Role of connexins in metastatic breast cancer and melanoma brain colonization

Breaking boundaries: role of the brain barriers in metastatic process

Brain metastases (BMs) are the most common intracranial tumors in adults and occur 3-10 times more frequently than primary brain tumors. Despite intensive multimodal therapies, including resection, radiotherapy, and chemotherapy, BMs are associated with poor prognosis and remain challenging to treat. BMs predominantly originate from primary lung (20-56%), breast (5-20%), and melanoma (7-16%) tumors, although they can arise from other cancer types less frequently. The metastatic cascade is a multistep process involving local invasion, intravasation into the bloodstream or lymphatic system, extravasation into normal tissue, and colonization of the distal site. After reaching the brain, circulating tumor cells (CTCs) breach the blood-brain barrier (BBB).The selective permeability of the BBB poses a significant challenge for therapeutic compounds, limiting the treatment efficacy of BMs. Understanding the mechanisms of tumor cell interactions with the BBB is crucial for the development of effective treatments. This review provides an in-depth analysis of the brain barriers, including the BBB, blood-spinal cord barrier, blood-meningeal barrier, blood-arachnoid barrier, and blood-cerebrospinal fluid barrier. It explores the molecular and cellular components of these barriers and their roles in brain metastasis, highlighting the importance of this knowledge for identifying druggable targets to prevent or limit BM formation.

Connexin-43 in Cancer: Above and Beyond Gap Junctions!

Connexin-43 (Cx43) is the most characterized gap junction protein, primarily involved in the Gap Junctional Intercellular Communication (GJIC) between adjacent cells to facilitate molecule exchange and the formation of a signaling network. It is increasingly evident that the importance of Cx43 is not only limited to its GJIC function, but rather includes its role in connecting the intracellular and extracellular environment by forming membrane hemichannels, as well as its intracellular signaling function mediated by its C-terminal tail (Cx43-CT). Notably, Cx43 has been implicated in a variety of cancers, with earlier notions suggesting a tumor-suppressor function, whereas new studies shed light on its pro-tumorigenic role. Moreover, apart from GJIC-based activities, the relevance of the non-canonical functions of Cx43 in tumor progression is being actively studied. This review provides an analysis of the current research on the pro-tumorigenic roles of Cx43, with a focus on Cx43-CT interactions and the function of hemichannels in cancer progression. A better understanding of the multifaceted functions of Cx43 in cancer biology could foster its recognition as a pivotal target for the development of innovative therapeutic strategies.

Hypersensitive intercellular responses of endometrial stromal cells drive invasion in endometriosis

Endometriosis is a debilitating disease affecting 190 million women worldwide and the greatest single contributor to infertility. The most broadly accepted etiology is that uterine endometrial cells retrogradely enter the peritoneum during menses, and implant and form invasive lesions in a process analogous to cancer metastasis. However, over 90% of women suffer retrograde menstruation, but only 10% develop endometriosis, and debate continues as to whether the underlying defect is endometrial or peritoneal. Processes implicated in invasion include: enhanced motility; adhesion to, and formation of gap junctions with, the target tissue. Endometrial stromal (ESCs) from 22 endometriosis patients at different disease stages show much greater invasiveness across mesothelial (or endothelial) monolayers than ESCs from 22 control subjects, which is further enhanced by the presence of EECs. This is due to the enhanced responsiveness of endometriosis ESCs to the mesothelium, which induces migration and gap junction coupling. ESC-PMC gap junction coupling is shown to be required for invasion, while coupling between PMCs enhances mesothelial barrier breakdown.

Differential stiffness between brain vasculature and parenchyma promotes metastatic infiltration through vessel co-option

In brain metastasis, cancer cells remain in close contact with the existing vasculature and can use vessels as migratory paths-a process known as vessel co-option. However, the mechanisms regulating this form of migration are poorly understood. Here we use ex vivo brain slices and an organotypic in vitro model for vessel co-option to show that cancer cell invasion along brain vasculature is driven by the difference in stiffness between vessels and the brain parenchyma. Imaging analysis indicated that cells move along the basal surface of vessels by adhering to the basement membrane extracellular matrix. We further show that vessel co-option is enhanced by both the stiffness of brain vasculature, which reinforces focal adhesions through a talin-dependent mechanism, and the softness of the surrounding environment that permits cellular movement. Our work reveals a mechanosensing mechanism that guides cell migration in response to the tissue's intrinsic mechanical heterogeneity, with implications in cancer invasion and metastasis.

Integrative analyses identified gap junction beta-2 as a prognostic biomarker and therapeutic target for breast cancer

Background

Increasing evidence has shown that connexins are involved in the regulation of tumor development, immune escape, and drug resistance. This study investigated the gene expression patterns, prognostic values, and potential mechanisms of connexins in breast cancer.

Methods

We conducted a comprehensive analysis of connexins using public gene and protein expression databases and clinical samples from our institution. Connexin mRNA expressions in breast cancer and matched normal tissues were compared, and multiomics studies were performed.

Results

Gap junction beta-2 mRNA was overexpressed in breast cancers of different pathological types and molecular subtypes, and its high expression was associated with poor prognosis. The tumor membrane of the gap junction beta-2 mutated group was positive, and the corresponding protein was expressed. Somatic mutation and copy number variation of gap junction beta-2 are rare in breast cancer. The gap junction beta-2 transcription level in the p110α subunit of the phosphoinositide 3-kinase mutant subgroup was higher than that in the wild-type subgroup. Gap junction beta-2 was associated with the phosphoinositide 3-kinase-Akt signaling pathway, extracellular matrix-receptor interaction, focal adhesion, and proteoglycans in cancer. Furthermore, gap junction beta-2 overexpression may be associated with phosphoinositide 3-kinase and histone deacetylase inhibitor resistance, and its expression level correlated with infiltrating CD8+ T cells, macrophages, neutrophils, and dendritic cells.

Conclusions

Gap junction beta-2 may be a promising therapeutic target for targeted therapy and immunotherapy and may be used to predict breast cancer prognosis.

Intraglandular dissemination: a special pathological feature

Intraglandular dissemination is an important pathological feature of thyroid cancer, yet the biological characteristics of this phenomenon remain relatively underexplored. This paper aims to provide a comprehensive overview of its biological behaviors, protein expressions, and identification methods. Several retrospective studies have found that thyroid cancers with intraglandular dissemination have higher rates of lymph node metastasis, capsule invasion, and vascular invasion, exhibiting more aggressive biological behavior. Immunohistochemistry results show abnormal expression of proteins such as FKBP5, CENPF, CX26, KIF11, PTK7, which are associated with poor prognosis in thyroid cancers with intraglandular dissemination, offering potential guidance for specific targeted therapy in the future. Moreover, adjunctive techniques including ultrasound, fine-needle aspiration, and genetic testing offer valuable support in accurately identifying these cases, facilitating moreproactive treatment and closer follow-up.

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Vascular co-option in resistance to anti-angiogenic therapy

Three different mechanisms of neovascularization have been described in tumor growth, including sprouting angiogenesis, intussusceptive microvascular growth and glomeruloid vascular proliferation. Tumors can also grow by means of alternative mechanisms including vascular co-option, vasculogenic mimicry, angiotropism, and recruitment of endothelial precursor cells. Vascular co-option occurs in tumors independently of sprouting angiogenesis and the non-angiogenic cancer cells are described as exploiting pre-existing vessels. Vascular co-option is more frequently observed in tumors of densely vascularized organs, including the brain, lung and liver, and vascular co-option represents one of the main mechanisms involved in metastasis, as occurs in liver and lung, and resistance to anti-angiogenic therapy. The aim of this review article is to analyze the role of vascular co-option as mechanism through which tumors develop resistance to anti-angiogenic conventional therapeutic approaches and how blocking co-option can suppress tumor growth.

Leveraging translational insights toward precision medicine approaches for brain metastases

Due to increasing incidence and limited treatments, brain metastases (BM) are an emerging unmet need in modern oncology. Development of effective therapeutics has been hindered by unique challenges. Individual steps of the brain metastatic cascade are driven by distinctive biological processes, suggesting that BM possess intrinsic biological differences compared to primary tumors. Here, we discuss the unique physiology and metabolic constraints specific to BM as well as emerging treatment strategies that leverage potential vulnerabilities.

Tumor vessel co-option: The past & the future

Tumor vessel co-option (VCO) is a non-angiogenic vascularization mechanism that is a possible cause of resistance to anti-angiogenic therapy (AAT). Multiple tumors are hypothesized to primarily rely on growth factor signaling-induced sprouting angiogenesis, which is often inhibited during AAT. During VCO however, tumors invade healthy tissues by hijacking pre-existing blood vessels of the host organ to secure their blood and nutrient supply. Although VCO has been described in the context of AAT resistance, the molecular mechanisms underlying this process and the profile and characteristics of co-opted vascular cell types (endothelial cells (ECs) and pericytes) remain poorly understood, resulting in the lack of therapeutic strategies to inhibit VCO (and to overcome AAT resistance). In the past few years, novel next-generation technologies (such as single-cell RNA sequencing) have emerged and revolutionized the way of analyzing and understanding cancer biology. While most studies utilizing single-cell RNA sequencing with focus on cancer vascularization have centered around ECs during sprouting angiogenesis, we propose that this and other novel technologies can be used in future investigations to shed light on tumor EC biology during VCO. In this review, we summarize the molecular mechanisms driving VCO known to date and introduce the models used to study this phenomenon to date. We highlight VCO studies that recently emerged using sequencing approaches and propose how these and other novel state-of-the-art methods can be used in the future to further explore ECs and other cell types in the VCO process and to identify potential vulnerabilities in tumors relying on VCO. A better understanding of VCO by using novel approaches could provide new answers to the many open questions, and thus pave the way to develop new strategies to control and target tumor vascularization.

Prevalence and Clinicopathologic Features of Canine Metastatic Melanoma Involving the Central Nervous System: A Retrospective Analysis and Comparative Review

Background

Central nervous system (CNS) involvement is the leading cause of death in malignant melanoma. Rodent models, while vital to mechanistic investigation, have had limited success identifying effective therapies for melanoma brain metastases. The companion dog with de novo melanoma is a promising complementary model for developmental therapeutic investigation, as these tumors occur in an immunologically outbred host that has shared environmental exposures with humans. However, relatively little is known regarding the prevalence and clinicopathological features of canine melanoma metastasis to the CNS. To further validate the dog as an appropriate model for human metastatic melanoma, the aims of this study were to determine the rate of CNS metastasis and associated clinicopathologic features in canine malignant melanoma.

Methods

Medical records of dogs diagnosed with malignant melanoma from 1985-2019 at the University of California Davis Veterinary Medical Teaching Hospital were assessed retrospectively. Clinicopathologic features were compared between dogs with CNS metastasis (CNS+) and dogs without CNS metastasis (CNS-). Site of CNS involvement and associated neurological signs were analyzed via Wilcoxon-Mann-Whitney rank sum and Fisher’s exact tests. Survival data were analyzed via Kaplan-Meier estimates.

Results

CNS metastasis was identified in 38% of dogs in this study (20/53). The oral cavity was the most common site of primary melanoma in both groups [CNS+: n=12 (60%) vs. CNS-: n=22 (67%); p>0.99]. The total burden of metastatic disease was higher in the CNS+ group (CNS+: 4, 95% CI 3-5 vs. CNS-: 3, 95% CI 1-3; p<0.001). The cerebrum was the most common site of CNS metastasis (n=15, 75%) and seizures were the most observed neurological sign (n=9, 64%). There was no difference in overall survival between CNS+ and CNS- groups. However, the median survival time following onset of neurological signs was 9.5 days (95% CI 1-43), with 5 dogs euthanized within 24 hours of the onset of neurological signs.

Conclusions

Canine and human MM patients share similar rates of CNS metastasis and clinical presentation. This study will guide clinical management of canines with malignant melanoma and inform future studies using dogs with spontaneously occurring melanoma as a preclinical model for human melanoma brain metastases.

Exosomes Derived from Glioma Cells under Hypoxia Promote Angiogenesis through Up-regulated Exosomal Connexin 43

Glioblastoma multiform (GBM) is a highly aggressive primary brain tumor. Exosomes derived from glioma cells under a hypoxic microenvironment play an important role in tumor biology including metastasis, angiogenesis and chemoresistance. However, the underlying mechanisms remain to be elucidated. In this study, we aimed to explore the role of connexin 43 on exosomal uptake and angiogenesis in glioma under hypoxia. U251 cells were exposed to 3% oxygen to achieve hypoxia, and the expression levels of HIF-1α and Cx43, involved in the colony formation and proliferation of cells were assessed. Exosomes were isolated by differential velocity centrifugation from U251 cells under normoxia and hypoxia (Nor-Exos and Hypo-Exos), respectively. Immunofluorescence staining, along with assays for CCK-8, tube formation and wound healing along with a transwell assay were conducted to profile exosomal uptake, proliferation, tube formation, migration and invasion of HUVECs, respectively. Our results revealed that Hypoxia significantly up-regulated the expression of HIF-1α in U251 cells as well as promoting proliferation and colony number. Hypoxia also increased the level of Cx43 in U251 cells and in the exosomes secreted. The uptake of Dio-stained Hypo-Exos by HUVECs was greater than that of Nor-Exos, and inhibition of Cx43 by ^37,43gap27 or lenti-Cx43-shRNA efficiently prevented the uptake of Hypo-Exos by recipient endothelial cells. In addition, the proliferation and total loops of HUVECs were remarkably increased at 24 h, 48 h, and 10 h after Hypo-Exos, respectively. Notably, ^37,43gap27, a specific Cx-mimetic peptide blocker of Cx37 and Cx43, efficiently alleviated Hypo-Exos-induced proliferation and tube formation by HUVECs. Finally, ^37,43gap27 also significantly attenuated Hypo-Exos-induced migration and invasion of HUVECs. These findings demonstrate that exosomal Cx43 contributes to glioma angiogenesis mediated by Hypo-Exos, and suggests that exosomal Cx43 might serve as a potential therapeutic target for glioblastoma.

Astrocytic laminin-211 drives disseminated breast tumor cell dormancy in brain

Although dormancy is thought to play a key role in the metastasis of breast tumor cells to the brain, our knowledge of the molecular mechanisms regulating disseminated tumor cell (DTC) dormancy in this organ is limited. Here using serial intravital imaging of dormant and metastatic triple-negative breast cancer lines, we identify escape from the single-cell or micrometastatic state as the rate-limiting step towards brain metastasis. We show that every DTC occupies a vascular niche, with quiescent DTCs residing on astrocyte endfeet. At these sites, astrocyte-deposited laminin-211 drives DTC quiescence by inducing the dystroglycan receptor to associate with yes-associated protein, thereby sequestering it from the nucleus and preventing its prometastatic functions. These findings identify a brain-specific mechanism of DTC dormancy and highlight the need for a more thorough understanding of tumor dormancy to develop therapeutic approaches that prevent brain metastasis.

Role of gap junctions and connexin 43 in cancerogenesis and tumor metastasis

Gap junctions are intercellular connections that enable direct communication between neighboring cells. They are important in tissue homeostasis, cell growth, and differentiation. They are composed of connexin proteins, of which the most common and most studied is connexin 43. The role of connexin 43 in the development and progression of tumors is contradictory. The aim of this paper is to summarize the current state of knowledge on the expression of connexin 43 in various primary and secondary tumors, in order to explain its role in the development and progression of malignant tumors. Previous studies have examined the expression of connexin 43 in various primary and secondary tumors, as well as its association with prognosis. The expression of connexin 43 has been shown to be associated with various aspects of tumor behavior. However, it has been shown that the expression of connexin 43 differs between different types and localizations of tumors, as well as between different stages in tumor progression, which indicates the complex role of connexin 43 in tumor evolution. Since gap junctions play a role in carcinogenesis, invasion, and metastasis of malignant cells, further studies should clarify whether connexin 43 can be used as a diagnostic biomarker.

Animal models of brain metastasis

Modeling of metastatic disease in animal models is a critical resource to study the complexity of this multi-step process in a relevant system. Available models of metastatic disease to the brain are still far from ideal but they allow to address specific aspects of the biology or mimic clinically relevant scenarios. We not only review experimental models and their potential improvements but also discuss specific answers that could be obtained from them on unsolved aspects of clinical management.

Connexins in melanoma: Potential role of Cx46 in its aggressiveness

Melanoma is the most aggressive skin cancer, and in metastatic advanced states, it is completely refractory to chemotherapy. Therefore, it is relevant to understand the molecular bases that rule their aggressiveness. Connexins (Cxs) are proteins that under normal physiological conditions participate in intercellular communication, via the exchange of signaling molecules between the cytoplasm and extracellular milieu and the exchange of ions/second messengers between the cytoplasm of contacting cells. These proteins have shown important roles in cancer progression, chemo- and radiotherapy resistance, and metastasis. Accordingly, Cx26 and Cx43 seem to play important roles in melanoma progression and metastasis. On the other hand, Cx46 is typically expressed in the eye lens, where it seems to be associated with oxidative stress protection in fiber lens cells. However, in the last decade, Cx46 expression has been associated with breast and brain cancers, due to its role in potentiation of both extracellular vesicle release and cancer stem cell-like properties. In this review, we analyzed a potential role of Cx46 as a new biomarker and therapeutic target in melanoma.

Radioimmunotherapy for Brain Metastases: The Potential for Inflammation as a Target of Choice

Brain metastases (BM) are frequently detected during the follow-up of patients with malignant tumors, particularly in those with advanced disease. Despite a major progress in systemic anti-cancer treatments, the average overall survival of these patients remains limited (6 months from diagnosis). Also, cognitive decline is regularly reported especially in patients treated with whole brain external beam radiotherapy (WBRT), due to the absorbed radiation dose in healthy brain tissue. New targeted therapies, for an earlier and/or more specific treatment of the tumor and its microenvironment, are needed. Radioimmunotherapy (RIT), a combination of a radionuclide to a specific antibody, appears to be a promising tool. Inflammation, which is involved in multiple steps, including the early phase, of BM development is attractive as a relevant target for RIT. This review will focus on the (1) early biomarkers of inflammation in BM pertinent for RIT, (2) state of the art studies on RIT for BM, and (3) the importance of dosimetry to RIT in BM. These two last points will be addressed in comparison to the conventional EBRT treatment, particularly with respect to the balance between tumor control and healthy tissue complications. Finally, because new diagnostic imaging techniques show a potential for the detection of BM at an early stage of the disease, we focus particularly on this therapeutic window.

Metabolome Shift in Both Metastatic Breast Cancer Cells and Astrocytes Which May Contribute to the Tumor Microenvironment

The role of astrocytes in the periphery of metastatic brain tumors is unclear. Since astrocytes regulate central nervous metabolism, we hypothesized that changes in astrocytes induced by contact with cancer cells would appear in the metabolome of both cells and contribute to malignant transformation. Coculture of astrocytes with breast cancer cell supernatants altered glutamate (Glu)-centered arginine–proline metabolism. Similarly, the metabolome of cancer cells was also altered by astrocyte culture supernatants, and the changes were further amplified in astrocytes exposed to Glu. Inhibition of Glu uptake in astrocytes reduces the variability in cancer cells. Principal component analysis of the cancer cells revealed that all these changes were in the first principal component (PC1) axis, where the responsible metabolites were involved in the metabolism of the arginine–proline, pyrimidine, and pentose phosphate pathways. The contribution of these changes to the tumor microenvironment needs to be further pursued.

GJB5 association with BRAF mutation and survival in cutaneous malignant melanoma

BACKGROUND

Gap junctional intercellular communication is crucial for epidermal cellular homeostasis. Inability to establish melanocyte-keratinocytes contacts and loss of intercellular junction's integrity may contribute to melanoma development. Connexins, laminins and desmocollins have been implicated in the control of melanoma growth, where their reduced expression has been reported in metastatic lesions.

OBJECTIVES

The aim of this study was to investigate Connexin 31.1 (GJB5) expression and identify any association with BRAF mutational status, melanoma patient prognosis and MAPK inhibitors (MAPKi) treatment.

MATERIAL AND METHODS

GJB5 expression was measured at RNA and protein level in melanoma clinical samples and established cell lines treated or not with BRAF and MEK inhibitors, as well as in cell lines which developed MAPK inhibitors resistance. Findings were further validated and confirmed by analysis of independent datasets.

RESULTS

Our analysis reveals significant downregulation of GJB5 expression in metastatic melanoma lesions compared to primary ones and in BRAF mutated versus BRAF wild-type melanomas. Likewise, GJB5 expression is significantly lower in BRAF^V600E compared with BRAF^WT cell lines and increases upon MAPKi treatment. MAPKi-resistant melanoma cells display a similar expression pattern compared to BRAF^WT cells, with increased GJB5 expression associated with morphological changes. Enhancement of BRAF^V600E expression in BRAF^WT melanoma cells significantly upregulates miR-335-5p expression with consequent downregulation of GJB5, one of its targets. Furthermore, overexpression of miR-335-5p in two BRAF^WT cell lines confirms specific GJB5 protein downregulation. RT-qPCR analysis also revealed upregulation of miR-335 in BRAF^V600E melanoma cells, which is significantly downregulated in cells resistant to MEK inhibitors. Our data were further validated using the TCGA-SKCM dataset, where BRAF mutations associate with increased miR-335 expression and inversely correlate with GJB5 expression. In clinical samples, GJB5 underexpression is also associated with patient overall worse survival, especially at early stages.

CONCLUSION

We identified a significant association between metastases / BRAF mutation and low GJB5 expression in melanoma. Our results identify a novel mechanism of Gap-junctional protein regulation, suggesting a prognostic role for GJB5 in cutaneous melanoma.

Behind Brain Metastases Formation: Cellular and Molecular Alterations and Blood-Brain Barrier Disruption

Breast cancer (BC) brain metastases is a life-threatening condition to which accounts the poor understanding of BC cells' (BCCs) extravasation into the brain, precluding the development of preventive strategies. Thus, we aimed to unravel the players involved in the interaction between BCCs and blood-brain barrier (BBB) endothelial cells underlying BBB alterations and the transendothelial migration of malignant cells. We used brain microvascular endothelial cells (BMECs) as a BBB in vitro model, under conditions mimicking shear stress to improve in vivo-like BBB features. Mixed cultures were performed by the addition of fluorescently labelled BCCs to distinguish individual cell populations. BCC-BMEC interaction compromised BBB integrity, as revealed by junctional proteins (β-catenin and zonula occludens-1) disruption and caveolae (caveolin-1) increase, reflecting paracellular and transcellular hyperpermeability, respectively. Both BMECs and BCCs presented alterations in the expression pattern of connexin 43, suggesting the involvement of the gap junction protein. Myosin light chain kinase and phosphorylated myosin light chain were upregulated, revealing the involvement of the endothelial cytoskeleton in the extravasation process. β4-Integrin and focal adhesion kinase were colocalised in malignant cells, reflecting molecular interaction. Moreover, BCCs exhibited invadopodia, attesting migratory properties. Collectively, hub players involved in BC brain metastases formation were unveiled, disclosing possible therapeutic targets for metastases prevention.

Differential Expression of BOC, SPOCK2, and GJD3 Is Associated with Brain Metastasis of ER-Negative Breast Cancers

Simple Summary

Brain metastasis is diagnosed in 30–50% of metastatic breast cancer patients with currently limited treatment strategies and usually short survival rates. In the present study, we aim to identify genes specifically associated with the development of brain metastasis in breast cancer. Therefore, we compared RNA expression profiles from two groups of patients with metastatic breast cancer, with and without brain involvement. Three genes BOC, SPOCK2, and GJD3 were overexpressed in the group of primary breast cancers which developed brain metastasis. Expression profiles were confirmed in an independent breast cancer cohort for both BOC and SPOCK2. In addition, differential overexpression of SPOCK2 and GJD3 mRNA levels were found to be associated with the development of brain metastasis in an external online database of 204 primary breast cancers. Verification of these genes as biomarkers for brain metastasis development in primary breast cancer is warranted.

Abstract

Background: Brain metastasis is considered one of the major causes of mortality in breast cancer patients. To invade the brain, tumor cells need to pass the blood-brain barrier by mechanisms that are partially understood. In primary ER-negative breast cancers that developed brain metastases, we found that some of the differentially expressed genes play roles in the T cell response. The present study aimed to identify genes involved in the formation of brain metastasis independently from the T cell response. Method: Previously profiled primary breast cancer samples were reanalyzed. Genes that were found to be differentially expressed were confirmed by RT-PCR and by immunohistochemistry using an independent cohort of samples. Results: BOC, SPOCK2, and GJD3 were overexpressed in the primary breast tumors that developed brain metastasis. BOC expression was successfully validated at the protein level. SPOCK2 was validated at both mRNA and protein levels. SPOCK2 and GJD3 mRNA overexpression were also found to be associated with cerebral metastasis in an external online database consisting of 204 primary breast cancers. Conclusion: The overexpression of BOC, SPOCK2, and GJD3 is associated with the invasion of breast cancer into the brain. Further studies to determine their specific function and potential value as brain metastasis biomarkers are required.

GJA1 rs2071165 A > G Variant Increased Gastric Cancer Risk in Females of Northwest China: A Case-Control Study

Gastric cancer (GC) is one of the most common malignancies, and its incidence rates vary widely between men and women. Previous studies have suggested that connexin 43 (Cx43, encoded by gap junction protein alpha 1 (GJA1)) and secretory carrier membrane protein 1 (SCAMP1) are key functional proteins in tumors. Herein, the association between GJA1 and SCAMP1 polymorphisms and GC susceptibility and prognosis was evaluated. A total of three single-nucleotide polymorphisms among 681GC patients and 756 controls were tested using the Agena MassARRAY RS1000 system, including GJA1 rs2071165, SCAMP1 rs4530741, and SCAMP1 rs6874309. The strength of the association with GC risk was assessed by the odds ratios (ORs) and 95% confidence intervals (CIs) generated from the logistic regression model. Kaplan–Meier curve, long-rank tests, and a multivariate Cox proportional hazard model were used for prognosis analysis. The expression of GJA1 was assessed by immunohistochemistry. The GJA1 rs2071165 AA/AG genotype significantly increased the risk of GC in the female Chinese population (OR = 1.55, 95% CI = 1.03–2.32, p=0.034). Furthermore, the risk effect of GJA1 rs2071165 was more evident in the subgroups of female patients with GC, stratified by age, clinical stage, tumor size, and recurrence/metastasis. However, no obvious differences in Cx43 expression in GC tissues were observed between males and females. Furthermore, no significant association between SCAMP1 rs4530741 and rs6874309 polymorphisms and GC risk or prognosis was observed. In conclusion, this study suggests for the first time that the GJA1 rs2071165 polymorphism is associated with increased GC risk in females, revealing a potential new clinical marker for assessing GC risk in females.

Dysregulation of lncRNA-CCRR contributes to brain metastasis of breast cancer by intercellular coupling via regulating connexin 43 expression

Cardiac conduction regulatory RNA (CCRR) is down‐regulated in the pathogenesis of heart failure (HF), which accordingly suppresses cardiac conduction while promoting arrhythmogenicity. Meanwhile, CX43 was reported to play a role in the pathogenesis of metastatic breast cancer and melanoma brain colonization. In this study, we studied the role of long non‐coding RNA CCRR and its interaction with CX43 in brain metastasis of breast cancer. Breast cancer patients were grouped according to the metastasis status. Real‐time PCR and IHC assay were used to measure the expression of lncRNA‐CCRR and CX43 in patients. Western blot was conducted to observe the effect of lncRNA‐CCRR on the expression of CX43 in MDA‐MB‐231BR and BT‐474BR cells. Compared with the non‐metastasis group, the mRNA expression of tissue lncRNA‐CCRR, cerebrospinal fluid (CSF) lncRNA‐CCRR, tissue CX43 and tissue protein expression of CX43 were both evidently up‐regulated in metastasis patients, especially in patients with brain metastasis. The expression of lncRNA‐CCRR was positively correlated with the up‐regulated expression of CX43. Moreover, CX43 expression was significantly lower in MDA‐MB‐231WT cells compared with that in MDA‐MB‐231BR cells. Also, the overexpression of lncRNA‐CCRR evidently increased dye transfer rate from astrocytes to MDA‐MB‐231BR/BT‐474BR cells but reduced lncRNA‐CCRR expression and suppressed the transmigration of MDA‐MB‐231BR/BT‐474BR cells in a blood‐brain barrier (BBB) model. In this study, we demonstrated that the presence of lncRNA‐CCRR could up‐regulate the expression of CX43, which promoted gap junction formation in brain metastasis of breast cancer. Accordingly, the communication between breast cancer cells and astrocytes was also promoted.

Clinical Significance of PDCD4 in Melanoma by Subcellular Expression and in Tumor-Associated Immune Cells

Simple Summary

While targeting programmed cell death (PDCD) 1 is a central treatment against melanoma, little is known about the related protein PDCD4. We defined differences in melanoma PDCD4 subcellular localization (either total cellular or nuclear-only) during oncogenesis, evaluated its presence on tumor-infiltrating immune cells, and determined its impact on survival. High PDCD4 expression resulted in improved survival in patients with primary and intracranial but not extracranial metastatic melanoma. High PDCD4 levels in surrounding tumor tissue were also associated with increased infiltrating immune cells. PDCD4 may be a potentially useful biomarker in melanoma to help guide our understanding of patient prognosis. Methods to increase PDCD4 in those with melanoma brain metastases may also help improve disease response.

Abstract

Little is known about the subcellular localization and function of programmed cell death 4 (PDCD4) in melanoma. Our past studies suggest PDCD4 interacts with Pleckstrin Homology Domain Containing A5 (PLEKHA5) to influence melanoma brain metastasis outcomes, as high intracranial PDCD4 expression leads to improved survival. We aimed to define the subcellular distribution of PDCD4 in melanoma and in the tumor microenvironment during neoplastic progression and its impact on clinical outcomes. We analyzed multiple tissue microarrays with well-annotated clinicopathological variables using quantitative immunofluorescence and evaluated single-cell RNA-sequencing on a brain metastasis sample to characterize PDCD4+ immune cell subsets. We demonstrate differences in PDCD4 expression during neoplastic progression, with high tumor and stromal PDCD4 levels associated with improved survival in primary melanomas and in intracranial metastases, but not in extracranial metastatic disease. While the expression of PDCD4 is well-documented on CD8+ T cells and natural killer cells, we show that it is also found on B cells and mast cells. PDCD4 expression in the tumor microenvironment is associated with increased immune cell infiltration. Further studies are needed to define the interaction of PDCD4 and PLEKHA5 and to evaluate the utility of this pathway as a therapeutic target in melanoma brain metastasis.

Picturing Breast Cancer Brain Metastasis Development to Unravel Molecular Players and Cellular Crosstalk

Simple Summary

Breast cancer is a devastating disorder affecting millions of women worldwide. With improved therapeutics for the primary tumor, the appearance of metastasis has been increasing. Breast cancer frequently metastasizes to the brain, constituting a major hurdle without cure and with a poor survival. It is imperative to better understand the mechanisms involved in malignant cell transposition of the brain microvasculature and parenchymal colonization by deciphering the alterations occurring in the tumor and microvascular cells, as well as the occurrence of intercellular communication during the process. We aimed to profile the process of the formation of breast cancer brain metastasis and the timeline of events governing it. We used a specific mouse model of the disease to perform extensive microscopic analyses. We identified phenotypic changes and the activation of relevant molecular players in tumorigenesis, together with vascular alterations, and the occurrence of crosstalk. Our findings unravel putative therapeutic targets to tackle breast cancer brain metastasis.

Abstract

With breast cancer (BC) therapy improvements, the appearance of brain metastases has been increasing, representing a life-threatening condition. Brain metastasis formation involves BC cell (BCC) extravasation across the blood–brain barrier (BBB) and brain colonization by unclear mechanisms. We aimed to disclose the actors involved in BC brain metastasis formation, focusing on BCCs’ phenotype, growth factor expression, and signaling pathway activation, correlating with BBB alterations and intercellular communication. Hippocampi of female mice inoculated with 4T1 BCCs were examined over time by hematoxylin-eosin, immunohistochemistry and immunofluorescence. Well-established metastases were observed at seven days, increasing thereafter. BCCs entering brain parenchyma presented mesenchymal, migratory, and proliferative features; however, with time, they increasingly expressed epithelial markers, reflecting a mesenchymal–epithelial transition. BCCs also expressed platelet-derived growth factor-B, β₄ integrin, and focal adhesion kinase, suggesting autocrine and/or paracrine regulation with adhesion signaling activation, while balance between Rac1 and RhoA was associated with the motility status. Intercellular communication via gap junctions was clear among BCCs, and between BCCs and endothelial cells. Thrombin accumulation, junctional protein impairment, and vesicular proteins increase reflect BBB alterations related with extravasation. Expression of plasmalemma vesicle-associated protein was increased in BCCs, along with augmented vascularization, whereas pericyte contraction indicated mural cells’ activation. Our results provide further understanding of BC brain metastasis formation, disclosing potential therapeutic targets.

Connexins and cAMP Cross-Talk in Cancer Progression and Metastasis

Simple Summary

Different connexins play diverse roles in cancers, either tumor-suppressing or tumor-promoting. In lung cancer, Cx43 serves as a tumor suppressor at the early stage, but it can also be a tumor-promotor at an advanced stage and during metastasis. Moreover, other connexins, including Cx26, Cx31.1, and Cx32, can be tumor suppressors. In contrast, Cx30.3 can be a tumor-promotor. The roles of different connexins in different cancers have also been established. Cx43 acts as a tumor suppressor in colorectal cancer, breast cancer, and glioma, whereas Cx32 can be a suppressor in liver tumors and hepatocarcinogenesis. Cx26 can be a tumor suppressor in mammary tumors; in contrast, it can be a tumor-promotor in melanoma. Existing drugs/molecules targeting the cAMP/PKA/connexin axis act to regulate channel opening/closing. Mimic peptides, such as Gap19, Gap26, and Gap 27 block hemichannels, mimetic peptides, and CT9/CT10 and promote hemichannel opening and also hemichannel closing.

Abstract

Connexin-containing gap junctions mediate the direct exchange of small molecules between cells, thus promoting cell–cell communication. Connexins (Cxs) have been widely studied as key tumor-suppressors. However, certain Cx subtypes, such as Cx43 and Cx26, are overexpressed in metastatic tumor lesions. Cyclic adenosine monophosphate (cAMP) signaling regulates Cx expression and function via transcriptional control and phosphorylation. cAMP also passes through gap junction channels between adjacent cells, regulating cell cycle progression, particularly in cancer cell populations. Low levels of cAMP are sufficient to activate key effectors. The present review evaluates the mechanisms underlying Cx regulation by cAMP signaling and the role of gap junctions in cancer progression and metastasis. A deeper understanding of these processes might facilitate the development of novel anticancer drugs.

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.

Connexin 32 induces pro-tumorigenic features in MCF10A normal breast cells and MDA-MB-231 metastatic breast cancer cells

Connexins (Cx), the basic subunit of gap junctions, play important roles in cell homeostasis, and their abnormal expression and function are associated with human hereditary diseases and cancers. In tumorigenesis, connexins were observed to have both anti-tumorigenic and pro-tumorigenic roles in a context- and stage-dependent manner. Initially, Cx26 and Cx43 were thought to be the only connexins involved in normal breast homeostasis and breast cancer. Later on, association of Cx32 expression with lymph node metastasis of breast cancer and subsequent demonstration of its expression in normal breast tissue suggested that Cx32 contributes to breast tissue homeostasis. Here, we aimed to determine the effects of Cx32 on normal breast cells, MCF10A, and on breast cancer cells, MDA-MB-231. Cx32 overexpression had profound effects on MCF10A cells, decreasing cell proliferation by increasing the doubling time of MCF10A. Furthermore, MCF10A cells acquired mesenchymal-like appearance upon Cx32 expression and had increased migration capacity and expression of both E-cadherin and vimentin. In contrast, Cx32 overexpression altered the EMT markers of MDA-MB-231 by increasing the expression of mesenchymal markers, such as slug and vimentin, and decreasing E-cadherin expression without affecting their proliferation and morphology. Our results indicate, for the first time in the literature, that Cx32 has tumor-promoting roles in MCF10A and MDA-MB-231 cells.

Connexins-Therapeutic Targets in Cancers

Connexins (Cx) are members of a protein family that forms intercellular channels localised in gap junction (GJ) plaques and single transmembrane channels called hemichannels. They participate in intercellular communication or communication between the intracellular and extracellular environments. Connexins affect cell homeostasis, growth and differentiation by enabling the exchange of metabolites or by interfering with various signalling pathways. Alterations in the functionality and the expression of connexins have been linked to the occurrence of many diseases. Connexins have been already linked to cancers, cardiac and brain disorders, chronic lung and kidney conditions and wound healing processes. Connexins have been shown either to suppress cancer tumour growth or to increase tumorigenicity by promoting cancer cell growth, migration and invasiveness. A better understanding of the complexity of cancer biology related to connexins and intercellular communication could result in the design of novel therapeutic strategies. The modulation of connexin expression may be an effective therapeutic approach in some types of cancers. Therefore, one important challenge is the search for mechanisms and new drugs, selectively modulating the expression of various connexin isoforms. We performed a systematic literature search up to February 2020 in the electronic databases PubMed and EMBASE. Our search terms were as follows: connexins, hemichannels, cancer and cancer treatment. This review aims to provide information about the role of connexins and gap junctions in cancer, as well as to discuss possible therapeutic options that are currently being studied.

GJA1 Expression and Its Prognostic Value in Cervical Cancer

Gap Junction Protein Alpha 1 (GJA1) belongs to the gap junction family and has been widely studied in cancers. We evaluated the role of GJA1 in cervical cancer (CC) using public data from The Cancer Genome Atlas (TCGA) and the Gene Expression Omnibus (GEO) database. The difference of GJA1 expression level between CC and normal tissues was analyzed by the Gene Expression Profiling Interactive Analysis (GEPIA), six GEO datasets, and the Human Protein Atlas (HPA). The relationship between clinicopathological features and GJA1 expression was analyzed by the chi-squared test and the logistic regression. Kaplan–Meier survival analysis and Cox proportional hazard regression analysis were used to assessing the effect of GJA1 expression on survival. Gene set enrichment analysis (GSEA) was used to screen the signaling pathways regulated by GJA1. Immune Cell Abundance Identifier (ImmuCellAI) was chosen to analyze the immune cells affected by GJA1. The expression of GJA1 in CC was significantly lower than that in normal tissues based on the GEPIA, GEO datasets, and HPA. Both the chi-squared test and the logistic regression showed that high-GJA1 expression was significantly correlated with keratinization, hormone use, tumor size, and FIGO stage. The Kaplan–Meier curves suggested that high-GJA1 expression could indicate poor prognosis (p = 0.0058). Multivariate analysis showed that high-GJA1 expression was an independent predictor of poor overall survival (HR, 4.084; 95% CI, 1.354-12.320; p = 0.013). GSEA showed many cancer-related pathways, such as the p53 signaling pathway and the Wnt signaling pathway, were enriched in the high-GJA1-expression group. Immune cell abundance analysis revealed that the abundance of CD8 naive, DC, and neutrophil was significantly increased in the high-GJA1-expression group. In conclusion, GJA1 can be regarded as a potential prognostic marker of poor survival and therapeutic target in CC. Moreover, many cancer-related pathways may be the critical pathways regulated by GJA1. Furthermore, GJA1 can affect the abundance of immune cells.

Brain metastases-derived extracellular vesicles induce binding and aggregation of low-density lipoprotein

BACKGROUND

Cancer cell-derived extracellular vesicles (EVs) have previously been shown to contribute to pre-metastatic niche formation. Specifically, aggressive tumors secrete pro-metastatic EVs that travel in the circulation to distant organs to modulate the microenvironment for future metastatic spread. Previous studies have focused on the interface between pro-metastatic EVs and epithelial/endothelial cells in the pre-metastatic niche. However, EV interactions with circulating components such as low-density lipoprotein (LDL) have been overlooked.

RESULTS

This study demonstrates that EVs derived from brain metastases cells (Br-EVs) and corresponding regular cancer cells (Reg-EVs) display different interactions with LDL. Specifically, Br-EVs trigger LDL aggregation, and the presence of LDL accelerates Br-EV uptake by monocytes, which are key components in the brain metastatic niche.

CONCLUSIONS

Collectively, these data are the first to demonstrate that pro-metastatic EVs display distinct interactions with LDL, which impacts monocyte internalization of EVs.

Collective invasion induced by an autocrine purinergic loop through connexin-43 hemichannels

Progression of epithelial cancers predominantly proceeds by collective invasion of cell groups with coordinated cell-cell junctions and multicellular cytoskeletal activity. Collectively invading breast cancer cells express the gap junction protein connexin-43 (Cx43), yet whether Cx43 regulates collective invasion remains unclear. We here show that Cx43 mediates gap-junctional coupling between collectively invading breast cancer cells and, via hemichannels, adenosine nucleotide/nucleoside release into the extracellular space. Using molecular interference and rescue strategies, we identify that Cx43 hemichannel function, but not intercellular communication, induces leader cell activity and collective migration through the engagement of the adenosine receptor 1 (ADORA1) and AKT signaling. Accordingly, pharmacological inhibition of ADORA1 or AKT signaling caused leader cell collapse and halted collective invasion. ADORA1 inhibition further reduced local invasion of orthotopic mammary tumors in vivo, and joint up-regulation of Cx43 and ADORA1 in breast cancer patients correlated with decreased relapse-free survival. This identifies autocrine purinergic signaling, through Cx43 hemichannels, as a critical pathway in leader cell function and collective invasion.

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.

YAP Enhances Tumor Cell Dissemination by Promoting Intravascular Motility and Reentry into Systemic Circulation

The oncogene YAP has been shown previously to promote tumor growth and metastasis. However, how YAP influences the behavior of tumor cells traveling within the circulatory system has not been as well explored. Given that rate-limiting steps of metastasis are known to occur while tumor cells enter, travel through, or exit circulation, we sought to study how YAP influences tumor cell behavior within the circulatory system. Intravital imaging in live zebrafish embryos revealed that YAP influenced the distribution of tumor cells within the animal following intravenous injection. Control cells became lodged in the first capillary bed encountered in the tail, whereas cells overexpressing constitutively active YAP were able to travel through this capillary plexus, reenter systemic circulation, and seed in the brain. YAP controlled transit through these capillaries by promoting active migration within the vasculature. These results were corroborated in a mouse model following intravenous injection, where active YAP increased the number of circulating tumor cells over time. Our results suggest a possible mechanism whereby tumor cells can spread to organs beyond the first capillary bed downstream from the primary tumor. These results also show that a specific gene can affect the distribution of tumor cells within an animal, thereby influencing the global pattern of metastasis in that animal. SIGNIFICANCE: These findings demonstrate that YAP endows tumor cells with the ability to move through capillaries, allowing them to return to and persist in circulation, thereby increasing their metastatic spread.See related commentary by Davidson, p. 3797.

Emerging functions and clinical prospects of connexins and pannexins in melanoma

Cellular communication through gap junctions and hemichannels formed by connexins and through channels made by pannexins allows for metabolic cooperation and control of cellular activity and signalling. These channel proteins have been described to be tumour suppressors that regulate features such as cell death, proliferation and differentiation. However, they display cancer type-dependent and stage-dependent functions and may facilitate tumour progression through junctional and non-junctional pathways. The accumulated knowledge and emerging strategies to target connexins and pannexins are providing novel clinical opportunities for the treatment of cancer. Here, we provide an updated overview of the role of connexins and pannexins in malignant melanoma. We discuss how targeting of these channel proteins may be used to potentiate antitumour effects in therapeutic settings, including through improved immune-mediated tumour elimination.

High GJB2 mRNA expression and its prognostic significance in lung adenocarcinoma: a study based on the TCGA database

Lung adenocarcinoma (LUAD), a form of lung cancer, is reported to cause first and second-order cancer morbidity to men and women in China, respectively. We assessed the mRNA expression of GJB2 in LUAD patients in our study, based on data acquired from the cancer genome atlas (TCGA) and so as to increase further knowledge into the biological pathways involved in LUAD pathogenesis related to GJB2.Information on gene expression and comparing clinical data were recognized and downloaded from TCGA. Gene set enrichment analysis (GSEA) created an arranged list of all genes is indicated by their connection with GJB2 expression.Our study cohort included 265 (54.5%) female and 221 (36.0%) male patients. The scatter plot and paired plot showed the difference of GJB2 expression between normal and tumor samples (P < .01). Overall survival (OS) analysis demonstrated that LUAD with GJB2 -high had a more terrible prognosis than that with GJB2 -low (P < .01). Multivariate analysis with the cox proportional hazards model indicated that the expression of Cx26 (HR: 1.00; 95%CI: 1.00-1.01; P = .041) and stage (HR: 1.95; 95%CI: 1.23-3.09; P = .003) were independent prognostic factors for patients with LUAD. The GSEA results showed that cytosolic DNA sensing pathway, apoptosis, cytokine-cytokine receptor interaction, natural killer cell mediated cytotoxicity, regulation of actin cytoskeleton, toll-like receptor signaling pathway, small cell lung cancer and pathways in cancer are differentially enriched in GJB2 high expression phenotype.Our study confirmed the significantly high levels of Cx26 expression in LUAD patients with several observed clinical features. GJB2 may be a potentially useful prognostic molecular biomarker of bad survival in LUAD, while further experimental ought to be performed to demonstrate the biologic effect of GJB2.

Gap Junctions and Breast Cancer Dormancy

Breast cancer (BC) relapse, despite clinical advancement, remains one of the biggest issues in the field. Intercellular communication, specifically via connexin (Cx)-mediated gap junctions (GJs), play a key role in the long-term survival of these, treatment-resistant breast cancer stem cells (CSCs), allowing for relapse. Both basic and clinical evidence reveal dual roles for GJs, in tumor suppression, generally referred to as dormancy, and progression and metastasis. GJ intercellular communication (GJIC) can be mediated by multiple types of Cxs, depending on the organ to which the BC cells metastasize. This review expands on the differential expression of Cx-mediated GJIC between CSCs and niche cells within a given microenvironment.

Models and molecular mechanisms of blood vessel co-option by cancer cells

Cancer cells have diverse mechanisms for utilizing the vasculature; they can initiate the formation of new blood vessels from preexisting ones (sprouting angiogenesis) or they can form cohesive interactions with the abluminal surface of preexisting vasculature in the absence of sprouting (co-option). The later process has received renewed attention due to the suggested role of blood vessel co-option in resistance to antiangiogenic therapies and the reported perivascular positioning and migratory patterns of cancer cells during tumor dormancy and invasion, respectively. However, only a few molecular mechanisms have been identified that contribute to the process of co-option and there has not been a formal survey of cell lines and laboratory models that can be used to study co-option in different organ microenvironments; thus, we have carried out a comprehensive literature review on this topic and have identified cell lines and described the laboratory models that are used to study blood vessel co-option in cancer. Put into practice, these models may help to shed new light on the molecular mechanisms that drive blood vessel co-option during tumor dormancy, invasion, and responses to different therapies.

PLEKHA5 regulates tumor growth in metastatic melanoma

BACKGROUND

PLEKHA5 has previously been identified as a novel molecule implicated in melanoma brain metastasis, a disease that continues to portend a poor prognosis. The aim of this study was to further investigate the functional role of PLEKHA5 in disseminated melanoma.

METHODS

The impact of PLEKHA5 on proliferation and tumor growth was examined in vitro and in melanoma xenograft models, including brain-tropic melanomas (melanomas tending to disseminate to the brain). In vitro loss- and gain-of-function studies were used to explore the underlying mechanisms of PLEKHA5-mediated tumor growth and the crosstalk between PLEKHA5 and PI3K/AKT/mTOR or MAPK/ERK signaling. The clinical relevance of PLEKHA5 dysregulation was further investigated in a cohort of matched cranial and extracranial melanoma metastases.

RESULTS

PLEKHA5 stable knockdown negatively regulated cell proliferation by inhibiting the G₁ -to-S cell cycle transition, which coincided with upregulation of the cell cycle regulator PDCD4. Conversely, ectopic PLEKHA5 expression exhibited the inverse effect. PLEKHA5 knockdown significantly inhibited tumor growth, whereas its overexpression upregulated the growth of tumors, which was induced by cranial and subcutaneous inoculation of cells in nude mice. PLEKHA5 modulation affected PDCD4 protein stability and was coupled with changes in PI3K/AKT/mTOR pathway signaling. High PDCD4 expression in cerebral specimens was associated with better overall survival.

CONCLUSIONS

This study further supports the role of PLEKHA5 as a regulator of melanoma growth at distant sites, including the brain. Furthermore, the results highlight the significance of PDCD4 dysregulation in disseminated melanoma and implicate PDCD4 as a possible causal link between PLEKHA5 and cell proliferation and growth.

Betacellulin enhances ovarian cancer cell migration by up-regulating Connexin43 via MEK-ERK signaling

Epithelial ovarian cancer is the fifth common cause of cancer death in women and the most lethal gynecological malignancies. Our previous studies have shown that up-regulation of Connexin43, a gap-junction subunit crucial for cell-cell communication, enhances ovarian cancer cell migration. Betacellulin is a member of the epidermal growth factor (EGF) family which can bind to multiple EGF family receptors. Overexpression of betacellulin is found in a variety of cancers and is associated with reduced survival. However, the specific roles and molecular mechanisms of betacellulin in ovarian cancer progression are poorly understood. In the current study, we tested the hypothesis that betacellulin induces ovarian cancer cell migration by up-regulating Connexin43. Our results showed that treatment with betacellulin significantly increased Connexin43 expression and cell migration in both OVCAR4 and SKOV3 ovarian cancer cell lines. Moreover, betacellulin induced the activation of MEK-ERK signaling, and its effects on Connexin43 were inhibited by pre-treatment with U0126. Pre-treatment with AG1478 totally blocked the activation of MEK-ERK signaling but only partially inhibited betacellulin-induced Connexin43 expression and cell migration. Most importantly, betacellulin-induced cell migration was attenuated by knockdown of Connexin43, and co-treatment with gap junction inhibitor carbenoxolone did not alter this effect. Our results suggest a bilateral role of Connexin43 in ovarian cancer migration, and also demonstrate a gap junction-independent mechanism of betacellulin.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Gap Junction Intercellular Communication in the Carcinogenesis Hallmarks: Is This a Phenomenon or Epiphenomenon?

If occupational tumors are excluded, cancer causes are largely unknown. Therefore, it appeared useful to work out a theory explaining the complexity of this disease. More than fifty years ago the first demonstration that cells communicate with each other by exchanging ions or small molecules through the participation of connexins (Cxs) forming Gap Junctions (GJs) occurred. Then the involvement of GJ Intercellular Communication (GJIC) in numerous physiological cellular functions, especially in proliferation control, was proven and accounts for the growing attention elicited in the field of carcinogenesis. The aim of the present paper is to verify and discuss the role of Cxs, GJs, and GJIC in cancer hallmarks, pointing on the different involved mechanisms in the context of the multi-step theory of carcinogenesis. Functional GJIC acts both as a tumor suppressor and as a tumor enhancer in the metastatic stage. On the contrary, lost or non-functional GJs allow the uncontrolled proliferation of stem/progenitor initiated cells. Thus, GJIC plays a key role in many biological phenomena or epiphenomena related to cancer. Depending on this complexity, GJIC can be considered a tumor suppressor in controlling cell proliferation or a cancer ally, with possible preventive or therapeutic implications in both cases.

Microenvironmental Regulation of Tumor Progression and Therapeutic Response in Brain Metastasis

Cellular and non-cellular components of the tumor microenvironment (TME) are emerging as key regulators of primary tumor progression, organ-specific metastasis, and therapeutic response. In the era of TME-targeted- and immunotherapies, cancer-associated inflammation has gained increasing attention. In this regard, the brain represents a unique and highly specialized organ. It has long been regarded as an immunological sanctuary site where the presence of the blood brain barrier (BBB) and blood cerebrospinal fluid barrier (BCB) restricts the entry of immune cells from the periphery. Consequently, tumor cells that metastasize to the brain were thought to be shielded from systemic immune surveillance and destruction. However, the detailed characterization of the immune landscape within border-associated areas of the central nervous system (CNS), such as the meninges and the choroid plexus, as well as the discovery of lymphatics and channels that connect the CNS with the periphery, have recently challenged the dogma of the immune privileged status of the brain. Moreover, the presence of brain metastases (BrM) disrupts the integrity of the BBB and BCB. Indeed, BrM induce the recruitment of different immune cells from the myeloid and lymphoid lineage to the CNS. Blood-borne immune cells together with brain-resident cell-types, such as astrocytes, microglia, and neurons, form a highly complex and dynamic TME that affects tumor cell survival and modulates the mode of immune responses that are elicited by brain metastatic tumor cells. In this review, we will summarize recent findings on heterotypic interactions within the brain metastatic TME and highlight specific functions of brain-resident and recruited cells at different rate-limiting steps of the metastatic cascade. Based on the insight from recent studies, we will discuss new opportunities and challenges for TME-targeted and immunotherapies for BrM.

Inhibition of Gap Junctions Sensitizes Primary Glioblastoma Cells for Temozolomide

Gap junctions have recently been shown to interconnect glioblastoma cells to a multicellular syncytial network, thereby allowing intercellular communication over long distances as well as enabling glioblastoma cells to form routes for brain microinvasion. Against this backdrop gap junction-targeted therapies might provide for an essential contribution to isolate cancer cells within the brain, thus increasing the tumor cells’ vulnerability to the standard chemotherapeutic agent temozolomide. By utilizing INI-0602—a novel gap junction inhibitor optimized for crossing the blood brain barrier—in an oncological setting, the present study was aimed at evaluating the potential of gap junction-targeted therapy on primary human glioblastoma cell populations. Pharmacological inhibition of gap junctions profoundly sensitized primary glioblastoma cells to temozolomide-mediated cell death. On the molecular level, gap junction inhibition was associated with elevated activity of the JNK signaling pathway. With the use of a novel gap junction inhibitor capable of crossing the blood–brain barrier—thus constituting an auspicious drug for clinical applicability—these results may constitute a promising new therapeutic strategy in the field of current translational glioblastoma research.

Connexins in cancer: bridging the gap to the clinic

Gap junctions comprise arrays of intercellular channels formed by connexin proteins and provide for the direct communication between adjacent cells. This type of intercellular communication permits the coordination of cellular activities and plays key roles in the control of cell growth and differentiation and in the maintenance of tissue homoeostasis. After more than 50 years, deciphering the links among connexins, gap junctions and cancer, researchers are now beginning to translate this knowledge to the clinic. The emergence of new strategies for connexin targeting, combined with an improved understanding of the molecular bases underlying the dysregulation of connexins during cancer development, offers novel opportunities for clinical applications. However, different connexin isoforms have diverse channel-dependent and -independent functions that are tissue and stage specific. This can elicit both pro- and anti-tumorigenic effects that engender significant challenges in the path towards personalised medicine. Here, we review the current understanding of the role of connexins and gap junctions in cancer, with particular focus on the recent progress made in determining their prognostic and therapeutic potential.

Spatial Arrangements of Connexin43 in Cancer Related Cells and Re-Arrangements under Treatment Conditions: Investigations on the Nano-Scale by Super-Resolution Localization Light Microscopy

Cancer studies suggest that the spatial localization of connexin43 (Cx43) could play an important role during tumor genesis and the formation of metastasis. Cx43 has been shown to be upregulated in cancer cells; thereby a shift from Cx43 normal localization in gap junctions in the cell membrane towards a primarily cytoplasmic localization was observed in many studies. So far neither the spatial arrangements of Cx43 in breast cancer cells nor the effects of treatment outcome (ionizing radiation and antibody therapy) on the spatial arrangements of Cx43, have been microscopically studied on the nanoscale. This has brought up the idea to study the micro- and nanoscaled spatial Cx43 arrangements in a model of breast cancer-related cell types, i.e., SkBr3 breast cancer cells, BJ fibroblasts, and primary human internal mammary artery endothelial cells (HIMAECs). The cells were treated with neuregulin1 (NRG1), trastuzumab (Herceptin), or 6MeV-photon irradiation at a dose of 4 Gy. NRG1 stimulates further NRG1 release in the tumor endothelium that may lead to an enhanced tumor protective effect whereas Herceptin, used in antibody treatment, works in an antagonistic fashion to NRG1. After fluorescent labelling with specific antibodies, the molecular positions of Cx43 in the perinuclear cytosol and in the cell periphery at the membrane were determined for the three treatment related applications (NRG1, trastuzumab, 4 Gy irradiation) using confocal laser scanning microscopy (CLSM) and single molecule localization microscopy (SMLM). These techniques enable investigations of Cx43 enrichment and topological arrangements of Cx43 molecules from the micro-scale of a whole cell to the nano-scale of single molecules. In SkBr3 cells with and without radiation treatment high density accumulations were detected which seem to be diluted after NRG1 and trastuzumab treatment although the SMLM distance frequency distributions did not significantly vary. In BJ fibroblasts and HIMAECs differences between periphery and perinuclear cytosol were observed after the different treatment processes. HIMAECs showed significant Cx43 accumulation after NRG1, trastuzumab, and radiation treatment in the perinuclear region whereas in the periphery radiation has less influence as compared to the control. BJ cells were reacting to the treatments by Cx43 accumulations in the perinuclear region but also in the periphery. In conclusion, it was shown that by using CLSM and super-resolution SMLM, treatment effects on the spatial and thus functional arrangements of Cx43 became detectable for investigations of tumor response mechanisms.

Facets of Communication: Gap Junction Ultrastructure and Function in Cancer Stem Cells and Tumor Cells

Gap junction proteins are expressed in cancer stem cells and non-stem cancer cells of many tumors. As the morphology and assembly of gap junction channels are crucial for their function in intercellular communication, one focus of our review is to outline the data on gap junction plaque morphology available for cancer cells. Electron microscopic studies and freeze-fracture analyses on gap junction ultrastructure in cancer are summarized. As the presence of gap junctions is relevant in solid tumors, we exemplarily outline their role in glioblastomas and in breast cancer. These were also shown to contain cancer stem cells, which are an essential cause of tumor onset and of tumor transmission into metastases. For these processes, gap junctional communication was shown to be important and thus we summarize, how the expression of gap junction proteins and the resulting communication between cancer stem cells and their surrounding cells contributes to the dissemination of cancer stem cells via blood or lymphatic vessels. Based on their importance for tumors and metastases, future cancer-specific therapies are expected to address gap junction proteins. In turn, gap junctions also seem to contribute to the unattainability of cancer stem cells by certain treatments and might thus contribute to therapeutic resistance.

Exploring Differential Connexin Expression across Melanocytic Tumor Progression Involving the Tumor Microenvironment

The incidence of malignant melanoma, one of the deadliest cancers, continues to increase. Here we tested connexin (Cx) expression in primary melanocytes, melanoma cell lines and in a common nevus, dysplastic nevus, and thin, thick, and metastatic melanoma tumor progression series involving the tumor microenvironment by utilizing in silico analysis, qRT-PCR, immunocyto-/histochemistry and dye transfer tests. Primary melanocytes expressed GJA1/Cx43, GJA3/Cx46 and low levels of GJB2/Cx26 and GJC3/Cx30.2 transcripts. In silico data revealed downregulation of GJA1/Cx43 and GJB2/Cx26 mRNA, in addition to upregulated GJB1/Cx32, during melanoma progression. In three melanoma cell lines, we also showed the loss of GJA1/Cx43 and the differential expression of GJB1/Cx32, GJB2/Cx26, GJA3/Cx46 and GJC3/Cx30.2. The dominantly paranuclear localization of connexin proteins explained the ~10–90 times less melanoma cell coupling compared to melanocytes. In melanocytic tumor tissues, we confirmed the loss of Cx43 protein, fall of cell membrane and elevated paranuclear Cx32 with moderately increased cytoplasmic Cx26 and paranuclear Cx30.2 positivity during tumor progression. Furthermore, we found Cx43, Cx26 and Cx30 proteins upregulated in the melanoma adjacent epidermis, and Cx43 in the tumor flanking vessels. Therefore, differential connexin expression is involved in melanocytic tumor progression where varying connexin isotypes and levels reflect tumor heterogeneity-related bidirectional adaptive interactions with the microenvironment.