tissues is also down-regulated. Collectively, Cx43 over-expression induced an epithelial-like phenotype in MDA-MB-231 cells and suppressed tumor growth and metastasis to secondary organs in vivo. In contrast, Cx43 knock-down in MDA-MB-231 cells induced a mesenchymal phenotype with increased cell invasion leading to an enhanced metastatic phenotype. These data provide evidence for a pivotal role of Cx43 in breast cancer metastasis and support the potential targeting of connexins in breast cancer therapy.

Emerging roles of gap junction proteins connexins in cancer metastasis, chemoresistance and clinical application

Connexin, a four-pass transmembrane protein, contributes to assembly of gap junctions among neighboring cells and thus facilitates gap junctional intercellular communication (GJIC). Traditionally, the roles of connexins were thought to mediate formation of hemichannels and GJIC assembly for transportation of ions and small molecules. Many studies have observed loss of GJIC, due to reduced expression or altered cytoplasmic localization of connexins, in primary tumor cells. Connexins are generally considered tumor-suppressive. However, recent studies of clinical samples suggested a different role of connexins in that expression levels and membrane localization of connexins, including Connexin 43 (Cx43, GJA1) and Connexin 26 (Cx26, GJB2), were found to be enhanced in metastatic lesions of cancer patients. Cx43- and Cx26-mediated GJIC was found to promote cancer cell migration and adhesion to the pulmonary endothelium. Regulatory circuits involved in the induction of connexins and their functional effects have also been reported in various types of cancer. Connexins expressed in stromal cells were correlated with metastasis and were implicated in regulating metastatic behaviors of cancer cells. Recent studies have revealed that connexins can contribute to cellular phenotypes via multiple ways, namely 1) GJIC, 2) C-terminal tail-mediated signaling, and 3) cell-cell adhesion during gap junction formation. Both expression levels and the subcellular localization could participate determining the functional roles of connexins in cancer. Compounds targeting connexins were thus tested as potential therapeutics intervening metastasis or chemoresistance. This review focuses on the recent findings in the correlation between the expression of connexins and patients’ prognosis, their roles in metastasis and chemoresistance, as well as the implications and concerns of using connexin-targeting drugs as anti-metastatic therapeutics. Overall, connexins may serve as biomarkers for cancer prognosis and as therapeutic targets for intervening metastasis and chemoresistance.

Cancer Connectors: Connexins, Gap Junctions, and Communication

Despite concerted clinical and research efforts, cancer is a leading cause of death worldwide. Surgery, radiation, and chemotherapy have remained the most common standard-of-care strategies against cancer for decades. However, the side effects of these therapies demonstrate the need to investigate adjuvant novel treatment modalities that minimize the harm caused to healthy cells and tissues. Normal and cancerous cells require communication amongst themselves and with their surroundings to proliferate and drive tumor growth. It is vital to understand how intercellular and external communication impacts tumor cell malignancy. To survive and grow, tumor cells, and their normal counterparts utilize cell junction molecules including gap junctions (GJs), tight junctions, and adherens junctions to provide contact points between neighboring cells and the extracellular matrix. GJs are specialized structures composed of a family of connexin proteins that allow the free diffusion of small molecules and ions directly from the cytoplasm of adjacent cells, without encountering the extracellular milieu, which enables rapid, and coordinated cellular responses to internal and external stimuli. Importantly, connexins perform three main cellular functions. They enable direct gap junction intercellular communication (GJIC) between cells, form hemichannels to allow cell communication with the extracellular environment, and serve as a site for protein-protein interactions to regulate signaling pathways. Connexins themselves have been found to promote tumor cell growth and invasiveness, contributing to the overall tumorigenicity and have emerged as attractive anti-tumor targets due to their functional diversity. However, connexins can also serve as tumor suppressors, and therefore, a complete understanding of the roles of the connexins and GJs in physiological and pathophysiological conditions is needed before connexin targeting strategies are applied. Here, we discuss how the three aspects of connexin function, namely GJIC, hemichannel formation, and connexin-protein interactions, function in normal cells, and contribute to tumor cell growth, proliferation, and death. Finally, we discuss the current state of anti-connexin therapies and speculate which role may be most amenable for the development of targeting strategies.

Relationship of vasculogenic mimicry, SphK1 expression, and Cx43 expression to metastasis and prognosis in colorectal cancer

OBJECTIVE

To determine the presence of vasculogenic mimicry (VM) and expression of Sphingosine kinase 1 (SphK1) and Connexin43 (Cx43) in colorectal cancer (CRC) tissues, and to identify their inter-relationships and associations with multiple pathologic parameters.

METHODS

Ninety-two CRC specimens and normal pericarcinoma tissues were analyzed for expression of SphK1 and Cx43 using immunohistochemistry, and for identification of VM using CD34-periodic acid-Schiff dual staining.

RESULTS

The positive rate of SphK1 expression was greater in CRC cells than pericarcinoma cells (85.87% vs. 33.70%, P < 0.05). In contrast, the positive rate of Cx43 expression was greater in pericarcinoma cells than in CRC cells (58.70% vs. 92.39%, P < 0.05). Analysis of CRC tissues indicated that expression of SphK1 was associated with poor differentiation, advanced tumor stage, lymph node metastasis, and the presence of VM (P < 0.05 for each comparison). Expression of Cx43 was associated with high differentiation and the presence of VM (P < 0.05 for each comparison). Patient sex, age, tumor size, depth of invasion, and distant metastasis were unrelated to the expression of either protein. There was a significant correlation between the expression of SphK1 and Cx43 (P < 0.05). Analysis of overall patient survival indicated that SphK1 positivity and the presence of VM were significantly associated with poor survival, but Cx43 positivity had no relationship with survival.

CONCLUSION

SphK1 protein expression was significantly greater in CRC tissues than pericarcinoma tissues, suggesting this protein may be associated with the pathogenesis of CRC. In addition, the significant correlation between expression of SphK1 and Cx43 in CRC tissues suggests their interaction may impact the pathogenesis of CRC.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

The effects of flutamide on cell-cell junctions in the testis, epididymis, and prostate

In this review, we summarize recent findings on the effect of the anti-androgen flutamide on cell-cell junctions in the male reproductive system. We outline developmental aspects of flutamide action on the testis, epididymis, and prostate, and describe changes in junction protein expression and organization of junctional complexes in the adult boar following prenatal and postnatal exposure. We also discuss findings on the mechanisms by which flutamide induces alterations in cell-cell junctions in reproductive tissues of adult males, with special emphasis on cytoplasmic effects. Based on the results from in vivo and in vitro studies in the rat, we propose that flutamide affects the expression of junction proteins and junction complex structure not only by inhibiting androgen receptor activity, but equally important by modulating protein kinase-dependent signaling in testicular cells. Additionally, results from studies on prostate cancer cell lines point to a role for the cellular molecular outfit in response to flutamide.

Connexins and Pannexins: Important Players in Tumorigenesis, Metastasis and Potential Therapeutics

Since their characterization more than five decades ago, gap junctions and their structural proteins-the connexins-have been associated with cancer cell growth. During that period, the accumulation of data and molecular knowledge about this association revealed an apparent contradictory relationship between them and cancer. It appeared that if gap junctions or connexins can down regulate cancer cell growth they can be also implied in the migration, invasion and metastatic dissemination of cancer cells. Interestingly, in all these situations, connexins seem to be involved through various mechanisms in which they can act either as gap-junctional intercellular communication mediators, modulators of signalling pathways through their interactome, or as hemichannels, which mediate autocrine/paracrine communication. This complex involvement of connexins in cancer progression is even more complicated by the fact that their hemichannel function may overlap with other gap junction-related proteins, the pannexins. Despite this complexity, the possible involvements of connexins and pannexins in cancer progression and the elucidation of the mechanisms they control may lead to use them as new targets to control cancer progression. In this review, the involvements of connexins and pannexins in these different topics (cancer cell growth, invasion/metastasis process, possible cancer therapeutic targets) are discussed.

Altered translation initiation of Gja1 limits gap junction formation during epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is activated during development, wound healing, and pathologies including fibrosis and cancer metastasis. Hallmarks of EMT are remodeling of intercellular junctions and adhesion proteins, including gap junctions. The GJA1 mRNA transcript encoding the gap junction protein connexin43 (Cx43) has been demonstrated to undergo internal translation initiation, yielding truncated isoforms that modulate gap junctions. The PI3K/Akt/mTOR pathway is central to translation regulation and is activated during EMT, leading us to hypothesize that altered translation initiation would contribute to gap junction loss. Using TGF-β-induced EMT as a model, we find reductions in Cx43 gap junctions despite increased transcription and stabilization of Cx43 protein. Biochemical experiments reveal suppression of the internally translated Cx43 isoform, GJA1-20k in a Smad3 and ERK-dependent manner. Ectopic expression of GJA1-20k does not halt EMT, but is sufficient to rescue gap junction formation. GJA1-20k localizes to the Golgi apparatus, and using superresolution localization microscopy we find retention of GJA1-43k at the Golgi in mesenchymal cells lacking GJA1-20k. NativePAGE demonstrates that levels of GJA1-20k regulate GJA1-43k hexamer oligomerization, a limiting step in Cx43 trafficking. These findings reveal alterations in translation initiation as an unexplored mechanism by which the cell regulates Cx43 gap junction formation during EMT.

High mobility group box 1 antagonist limits metastatic seeding in the lungs via reduction of cell-cell adhesion

Metastatic spread is the leading cause for cancer-related mortality, with the lungs being a major site for metastatic seeding. Available therapies for patients with metastatic disease are extremely limited. Therefore, there is a desperate need for new strategies to prevent or limit metastatic dissemination and treat existing metastases. The metastatic cascade is highly complex and is affected by multiple factors related to both tumor cells themselves and the microenvironment in the future site of metastasis. We hypothesized that modifying the lung microenvironment by blocking central ubiquitous signals may affect metastatic seeding in the lungs. Given the high basal levels of the Receptor for Advanced Glycation End products (RAGE) in the pulmonary tissue, and its pro-inflammatory properties, we investigated the consequences of interfering with its ligand; High Mobility Group Box 1 (HMGB1). To this end, we tested the effect of Carbenoxolone, an HMGB1 antagonist, on primary tumor growth and metastatic progression in several murine tumor models. We show that antagonizing HMGB1 prevents the adhesion and colonization of cancer cells in the lungs through the reduction of their adhesion and cell–cell interaction both in vitro and in vivo. We demonstrated that these activities are mediated by downregulation of the adhesion molecule Intercellular Adhesion Molecule 1 (ICAM1) and ultimately result in reduced metastatic burden. Carbenoxolone decreases significantly lung metastases formation and can be used potentially as prophylactic therapy for metastatic diseases.

A short-term intervention with selenium affects expression of genes implicated in the epithelial-to-mesenchymal transition in the prostate

In parallel with the inconsistency in observational studies and chemoprevention trials, the mechanisms by which selenium affects prostate cancer risk have not been elucidated. We conducted a randomized, placebo-controlled trial to examine the effects of a short-term intervention with selenium on gene expression in non-malignant prostate tissue. Twenty-three men received 300 μg selenium per day in the form of selenized yeast (n=12) or a placebo (n=11) during 5 weeks. Prostate biopsies collected from the transition zone before and after intervention were analysed for 15 participants (n=8 selenium, n=7 placebo). Pathway analyses revealed that the intervention with selenium was associated with down-regulated expression of genes involved in cellular migration, invasion, remodeling and immune responses. Specifically, expression of well-established epithelial markers, such as E-cadherin and epithelial cell adhesion molecule EPCAM, was up-regulated, while the mesenchymal markers vimentin and fibronectin were down-regulated after intervention with selenium. This implies an inhibitory effect of selenium on the epithelial-to-mesenchymal transition (EMT). Moreover, selenium was associated with down-regulated expression of genes involved in wound healing and inflammation; processes which are both related to EMT. In conclusion, our explorative data showed that selenium affected expression of genes implicated in EMT in the transition zone of the prostate.

In vitro inhibited effect of gap junction composed of Cx43 in the invasion and metastasis of testicular cancer resistanced to cisplatin

The effect of gap junction intercellular communication composed of connexin on cancer invasion/metastasis has been thoroughly explored; however, its effect on testicular cancer resistanced to chemotherapy is still unclear. In this study, we found that the capability of invasion and migration of I-10/DDP (cisplatin (DDP)-resistance) cells were elevated. Furthermore, the expression of Cx43 and the function of gap junction (GJ) in I-10/DDP cells were decreased compared with parental I-10 cells. Pharmacological inhibition of GJs by oleamide (Olea) enhanced invasion and migration. However, enhancement of GJs by retinoic acid (RA) decreased invasion and migration of I-10/DDP cells. To further clarify the invasion/migration inhibited effect of GJ in the testicular cancer resistanced to DDP, GJ function was modulated by overexpression and knockdown of Cx43 expression. Overexpression of Cx43 reduced invasion and migration of I-10/DDP cells. Conversely, knockdown of Cx43 expression increased invasion and migration of I-10/DDP cells. In summary, GJ composed of Cx43 inhibits I-10/DDP cells invasion and migration, and it may become the potential therapeutic target for testicular cancer chemotherapy.

Inhibition of estrogen receptor reduces connexin 43 expression in breast cancers

Connexins are widely supported as tumor suppressors due to their downregulation in cancers, nevertheless, more recent evidence suggests roles for connexins in facilitating tumor progression in later stages, including metastasis. One of the key factors regulating the expression, modification, stability, and localization of connexins is hormone receptors in hormone-dependent cancers. It is reasonable to consider that hormones/hormone receptors may modulate connexins expression and play critical roles in the cellular control of connexins during breast cancer progression. In estrogen receptor (ER)-positive breast cancers, tamoxifen and fulvestrant are widely used therapeutic agents and are considered to alter ER signaling. In this present study, we investigated the effects of fulvestrant and tamoxifen in Cx43 expression, and we also explored the role of Cx43 in ER-positive breast cancer migration and the relationship between Cx43 and ER. The involvement of estrogen/ER in Cx43 modulation was further verified by administering tyrosine kinase inhibitors and chemotherapeutic agents. We found that inhibition of ER promoted the binding of E3 ligase Nedd4 to Cx43, leading to Cx43 ubiquitination. Furthermore, inhibition of ER by fulvestrant and tamoxifen phosphorylated p38 MAPK, and inhibition of Rac, MKK3/6, and p38 reversed fulvestrant-reduced Cx43 expression. These findings suggest that Cx43 expression which may positively regulate cell migration is ER-dependent in ER-positive breast cancer cells.

Tramadol attenuates the sensitivity of glioblastoma to temozolomide through the suppression of Cx43‑mediated gap junction intercellular communication

Analgesics and antineoplastic drugs are often used concurrently for cancer patients. Our previous study reported that gap junctions composed of connexin32 (Cx32) was implicated in the effect of analgesics on cisplatin cytotoxicity. However, the effect of analgesic on the most widely expressed connexin (Cx), connexin43 (Cx43), and whether such effect mediates the influence on chemotherapeutic efficiency remain unknown. By manipulation of Cx43 expression or gap junction function, we found that there were gap junction-dependent and independent effect of Cx43 on temozolomide (TMZ) sensitivity in U87 glioblastoma cells. Studies on survival and apoptosis showed widely used analgesic tramadol significantly reduced TMZ-induced cytotoxicity in control and negative control cells but not shCx43-transfected cells. Proliferation assay demonstrated tramadol suppressed TMZ-induced cytotoxicity only on high density (with gap junction formation) but not on low density (without gap junction formation). Tramadol inhibited dye-coupling through gap junctions between U87 cells. Tramadol treatment for 72 h did not alter Cx43 expression, but decreased Cx43 phosphorylation accompanied with reduced p-ERK and p-JNK. Our results indicated that long-term treatment with tramadol reduced TMZ cytotoxicity in U87 cells by suppressing Cx43-composed gap junctions, suggesting identification and usage of antinociceptive drugs which do not downregulate connexin activity should have beneficial therapeutic consequences.

Normoxic cells remotely regulate the acid-base balance of cells at the hypoxic core of connexin-coupled tumor growths

ATP fuels the removal of metabolic end-products, including H⁺ ions that profoundly modulate biological activities. Energetic resources in hypoxic tumor regions are constrained by low-yielding glycolysis, and any means of reducing the cost of acid extrusion, without compromising pH homeostasis, would therefore be advantageous for cancer cells. Some cancers express connexin channels that allow solute exchange between cells, and we propose that, via this route, normoxic cells supply hypoxic neighbors with acid-neutralizing HCO₃^- ions. This hypothesis was tested by imaging cytoplasmic pH in spheroidal tissue growths of connexin43-positive pancreatic cancer Colo357 cells during light-controlled H⁺ uncaging at the hypoxic core. Cytoplasmic acid retention at the core was halved in the presence of CO₂/HCO₃^-, but this process requires a restorative HCO₃^- flux. The effect of CO₂/HCO₃^- was ablated by connexin43 inhibition or knockdown. In connexin-decoupled spheroids, 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), an inhibitor of HCO₃^- uptake, had no effect on cytoplasmic [H⁺] in the H⁺-uncaging region, indicating that DIDS-sensitive transport is not an adequate pH-regulatory strategy therein. With intact connexin-coupling, acid retention at the core increased upon DIDS treatment, indicating that HCO₃^- ions are taken up actively by peripheral cells and then transmitted passively to cells at the hypoxic core. Thus, the energetic burden of pH regulation is offloaded from hypoxic cells onto metabolically altruistic normoxic neighbors.-Dovmark, T. H., Hulikova, A., Niederer, S. A., Vaughan-Jones, R. D., Swietach, P. Normoxic cells remotely regulate the acid-base balance of cells at the hypoxic core of connexin-coupled tumor growths.

Connexins, important players in the dissemination of prostate cancer cells

Over the past 50years, increasing experimental evidences have established that connexins (Cxs) and gap junctional intercellular communication (GJIC) ensure an important role in both the onset and development of cancerous processes. In the present review, we focus on the impact of Cxs and GJIC during the development of prostate cancer (PCa), from the primary growth mainly localized in acinar glands and ducts to the distant metastasis mainly concentrated in bone. As observed in several other types of solid tumours, Cxs and especially Cx43 exhibit an ambivalent role with a tumour suppressor effect in the early stages and, conversely, a rather pro-tumoural profile for most of invasion and dissemination steps to secondary sites. We report here the current knowledge on the function of Cxs during PCa cells migration, cytoskeletal dynamics, proteinases activities and the cross talk with the surrounding stromal cells in the microenvironment of the tumour and the bones. In addition, we discuss the role of Cxs in the bone tropism even if the prostate model is rarely used to study the complete sequence of cancer dissemination compared to breast cancer or melanoma. Even if not yet fully understood, these recent findings on Cxs provide new insights into their molecular mechanisms associated with progression and bone targeted behaviour of PCa. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Myoendothelial gap junctions mediate regulation of angiopoietin-2-induced vascular hyporeactivity after hypoxia through connexin 43-gated cAMP transfer

Angiopoietin-2 (Ang-2) contributes to vascular hyporeactivity after hemorrhagic shock and hypoxia through upregulation of inducible nitric oxide synthase (iNOS) in a vascular endothelial cell (VEC)-specific and Ang-2/Tie2 receptor-dependent manner. While iNOS is primarily expressed in vascular smooth muscle cells (VSMCs), the mechanisms of signal transfer from VECs to VSMCs are unknown. A double-sided coculture model with VECs and VSMCs from Sprague-Dawley rats was used to investigate the role of myoendothelial gap junctions (MEGJs), the connexin (Cx) isoforms involved, and other relevant mechanisms. After hypoxia, VSMCs treated with exogenous Ang-2 showed increased iNOS expression and hyporeactivity, as well as MEGJ formation and communication. These Ang-2 effects were suppressed by the MEGJ inhibitor 18α-glycyrrhetic acid (18-GA), Tie2 siRNA, or Cx43 siRNA. Reagents antagonizing cAMP or protein kinase A (PKA) in VECs inhibited Cx43 expression in MEGJs, decreasing MEGJ formation and associated communication, after hypoxia following Ang-2 treatment. The increased cAMP levels in VSMCs and transfer of Alexa Fluor 488-labeled cAMP from VECs to VSMCs, after hypoxia following Ang-2 treatment, was antagonized by Cx43 siRNA. A cAMP antagonist added to VECs or VSMCs inhibited both increased iNOS expression and hyporeactivity in VSMCs subjected to hypoxia following Ang-2 treatment. Based on these findings, we propose that Cx43 was the Cx isoform involved in MEGJ-mediated VEC-dependent regulation of Ang-2, which induces iNOS protein expression and vascular hyporeactivity after hypoxia. Cx43 was upregulated by cAMP and PKA, permitting cAMP transfer between cells.

Tuning Collective Cell Migration by Cell-Cell Junction Regulation

Collective cell migration critically depends on cell-cell interactions coupled to a dynamic actin cytoskeleton. Important cell-cell adhesion receptor systems implicated in controlling collective movements include cadherins, immunoglobulin superfamily members (L1CAM, NCAM, ALCAM), Ephrin/Eph receptors, Slit/Robo, connexins and integrins, and an adaptive array of intracellular adapter and signaling proteins. Depending on molecular composition and signaling context, cell-cell junctions adapt their shape and stability, and this gradual junction plasticity enables different types of collective cell movements such as epithelial sheet and cluster migration, branching morphogenesis and sprouting, collective network migration, as well as coordinated individual-cell migration and streaming. Thereby, plasticity of cell-cell junction composition and turnover defines the type of collective movements in epithelial, mesenchymal, neuronal, and immune cells, and defines migration coordination, anchorage, and cell dissociation. We here review cell-cell adhesion systems and their functions in different types of collective cell migration as key regulators of collective plasticity.

Connexin-43 channels are a pathway for discharging lactate from glycolytic pancreatic ductal adenocarcinoma cells

Glycolytic cancer cells produce large quantities of lactate that must be removed to sustain metabolism in the absence of oxidative phosphorylation. The only venting mechanism described to do this at an adequate rate is H⁺-coupled lactate efflux on monocarboxylate transporters (MCTs). Outward MCT activity is, however, thermodynamically inhibited by extracellular acidity, a hallmark of solid tumours. This inhibition would feedback unfavourably on metabolism and growth, raising the possibility that other venting mechanisms become important in under-perfused tumours. We investigated connexin-assembled gap junctions as an alternative route for discharging lactate from pancreatic ductal adenocarcinoma (PDAC) cells. Diffusive coupling (calcein transmission) in vitro was strong between Colo357 cells, weaker yet hypoxia-inducible between BxPC3 cells, and very low between MiaPaCa2 cells. Coupling correlated with levels of connexin-43 (Cx43), a protein previously linked to late-stage disease. Evoked lactate dynamics, imaged in Colo357 spheroids using cytoplasmic pH as a read-out, indicated that lactate anions permeate gap junctions faster than highly-buffered H⁺ ions. At steady-state, junctional transmission of lactate (a chemical base) from the spheroid core had an alkalinizing effect on the rim, producing therein a milieu conducive for growth. Metabolite assays demonstrated that Cx43 knockdown increased cytoplasmic lactate retention in Colo357 spheroids (diameter ~150 μm). MiaPaCa2 cells, which are Cx43 negative in monolayer culture, showed markedly increased Cx43 immunoreactivity at areas of invasion in orthotopic xenograft mouse models. These tissue areas were associated with chronic extracellular acidosis (as indicated by the marker LAMP2 near/at the plasmalemma), which can explain the advantage of junctional transmission over MCT in vivo. We propose that Cx43 channels are important conduits for dissipating lactate anions from glycolytic PDAC cells. Furthermore, lactate entry into the better-perfused recipient cells has a favourable alkalinizing effect and supplies substrate for oxidative phosphorylation. Cx43 is thus a novel target for influencing metabolite handling in junctionally-coupled tumours.

Role of connexin 43 in cadmium-induced proliferation of human prostate epithelial cells

Connexins (Cxs), the subunits of gap junction channels, are involved in many physiological processes. Aberrant control of Cxs and gap junction intercellular communication may contribute to many diseases, including the promotion of cancer. Cd exposure is associated with increased risk of human prostate cancer and benign prostatic hyperplasia. The roles of Cxs in the effects of Cd on the prostate have, however, not been reported previously. In this study, the human prostate epithelial cell line RWPE-1 was exposed to Cd. A low dose of Cd stimulated cell proliferation along with a lower degree of gap junction intercellular communication and an elevated level of the protein Cx43. Cd exposure increased the levels of intracellular Ca²⁺ and phosphorylated Cx43 at the Ser368 site. Knockdown of Cx43 using siRNA blocked Cd-induced proliferation and interfered with the Cd-induced changes in the protein levels of cyclin D1, cyclin B1, p27^Kip1 (p27) and p21^Waf1/Cip1 (p21). The increase in Cx43 expression induced by Cd was presumably mediated by the androgen receptor, because it was abolished upon treatment with the androgen receptor antagonist, flutamide. Thus, a low dose of Cd promotes cell proliferation in RWPE-1, possibly mediated by Cx43 expression through an effect on cell cycle-associated proteins. Cx43 might be a target for prostatic diseases associated with Cd exposure. Copyright © 2017 John Wiley & Sons, Ltd.

cGAS-STING Activation in the Tumor Microenvironment and Its Role in Cancer Immunity

Stimulator of interferon (IFN) genes (STING) is a key mediator in the immune response to cytoplasmic DNA sensed by cyclic GMP-AMP (cGAMP) synthase (cGAS). After synthesis by cGAS, cGAMP acts as a second messenger activating STING in the cell harboring cytoplasmic DNA but also in adjacent cells through gap junction transfer. While the role of the cGAS-STING pathway in pathogen detection is now well established, its importance in cancer immunity has only recently started to emerge. Nonetheless, STING appears to be an essential component in the recruitment of immune cells to the tumor microenvironment, which is paramount to immune clearance of the tumor. This review presents an overview of the growing literature around the role of the cGAS-STING pathway in the tumor microenvironment, with a specific focus on the role that cancer cells may play in the direct activation of this pathway, and its amplification through cell-cell transfer of cGAMP.

The inhibition of indoleamine 2, 3-dioxygenase 1 by connexin 43

Upregulation of connexin 43 (Cx43) showed potential in enhancing immune surveillance that was suppressed in the tumor microenvironment. The expression of indoleamine 2, 3-dioxygenase (IDO) is one of the crucial factors contributing to tumor immune tolerance by depletion of tryptophan and IDO-mediated tryptophan metabolites. Here, we aim to investigate the role of Cx43 in IDO production in murine tumor by using Cx43 inducers. Resveratrol (trans-3, 5, 4 '-trihydroxystilbene) is a natural plant-derived polyphenol possessing positive effect against cancer. Salmonella enterica serovar choleraesuis (S.C.) was proved to target and inhibit tumor growth. Both of them regulated Cx43 expression in tumor cells and led to either chemosensitizing or immune-activating. In this study, the correlation between Cx43 and IDO were determined by the treatment of resveratrol and S.C. Our data showed an increase in Cx43 while IDO protein and IDO-mediated inhibited effects on T cell decreased after tumor cells are given with resveratrol and S.C.

TREATMENTS

All of which could be inhibited once the expression of Cx43 was blocked. Cx43 involved in IDO regulation might be useful in developing IDO-targeted cancer immune therapy.

Gap junctions and cancer: communicating for 50 years

Fifty years ago, tumour cells were found to lack electrical coupling, leading to the hypothesis that loss of direct intercellular communication is commonly associated with cancer onset and progression. Subsequent studies linked this phenomenon to gap junctions composed of connexin proteins. Although many studies support the notion that connexins are tumour suppressors, recent evidence suggests that, in some tumour types, they may facilitate specific stages of tumour progression through both junctional and non-junctional signalling pathways. This Timeline article highlights the milestones connecting gap junctions to cancer, and underscores important unanswered questions, controversies and therapeutic opportunities in the field.

Cx25 contributes to leukemia cell communication and chemosensitivity

Leukemia encompasses several hematological malignancies with shared phenotypes that include rapid proliferation, abnormal leukocyte self-renewal, and subsequent disruption of normal hematopoiesis. While communication between leukemia cells and the surrounding stroma supports tumor survival and expansion, the mechanisms underlying direct leukemia cell-cell communication and its contribution to tumor growth are undefined. Gap junctions are specialized intercellular connections composed of connexin proteins that allow free diffusion of small molecules and ions directly between the cytoplasm of adjacent cells. To characterize homotypic leukemia cell communication, we employed in vitro models for both acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) and measured gap junction function through dye transfer assays. Additionally, clinically relevant gap junction inhibitors, carbenoxolone (CBX) and 1-octanol, were utilized to uncouple the communicative capability of leukemia cells. Furthermore, a qRT-PCR screen revealed several connexins with higher expression in leukemia cells compared with normal hematopoietic stem cells. Cx25 was identified as a promising adjuvant therapeutic target, and Cx25 but not Cx43 reduction via RNA interference reduced intercellular communication and sensitized cells to chemotherapy. Taken together, our data demonstrate the presence of homotypic communication in leukemia through a Cx25-dependent gap junction mechanism that can be exploited for the development of anti-leukemia therapies.

Connexin's Connection in Breast Cancer Growth and Progression

Gap junctions are cell-to-cell junctions that are located in the basolateral surface of two adjoining cells. A gap junction channel is composed of a family of proteins called connexins. Gap junction channels maintain intercellular communication between two cells through the exchange of ions, small metabolites, and electrical signals. Gap junction channels or connexins are widespread in terms of their expression and function in maintaining the development, differentiation, and homeostasis of vertebrate tissues. Gap junction connexins play a major role in maintaining intercellular communication among different cell types of normal mammary gland for proper development and homeostasis. Connexins have also been implicated in the pathogenesis of breast cancer. Differential expression pattern of connexins and their gap junction dependent or independent functions provide pivotal cross talk of breast tumor cells with the surrounding stromal cell in the microenvironment. Substantial research from the last 20 years has accumulated ample evidences that allow us a better understanding of the roles that connexins play in the tumorigenesis of primary breast tumor and its metastatic progression. This review will summarize the knowledge about the connexins and gap junction activities in breast cancer highlighting the differential expression and functional dynamics of connexins in the pathogenesis of the disease.

The anti-metastatic micro-environment of the bone: Importance of osteocyte Cx43 hemichannels

Bone metastases of tumor cells are a common and life-threatening feature of a variety of late-stage cancers, including breast cancers. However, until now, much less has been known about the intrinsic anti-metastatic properties of the bones and how these could be exploited to prevent or treat bone metastases. Very recently, native Cx43 hemichannels present in osteocytes have been identified as important anti-metastatic signaling complexes by establishing high local extracellular ATP levels. Moreover, bisphosphonate drugs, applied as adjuvant therapies in the treatment of breast cancer patients and bone diseases, are known to display anti-metastatic properties. Now, it became clear that these compounds exert their effects through osteocyte Cx43 hemichannels, thereby triggering their opening and promoting ATP release in the extracellular micro-environment. Hence, endogenous osteocyte Cx43 hemichannels emerge as important and promising therapeutic targets for the prevention of bone metastases and/or clinical treatment of bone-metastasized breast cancers.