Cisplatin and oxaliplatin inhibit gap junctional communication by direct action and by reduction of connexin expression, thereby counteracting cytotoxic efficacy

Evaluation of Cx43 Gap Junction Inhibitors Using a Quantitative Structure-Activity Relationship Model

Gap junctions (GJs) made of connexin-43 (Cx43) are necessary for the conduction of electrical impulses in the heart. Modulation of Cx43 GJ activity may be beneficial in the treatment of cardiac arrhythmias and other dysfunctions. The search for novel GJ-modulating agents using molecular docking allows for the accurate prediction of binding affinities of ligands, which, unfortunately, often poorly correlate with their potencies. The objective of this study was to demonstrate that a Quantitative Structure-Activity Relationship (QSAR) model could be used for more precise identification of potent Cx43 GJ inhibitors. Using molecular docking, QSAR, and 3D-QSAR, we evaluated 16 known Cx43 GJ inhibitors, suggested the monocyclic monoterpene d-limonene as a putative Cx43 inhibitor, and tested it experimentally in HeLa cells expressing exogenous Cx43. The predicted concentrations required to produce 50% of the maximal effect (IC50) for each of these compounds were compared with those determined experimentally (pIC50 and eIC50, respectively). The pIC50ies of d-limonene and other Cx43 GJ inhibitors examined by our QSAR and 3D-QSAR models showed a good correlation with their eIC50ies (R = 0.88 and 0.90, respectively) in contrast to pIC50ies obtained from molecular docking (R = 0.78). However, molecular docking suggests that inhibitor potency may depend on their docking conformation on Cx43. Searching for new potent, selective, and specific inhibitors of GJ channels, we propose to perform the primary screening of new putative compounds using the QSAR model, followed by the validation of the most suitable candidates by patch-clamp techniques.

The roles of connexins and gap junctions in the progression of cancer

Gap junctions (GJs), which are composed of connexins (Cxs), provide channels for direct information exchange between cells. Cx expression has a strong spatial specificity; however, its influence on cell behavior and information exchange between cells cannot be ignored. A variety of factors in organisms can modulate Cxs and subsequently trigger a series of responses that have important effects on cellular behavior. The expression and function of Cxs and the number and function of GJs are in dynamic change. Cxs have been characterized as tumor suppressors in the past, but recent studies have highlighted the critical roles of Cxs and GJs in cancer pathogenesis. The complex mechanism underlying Cx and GJ involvement in cancer development is a major obstacle to the evolution of therapy targeting Cxs. In this paper, we review the post-translational modifications of Cxs, the interactions of Cxs with several chaperone proteins, and the effects of Cxs and GJs on cancer. Video Abstract.

Supporting Cells and Their Potential Roles in Cisplatin-Induced Ototoxicity

Cisplatin is a known ototoxic chemotherapy drug, causing irreversible hearing loss. Evidence has shown that cisplatin causes inner ear damage as a result of adduct formation, a proinflammatory environment and the generation of reactive oxygen species within the inner ear. The main cochlear targets for cisplatin are commonly known to be the outer hair cells, the stria vascularis and the spiral ganglion neurons. Further evidence has shown that certain transporters can mediate cisplatin influx into the inner ear cells including organic cation transporter 2 (OCT2) and the copper transporter Ctr1. However, the expression profiles for these transporters within inner ear cells are not consistent in the literature, and expression of OCT2 and Ctr1 has also been observed in supporting cells. Organ of Corti supporting cells are essential for hair cell activity and survival. Special interest has been devoted to gap junction expression by these cells as certain mutations have been linked to hearing loss. Interestingly, cisplatin appears to affect connexin expression in the inner ear. While investigations regarding cisplatin-induced hearing loss have been focused mainly on the known targets previously mentioned, the role of supporting cells for cisplatin-induced ototoxicity has been overlooked. In this mini review, we discuss the implications of supporting cells expressing OCT2 and Ctr1 as well as the potential role of gap junctions in cisplatin-induced cytotoxicity.

DOK5 as a Prognostic Biomarker of Gastric Cancer Immunoinvasion: A Bioinformatics Analysis

Background

Docking protein 5 (DOK5) is a member of the docking protein group of membrane proteins and is an adapter protein involved in signal transduction. Nevertheless, the role of DOK5 expression in the prognosis of gastric cancer (GC) remains unclear.

Methods

In this study, clinical prognostic parameters and survival data related to DOK5, in patients with GC, were analyzed using bioinformatics analysis comprising Oncomine and TIMER, UALCAN database, Kaplan-Meier plotter, GEPIA, GSEA, DAVID, and cBioPortal websites.

Results

In our study, GC contained various DOK5 expressions, which forecasted poor survival outcomes. Moreover, our research showed that high DOK5 could predict high-level infiltration of several GC immune cells, as evidenced by M1, TAM, M2, B cell, and T cell failure. Hence, DOK5 might become a new gastric cancer biomarker and therapeutic target. In the following analysis, in order to explore the prognostic value of DOK5 in GC, more clinical trials are needed to validate our results.

Conclusions

Through multiple database verifications, DOK5 was found to be part of the pathogenic genes for GC. Thus, it can change the formation and progression of tumors by acting on human immunity.

Cx32 promotes autophagy and produces resistance to SN‑induced apoptosis via activation of AMPK signalling in cervical cancer

The roles of gap junctions (GJs) and its components, connexins, in the autophagy of cervical cancer cells have been rarely investigated. Our previous study demonstrated that connexin 32 (Cx32) exerted an anti‑apoptotic effect on cervical cancer. However, as an important regulator of apoptosis, whether the autophagy is involved in the function of Cx32 on cervical cancer cells is not well defined. The present study aimed to investigate the role of Cx32 on autophagy and apoptosis inhibition in cervical cancer cells. The expression levels of Cx32 and the autophagy‑associated protein LC3‑Ⅱ in paracancerous cervical tissues (n=30) and cervical cancer (n=50) tissues were determined via western blotting. In total, 45 cervical cancer specimens were used to evaluate the clinical relevance of Cx32 and LC3‑Ⅱ. It was found that both Cx32 and LC3‑Ⅱ were upregulated in cervical cancer tissues compared with those in paracancerous cervical tissues. The effect of Cx32 on autophagy was examined by detecting the change of LC3‑Ⅱ using western blotting, transfection with enhanced green fluorescent protein‑LC3 plasmid and transmission electron microscopy analysis. Overexpression of Cx32 significantly enhanced autophagy in HeLa‑Cx32 cells, whereas knockdown of Cx32 suppressed autophagy in C‑33A cells. The flow cytometry results demonstrated that Cx32 inhibited the apoptosis of cervical cancer cells by promoting autophagy. Moreover, Cx32 triggered autophagy via the activation of the AMP‑activated protein kinase (AMPK) signalling, regardless of the presence or absence of GJs. Collectively, it was identified that Cx32 exerted its anti‑apoptotic effect by activating autophagy via the AMPK pathway in cervical cancer, which demonstrates a novel mechanism for Cx32 in human cervical cancer progression.

Gap Junction-Mediated Intercellular Communication of cAMP Prevents CDDP-Induced Ototoxicity via cAMP/PKA/CREB Pathway

In the cochlea, non-sensory supporting cells are directly connected to adjacent supporting cells via gap junctions that allow the exchange of small molecules. We have previously shown that the pharmacological regulation of gap junctions alleviates cisplatin (CDDP)-induced ototoxicity in animal models. In this study, we aimed to identify specific small molecules that pass through gap junctions in the process of CDDP-induced auditory cell death and suggest new mechanisms to prevent hearing loss. We found that the cyclic adenosine monophosphate (cAMP) inducer forskolin (FSK) significantly attenuated CDDP-induced auditory cell death in vitro and ex vivo. The activation of cAMP/PKA/CREB signaling was observed in organ of Corti primary cells treated with FSK, especially in supporting cells. Co-treatment with gap junction enhancers such as all-trans retinoic acid (ATRA) and quinoline showed potentiating effects with FSK on cell survival via activation of cAMP/PKA/CREB. In vivo, the combination of FSK and ATRA was more effective for preventing ototoxicity compared to either single treatment. Our study provides the new insight that gap junction-mediated intercellular communication of cAMP may prevent CDDP-induced ototoxicity.

Cx43 deficiency confers EMT-mediated tamoxifen resistance to breast cancer via c-Src/PI3K/Akt pathway

Tamoxifen (TAM) resistance has indicated a significant challenge during endocrine therapy for hormone-sensitive breast cancer. Thus, it is significant to elucidate the molecular events endowing TAM resistance to endocrine therapy. In this study, we found that epithelial-mesenchymal transition (EMT) was an important event to confer TAM resistance, and attenuating EMT by elevating connexin (Cx) 43 expression could reverse TAM resistance. Specifically, Cx43 overexpression improved TAM sensitivity, while Cx43 depletion facilitated TAM insensitivity by modulating EMT in T47D TAM-resistant and -sensitive cells, and transplanted xenografts. Importantly, we found a novel reciprocal regulation between Cx43 and c-Src/PI3K/Akt pathway contributing to EMT and TAM resistance in breast cancer. Moreover, we identified that Cx43 deficiency was significantly correlated with poor relapse-free survival in patients undergoing TAM treatment. Therefore, Cx43 represents a prognostic marker and an attractive target for breast cancer treatments. Therapeutic strategies designed to increase or maintain Cx43 function may be beneficial to overcome TAM resistance.

Analysis of Collagen type X alpha 1 (COL10A1) expression and prognostic significance in gastric cancer based on bioinformatics

Collagen type X alpha 1 (COL10A1) is a member of the collagen family and the main matrix component. However, COL10A1 expression and prognosis relationship remains unclear in gastric cancer (GC). Through the analysis of database of Oncomine, the Cancer Genome Atlas (TCGA) as well as the Gene Expression Omnibus (GEO), in contrast to the tissue of normal gastric, COL10A1 in gastric cancer, had been upregulated. The high expression of COL10A1 was obviously related to T stage (P = 0.025) and lymph node metastasis (P = 0.025). It has been illustrated by the analysis of logistic regression that COL10A1's heightened expression in gastric cancer had been essentially linked with pathological stage, tumor differentiation, and T classification. The Kaplan-Meier curve in the Kaplan-Meier plotter database (P = 0.0371) and GSE84437 (P = 0.002) indicate that patients with high COL10A1 expression possess poor prognosis, specifically GC patients with lymph node metastasis have it. TCGA's Multivariate analysis (P = 0.025) and GSE84437 dataset (P = 0.034) show that high expression COL10A1 is a key independent predictor of poor overall survival. Searching KEGG pathway enrichment by GSEA, the results suggested that 29 pathways were enriched. qRT-PCR technique was used for verification of the COL10A1's high expression in gastric cancer in contrast to the normal gastric tissues. In conclusion, COL10A1 is of great importance in predicting the survival rate of GC patients.

Nitric oxide affects cisplatin cytotoxicity oppositely in A2780 and A2780-CDDP cells via the connexin32/gap junction

Chemoresistance is a main obstacle in ovarian cancer therapy and new treatment strategies and further information regarding the mechanism of the medication cisplatin are urgently needed. Nitric oxide has a critical role in modulating the activity of chemotherapeutic drugs. Our previous work showed that connexin32 contributed to cisplatin resistance. However, whether nitric oxide is involved in connexin32-mediated cisplatin resistance remains unknown. In this study, using A2780 and A2780 cisplatin-resistant cells, we found that S-nitroso-N-acetyl-penicillamine, a nitric oxide donor, attenuated cisplatin toxicity by decreasing gap junctions in A2780 cells. Enhancement of gap junctions using retinoic acid reversed the effects of S-nitroso-N-acetyl-penicillamine on cisplatin toxicity. In A2780 cisplatin-resistant cells, however, S-nitroso-N-acetyl-penicillamine enhanced cisplatin toxicity by decreasing connexin32 expression. Downregulation of connexin32 expression by small interfering RNA exacerbated the effects of S-nitroso-N-acetyl-penicillamine on cisplatin cytotoxicity and upregulation of connexin32 expression by pcDNA transfection reversed the effects of S-nitroso-N-acetyl-penicillamine on cisplatin cytotoxicity. Our study suggests for the first time that combining cisplatin with nitric oxide in clinical therapies for ovarian cancer should be avoided before cisplatin resistance emerges. The present study provides a productive area of further study for increasing the efficacy of cisplatin by combining cisplatin with the specific inhibitors or enhancers of nitric oxide in clinical treatment.

Connexin 32 downregulation is critical for chemoresistance in oxaliplatin-resistant HCC cells associated with EMT

Background: Oxaliplatin (OXA)-based chemotherapy is critical in the management of advanced hepatocellular carcinoma (HCC); however, acquired drug resistance has largely restricted its clinical efficacy. This study aims to explore the key mechanisms and regulatory factors determining chemosensitivity in HCC.

Methods: We developed OXA-resistant (OR) HCC cells and used multiple methods, including real-time RT-PCR, Western blot, immunofluorescence, transwell invasion assay, wound-healing assay, MTT assay, gene transfection, and immunohistochemistry to achieve our goals.

Results: We found that OR HCC cells showed a typical epithelial–mesenchymal transition (EMT) phenotype. Meanwhile, the expression of Cx32, a major member of the liver connexin (Cx) family, was lowly expressed in OR HCC cells. Downregulation of Cx32 in parental HCC cells led to EMT induction and thereby reduced OXA cytotoxicity, while Cx32 upregulation in OR HCC cells could reverse the EMT phenotype and partially restore chemosensitivity to OXA. Finally, in human HCC tissue samples, Cx32 was positively correlated with the expression of the EMT marker E-cadherin and negatively correlated with the expression of Vimentin.

Conclusion: Our findings demonstrated that downregulation of Cx32 may be an important determinant for HCC cells to acquire EMT-related acquired drug resistance to OXA, and targeting Cx32 could be a novel strategy to overcome OXA resistance in HCC.

Involvement of gap junctions in propylthiouracil-induced cytotoxicity in BRL-3A cells

Gap junctions (GJs), which are important plasma membrane channels for the transfer of signaling molecules between adjacent cells, have been implicated in drug-induced liver injury. However, the influence and the underlying mechanisms of GJs in propylthiouracil (PTU)-induced hepatotoxicity are unclear. In the present study, distinct manipulations were performed to regulate GJ function in the BRL-3A rat liver cell line. The results indicated that the toxic effect of PTU in BRL-3A cells was mediated by GJ intercellular communication, as cell death was significantly attenuated in the absence of functional GJ channels. Furthermore, the specific knockdown of connexin-32 (Cx32; a major GJ component protein in hepatocytes) using small interfering RNA was observed to decrease necrosis, intracellular PTU content and the level of reactive oxygen species (ROS) following PTU exposure. These observations demonstrated that suppressing GJ Cx32 could confer protection against PTU-induced cytotoxicity through decreasing the accumulation of PTU and ROS. To the best of our knowledge, the present study is the first to demonstrate the role and possible underlying mechanisms of GJs in the regulation of PTU-induced toxicity in BRL-3A rat liver cells.

Enhanced phototoxicity of photodynamic treatment by Cx26-composed GJIC via ROS-, calcium- and lipid peroxide-mediated pathways

In spite of the promising initial treatment responses presented by photodynamic therapy (PDT), 5-year recurrence rates remain high level. Therefore, improvement in the efficacy of PDT is needed. There are reports showing that connexin(Cx) 26-composed gap junctional intercellular communication (GJIC) enhances the intercellular propagation of "death signal", thereby increasing chemotherapeutic cytotoxicity. However, it is unclear whether Cx26-formed GJIC has an effect on PDT phototoxicity. The results in the present study showed that Cx26-composed GJ formation at high density enhances the phototoxicity of Photofrin-PDT. When the Cx26 is not expressed or Cx26 channels are blocked, the phototoxicity in high-density cultures substantially reduces, indicating that the enhanced PDT phototoxicity at high density is mediated by Cx26-composed GJIC. The GJIC-mediated increase in PDT phototoxicity was associated with ROS, calcium and lipid peroxide-mediated stress signaling pathways. The work presents the ability of Cx26-composed GJIC to enhance the sensitivity of malignant cells to PDT, and indicates that maintenance or increase of Cx26-formed GJIC may be a profitable strategy towards the enhancement of PDT therapeutic efficiency. Picture: The survival response of Photofrin-PDT in Dox-treated (Cx26 expressing, GJ-formed) and Dox-untreated cells (Cx26 non-expressing, GJ-unformed) at high-cell density condition.

Deficiency of gap junction composed of connexin43 contributes to oxaliplatin resistance in colon cancer cells

Although comprehensive strategies in the treatment of colorectal cancer have been developed for a number of years, the five-year survival rate of metastatic colon cancer remains less than 10%. Oxaliplatin, a commonly used chemotherapeutic agent for metastatic colon cancer, improves the response rate of patients and prolongs patients' progression-free survival. However, the generation of resistance limits the clinical application of oxaliplatin, and the mechanisms of this remain unclear. The present study mainly investigated the effect of the gap junction (GJ) composed of connexin43 (Cx43) on oxaliplatin cytotoxicity in colon cancer cells. Three different methods with distinct mechanisms were used to change the function of Cx43 GJs, including cell culture at different densities, pretreatment with a specific inhibitor or enhancer, and special gene knockdown, to observe the cytotoxicity of oxaliplatin and the level of reactive oxygen species (ROS) mediated by Cx43 GJs. The results revealed that the cytotoxicity of oxaliplatin and the level of ROS were decreased with the downregulation of Cx43 GJ function, but exacerbated with the upregulation of Cx43 GJ function. Moreover, ROS scavenging with N-acetyl-L-cysteine and apocynin decreased the cytotoxicity of oxaliplatin. We concluded that the loss of GJ composed of Cx43 contributed to the resistance of oxaliplatin in colon cancer cells, and the mechanism was associated with intracellular ROS alternation.

Sufentanil attenuates oxaliplatin cytotoxicity via inhibiting connexin 43‑composed gap junction function

Comprehensive strategies for the treatment of colorectal cancer (CRC) have become increasingly important. One of the most important factors is pain relief. Therefore, patients with CRC are concurrently treated with analgesics and chemotherapeutic agents; however, the effects of analgesics on the therapeutic activity of chemotherapeutic agents remain largely unknown. The present study investigated the effects of three widely used analgesics in clinics: Fentanyl, remifentanil and sufentanil, on the cytotoxicity of oxaliplatin, a commonly used chemotherapeutic agent for CRC. Furthermore, the underlying mechanisms of those effects in association with connexin 43 (Cx43)‑composed gap junction (GJ) function were analyzed. The Lovo, Colo320, HCT116 and HT29 human CRC cell lines, with or without Cx43 expression, were used to examine the effects of the three analgesics on the cytotoxicity of oxaliplatin. The results demonstrated that in the cell lines expressing Cx43 (Lovo and Colo320), the cytotoxicity of oxaliplatin was attenuated and Cx43 GJ function was inhibited. Sufentanil, not fentanyl or remifentanil, inhibited Cx43 GJ function effectively, and reduced the cytotoxicity of oxaliplatin. In contrast, these effects were not observed in the other two colon cancer cell lines not expressing Cx43 (HCT116 and HT29). These results suggested that alternation of Cx43 GJ function may regulate the cytotoxicity of oxaliplatin in regard to CRC. Furthermore, sufentanil, not fentanyl or remifentanil, suppressed the cytotoxicity of oxaliplatin through inhibition of Cx43 GJ function. These results may be beneficial for the treatment of CRC and reduction of treatment resistance.

The cytoplasmic translocation of Cx32 mediates cisplatin resistance in ovarian cancer cells

Ovarian cancer is the most lethal gynecologic malignancy, and cisplatin is one of the first-line chemotherapeutic agents. However, acquired cisplatin resistance prevents the successful treatment of patients with ovarian cancer. Gap junction (GJ) and connexin (Cx) are closely related to tumor formation, but the relationship between cisplatin resistance and GJ or Cx are undetermined. In this study, we established the cisplatin-resistant human ovarian cancer cell line A2780-CDDP. Here we showed that cisplatin resistance was correlated to the loss of GJ and the upregulation of Cx32 expression. Enhancing GJ in A2780-CDDP cells could increase the apoptotic response to cisplatin treatment. Furthermore, although Cx32 expression was increased in A2780-CDDP cells, it was more localized to the cytoplasm rather than in the membrane, and knockdown of Cx32 in A2780-CDDP cells sensitized them to cisplatin treatment. In summary, Cx32 is involved in cisplatin resistance, and cytoplasmic Cx32 plays an important role in chemoresistance.

Connexin 43 Acts as a Proapoptotic Modulator in Cisplatin-Induced Auditory Cell Death

AIMS

Gap junction coupling is known to play a role in intercellular communication by the Good Samaritan effect or bystander effect. Nonjunctional connexins (Cxs) may also play certain gap junction-independent roles in cell death or survival. The purpose of the present study was to investigate the role of junctional and nonjunctional Cxs in ototoxic drug-induced auditory cell death by focusing on Cx43 in the cochlea.

RESULTS

Nonjunctional Cx43 conditions were prepared by low confluence culture (5 × 10³/cm²) or a trafficking inhibitor, brefeldin A (BFA), in auditory cells, and short lengthened Cx43s with amino-terminal (NT; amino acids 1-256) or carboxy-terminal (CT; amino acids 257-382) were transfected into Cx-deficient HeLa cells to avoid gap junction formation. Knockdown of nonchannel Cx43 (small interfering RNA [siRNA]) inhibited Cis-diamminedichloroplatinum (cisplatin)-induced cell death regardless of gap junction formation; however, a gap junction blocker, 18 alpha-glycyrrhetinic acid (18α-GA), showed inhibitory effect only under the junctional condition. BFA did not show any additive influence on the inhibitory effect of siRNA Cx43. Shortened Cx43-transfected HeLa cells also resulted in a significant increase in cell death under cisplatin. In the animal studies with cisplatin-treated rats, hearing thresholds of auditory brainstem response were significantly preserved by a gap junction blocker, carbenoxolone, showing much more preserved stereocilia of hair cells in scanning electron microscopic findings. Innovation and Conclusion: Cx43 plays a proapoptotic role in cisplatin-induced auditory cell death in both junctional and nonjunctional conditions. Targeting the Cx-mediated signaling control may be helpful in designing new therapeutic strategies for drug-induced ototoxicity. Antioxid. Redox Signal. 25, 623-636.

Gap junction composed of connexin43 modulates 5‑fluorouracil, oxaliplatin and irinotecan resistance on colorectal cancers

Chemotherapy is one of the most commonly used therapeutic strategies for metastatic colon cancer. However, the development of resistance to chemotherapeutic agents limits their application in clinical use. The underlying mechanisms of this resistance development require further elucidation. The current study investigated the effects of connexin43 (Cx43) gap junctions on 5‑fluorouracil (5‑FU), oxaliplatin and irinotecan in colon cancer cells. Three different methods were used to manipulate Cx43 gap junction function: i) Cell culture at different densities; ii) pretreatment with a Cx43 specific inhibitor or enhancer; and iii) Cx43 gene knock‑down. Results indicated that the cell toxicity of 5‑FU, oxaliplatin and irinotecan was cell density‑dependent, which was mediated by gap junctions. Downregulation of Cx43 gap junction functioning attenuated 5‑FU, oxaliplatin and irinotecan toxicity in colon cancer cells, which was increased in cells treated with a Cx43 gap junction function enhancer. Thus, the results of the present study suggest that resistance to 5‑FU, oxaliplatin and irinotecan in colon cancer cells was relative to Cx43 expression loss as cancer developed, which may indicate a novel basis for therapeutic strategy development to combat drug resistance in numerous cell types, in addition to colon cancer cells.

Propofol depresses cisplatin cytotoxicity via the inhibition of gap junctions

The general anesthetic, propofol, affects chemotherapeutic activity, however, the mechanism underlying its effects remains to be fully elucidated. Our previous study showed that tramadol and flurbiprofen depressed the cytotoxicity of cisplatin via the inhibition of gap junction (GJ) intercellular communication (GJIC) in connexin (Cx)32 HeLa cells. The present study investigated whether the effects of propofol on the cytotoxicity of cisplatin were mediated by GJ in U87 glioma cells and Cx26‑transfected HeLa cells. Standard colony formation assay was used to determine the cytotoxicity of cisplatin. Parachute dye coupling assay was used to measure GJ function, and western blot analysis was used to determine the expression levels of Cx32. The results revealed that exposure of the U87 glioma cells and the Cx26-transfected HeLa cells to cisplatin for 1 h reduced clonogenic survival in low density cultures (without GJs) and high density cultures (with GJs). However, the toxic effect was higher in the high density culture. In addition, pretreatment of the cells with propofol significantly reduced cisplatin‑induced cytotoxicity, but only in the presence of functional GJs. Furthermore, propofol significantly inhibited dye coupling through junctional channels, and a long duration of exposure of the cells to propofol downregulated the expression levels of Cx43 and Cx26. These results demonstrated that the inhibition of GJIC by propofol affected the therapeutic efficacy of chemotherapeutic drugs. The present study provides evidence of a novel mechanism underlying the effects of analgesics in counteracting chemotherapeutic efficiency.

Different gap junction-propagated effects on cisplatin transfer result in opposite responses to cisplatin in normal cells versus tumor cells

Previous work has shown that gap junction intercellular communication (GJIC) enhances cisplatin (Pt) toxicity in testicular tumor cells but decreases it in non-tumor testicular cells. In this study, these different GJIC-propagated effects were demonstrated in tumor versus non-tumor cells from other organ tissues (liver and lung). The downregulation of GJIC by several different manipulations (no cell contact, pharmacological inhibition, and siRNA suppression) decreased Pt toxicity in tumor cells but enhanced it in non-tumor cells. The in vivo results using xenograft tumor models were consistent with those from the above-mentioned cells. To better understand the mechanism(s) involved, we studied the effects of GJIC on Pt accumulation in tumor and non-tumor cells from the liver and lung. The intracellular Pt and DNA-Pt adduct contents clearly increased in non-tumor cells but decreased in tumor cells when GJIC was downregulated. Further analysis indicated that the opposite effects of GJIC on Pt accumulation in normal versus tumor cells from the liver were due to its different effects on copper transporter1 and multidrug resistance-associated protein2, membrane transporters attributed to intracellular Pt transfer. Thus, GJIC protects normal organs from cisplatin toxicity while enhancing it in tumor cells via its different effects on intracellular Pt transfer.

Propofol attenuated acute kidney injury after orthotopic liver transplantation via inhibiting gap junction composed of connexin 32

BACKGROUND

Postliver transplantation acute kidney injury (AKI) severely affects patient survival, whereas the mechanism is unclear and effective therapy is lacking. The authors postulated that reperfusion induced enhancement of connexin32 (Cx32) gap junction plays a critical role in mediating postliver transplantation AKI and that pretreatment/precondition with the anesthetic propofol, known to inhibit gap junction, can confer effective protection.

METHODS

Male Sprague-Dawley rats underwent autologous orthotopic liver transplantation (AOLT) in the absence or presence of treatments with the selective Cx32 inhibitor, 2-aminoethoxydiphenyl borate or propofol (50 mg/kg) (n = 8 per group). Also, kidney tubular epithelial (NRK-52E) cells were subjected to hypoxia-reoxygenation and the function of Cx32 was manipulated by three distinct mechanisms: cell culture in different density; pretreatment with Cx32 inhibitors or enhancer; Cx32 gene knock-down (n = 4 to 5).

RESULTS

AOLT resulted in significant increases of renal Cx32 protein expression and gap junction, which were coincident with increases in oxidative stress and impairment in renal function and tissue injury as compared to sham group. Similarly, hypoxia-reoxygenation resulted in significant cellular injury manifested as reduced cell growth and increased lactate dehydrogenase release, which was significantly attenuated by Cx32 gene knock-down but exacerbated by Cx32 enhancement. Propofol inhibited Cx32 function and attenuated post-AOLT AKI. In NRK-52E cells, propofol reduced posthypoxic reactive oxygen species production and attenuated cellular injury, and the cellular protective effects of propofol were reinforced by Cx32 inhibition but cancelled by Cx32 enhancement.

CONCLUSION

Cx32 plays a critical role in AOLT-induced AKI and that inhibition of Cx32 function may represent a new and major mechanism whereby propofol reduces oxidative stress and subsequently attenuates post-AOLT AKI.

Baicalein increases the cytotoxicity of cisplatin by enhancing gap junction intercellular communication

Drug resistance limits the clinical application of cisplatin, a widely used chemotherapeutic agent. Gap junction (GJ) is a channel that enhances cytotoxicity of certain chemotherapeutic agents. Baicalein is well known for its antitumor activity. This study investigated the effect of baicalein on cisplatin cytotoxicity and the relationship between this effect and the modulation of the GJ function in connexin 26 (Cx26)‑transfected HeLa cells. The sulforhodamine B (SRB) assay was used to examine the effect of baicalein on cell viability. A 'parachute' assay was used to investigate the effect of baicalein on GJ function. The effects of baicalein on cisplatin cytotoxicity and GJ function were assayed by standard colony‑forming assays. The expression of Cx26 was monitored by western blotting. It was observed that exposure of Cx26‑transfected cells to cisplatin reduced the number of colonies formed in low‑density cultures (no GJ formation) and in high‑density cultures (GJ formation), but the toxic effect was greater when cells were seeded at a high density. In the absence of connexin expression or with blockage of connexin channels however, cell density had no effect on cisplatin toxicity. Baicalein significantly enhanced cisplatin cytotoxicity, but this effect required the presence of functional GJs between the cells. In conclusion, the dependence of cisplatin toxicity on cell density is mediated by GJs. Baicalein increases cisplatin cytotoxicity through enhancing GJ intercellular communication.

Components of Panax notoginseng saponins enhance the cytotoxicity of cisplatin via their effects on gap junctions

Previously, Panax notoginseng saponin (PNS)-induced enhancement of gap junction (GJ) formation or function was observed to be responsible for the increased cytotoxic action of cisplatin. PNS has three constituents, ginsenoside Rg1 and Rb1, and notoginsenoside R1. The active compounds in PNS responsible for enhancing the cytotoxicity of cisplatin remain unknown. Thus, the effects of the main components of PNS on the cytotoxicity of cisplatin were investigated, as well as the correlation with the modulation of GJ function in transfected HeLa cells. The cytotoxicity of cisplatin (0.25-1 µg/ml) was increased in the presence of GJs. By contrast, the cytotoxicity of cisplatin was decreased when GJs were inhibited by a GJ blocker or by the inhibition of connexin expression. Ginsenoside Rg1 (100 µM) and notoginsenoside R1 (100 µM) were observed to significantly enhance cisplatin cytotoxicity in cells with functional GJs. Ginsenoside Rb1 had no effect on the cytotoxicity of cisplatin in the presence or absence of functional GJs. Cell exposure to ginsenoside Rg1 and notoginsenoside R1 for 4 h led to significant enhancement of a dye-coupled GJ in a dose-dependent manner; however, no effect was observed in cells exposed to ginsenoside Rb1. The present results indicate that ginsenoside Rg1 and notoginsenoside R1 are the active compounds responsible for enhancing the cytotoxic action of cisplatin induced by PNS in the presence of functional GJs.

Differential effects of paclitaxel and docetaxel on gap junctions affects their cytotoxicities in transfected HeLa cells

Gap junctions (GJs) enhance the cytotoxicity of specific cancer chemotherapeutic drugs and therefore, the inhibition of functional GJs may represent a mechanism by which the toxicity of chemotherapeutics in cancer cells can be reduced. In the present study, the effects and mechanisms of paclitaxel and docetaxel on GJ intercellular communication (GJIC) and the modulation of drug cytotoxicity were investigated in HeLa cells that were stably transfected with the connexin (Cx) 32 expression plasmid. Paclitaxel, but not docetaxel, was observed to inhibit dye‑coupling through junctional channels. Gating closure rather than the alteration of Cx32 expression or its membrane localization was responsible for the inhibitory action of paclitaxel on GJ function following short‑term exposure. The results revealed that the cytotoxicity of paclitaxel or docetaxel increased in the presence of functional GJs compared with that observed when GJIC was suppressed. In addition, paclitaxel‑induced downregulation of GJIC decreased the cytotoxicity of paclitaxel in the presence of functional GJs compared with that of docetaxel, which did not affect Cx32 channels. These observations demonstrated that the differential effects of paclitaxel and docetaxel on GJIC may affect the cytotoxicity of chemotherapeutic drugs. The present study provides a promising new approach to select antineoplastics and improve drug efficacy in carcinoma cells that form GJs.

The dynamics of connexin expression, degradation and localisation are regulated by gonadotropins during the early stages of in vitro maturation of swine oocytes

Gap junctional communication (GJC) plays a primordial role in oocyte maturation and meiotic resumption in mammals by directing the transfer of numerous molecules between cumulus cells and the oocyte. Gap junctions are made of connexins (Cx), proteins that regulate GJC in numerous ways. Understanding the dynamic regulation of connexin arrangements during in vitro maturation (IVM) could provide a powerful tool for controlling meiotic resumption and consequently in vitro development of fully competent oocytes. However, physiological events happening during the early hours of IVM may still be elucidated. The present study reports the dynamic regulation of connexin expression, degradation and localization during this stage. Cx43, Cx45 and Cx60 were identified as the main connexins expressed in swine COC. Cx43 and Cx45 transcripts were judged too static to be a regulator of GJC, while Cx43 protein expression was highly responsive to gonadotropins, suggesting that it might be the principal regulator of GJC. In addition, the degradation of Cx43 expressed after 4.5 h of IVM in response to equine chorionic gonadotropin appeared to involve the proteasomal complex. Cx43 localisation appeared to be associated with GJC. Taken together, these results show for the first time that gonadotropins regulate Cx43 protein expression, degradation and localisation in porcine COC during the first several hours of IVM. Regulation of Cx43 may in turn, via GJC, participate in the development of fully competent oocytes.

Connexin channel modulators and their mechanisms of action

Gap junction channels and hemichannels formed by the connexin family of proteins play important roles in many aspects of tissue homeostasis in the brain and in other organs. In addition, connexin channels have been proposed as pharmacological targets in the treatment of a number of human disorders. In this review, we describe the connexin-subtype selectivity and specificity of pharmacological agents that are commonly used to modulate connexin channels. We also highlight recent progress made toward identifying new agents for connexin channels that act in a selective and specific manner. Finally, we discuss developing insights into possible mechanisms by which these agents modulate connexin channel function. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

The effect of the PQ1 anti-breast cancer agent on normal tissues

Gap junctions are intercellular channels connecting adjacent cells, allowing cells to transport small molecules. The loss of gap junctional intercellular communication (GJIC) is one of the important hallmarks of cancer. Restoration of GJIC is related to the reduction of tumorigenesis and increase in drug sensitivity. Previous reports have shown that PQ1, a quinoline derivative, increases GJIC in T47D breast cancer cells, and subsequently attenuates xenograft breast tumor growth. Combinational treatment of PQ1 and tamoxifen can lower the effective dose of tamoxifen in cancer cells. In this study, the effects of PQ1 were examined in normal C57BL/6J mice, evaluating the distribution, toxicity, and adverse effects. The distribution of PQ1 was quantified by high-performance liquid chromatography and mass spectrometry. The expressions of survivin, caspase-8, cleaved caspase-3, aryl hydrocarbon receptor (AhR), and gap junction protein, connexin 43 (Cx43), were assessed using western blot analysis. Our results showed that PQ1 was absorbed and distributed to vital organs within 1 h and the level of PQ1 decreased after 24 h. Furthermore, PQ1 increased the expression of survivin, but decreased the expression of caspase-8 and caspase-3 activity. Interestingly, the expression of AhR increased in the presence of PQ1, suggesting that PQ1 may be involved in the AhR-mediated response. Previously, PQ1 caused an increase in Cx43 expression in breast cancer cells; however, PQ1 induced a decrease in Cx43 in normal tissues. Hemotoxylin and eosin staining of the tissues showed no histological change between the treated and the untreated organs. Our studies indicate that the administration of PQ1 by an oral gavage can be achieved with low toxicity to normal vital organs.

Panax notoginseng saponins enhances the cytotoxicity of cisplatin via increasing gap junction intercellular communication

Panax Notoginseng Saponins (PNS) have been well known to have anti-tumor activity and enhance cytotoxicity of some cancer chemotherapy agents, but the mechanisms underlying these effects are still unknown. This study investigates the effect of PNS on cytotoxicity of cisplatin and the relationship between this effect and the modulation of gap junctions (GJ) function by PNS in a transfected cell line. The cytotoxicity of cisplatin (0.25-1 µg/mL) was increased in the presence of GJ. Inhibition of gap junction by either GJ blocker or interception of Connexin (Cx) expression decreased the cytotoxicity of cisplatin. Increasing GJ function enhanced cytotoxicity of cisplatin, only in the cells with functional GJ. PNS (50-200 µg/mL) significantly enhanced cisplatin cytotoxicity, but this effect required functional gap junctions between the cells. Exposure of the cells to PNS (50-200 µg/mL) for 4 h leads to a significant enhance in dye coupling of GJ in a dose-dependent manner. These results suggest that PNS increases the cytotoxicity of cisplatin through enhancement of GJ activity.

Gap junction enhancer increases efficacy of cisplatin to attenuate mammary tumor growth

Cisplatin treatment has an overall 19% response rate in animal models with malignant tumors. Increasing gap junction activity in tumor cells provides the targets to enhance antineoplastic therapies. Previously, a new class of substituted quinolines (PQs) acts as gap junction enhancer, ability to increase the gap junctional intercellular communication, in breast cancer cells. We examined the effect of combinational treatment of PQs and antineoplastic drugs in an animal model, showing an increase in efficacy of antineoplastic drugs via the enhancement of gap junctions. Mice were implanted with estradiol-17ß (1.7 mg/pellet) before the injection of 1×10⁷ T47D breast cancer cells subcutaneously into the inguinal region of mammary fat pad. Animals were treated intraperitoneally with DMSO (control), cisplatin (3.5 mg/kg), PQ (25 mg/kg), or a combining treatment of cisplatin and PQ. Cisplatin alone decreased mammary tumor growth by 85% while combinational treatment of cisplatin and PQ1 or PQ7 showed an additional reduction of 77% and 22% of tumor growth after 7 treatments at every 2 days, respectively. Histological results showed a significant increase of gap junction proteins, Cx43 and Cx26, in PQ-treated tissues compared to control or cisplatin. Furthermore, evidence of highly stained caspase 3 in tumors of combinational treatment (PQ and cisplatin) was seen compared to cisplatin alone. We have showed for the first time an increase in the efficacy of antineoplastic drugs through a combinational treatment with PQs, a specific class of gap junction enhancers.

Dual Effects of Bilirubin on the Proliferation of Rat Renal NRK52E Cells and ITS Association with Gap Junctions

OBJECTIVE

The effect of bilirubin on renal pathophysiology is controversial. This study aimed to observe the effects of bilirubin on the proliferation of normal rat renal tubular epithelial cell line (NRK52E) and its potential interplay with gap junction function.

METHODS

Cultured NRK52E cells, seeded respectively at high- or low- densities, were treated with varying concentrations of bilirubin for 24 hours. Cell injury was assessed by measuring cell viability and proliferation, and gap junction function was assessed by Parachute dye-coupling assay. Connexin 43 protein was assessed by Western blotting.

RESULTS

At doses from 17.1 to 513μmol/L, bilirubin dose-dependently enhanced cell viability and colony-formation rates when cells were seeded at either high- or low- densities (all p<0.05 vs. solvent group) accompanied with enhanced intercellular fluorescence transmission and increased Cx43 protein expression in high-density cells. However, the above effects of BR were gradually reversed when its concentration increased from 684 to 1026μmol/L. In high-density cells, gap junction inhibitor 12-O-tetradecanoylphorbol 13-acetate attenuated bilirubin-induced enhancement of colony-formation and fluorescence transmission. However, in the presence of high concentration bilirubin (1026μmol/L), activation of gap junction with retinoid acid decreased colony-formation rates.

CONCLUSION

Bilirubin can confer biphasic effects on renal NRK52E cell proliferation potentially by differentially affecting gap junction functions.

Monocyte-endothelial adhesion is modulated by Cx43-stimulated ATP release from monocytes

Adhesion of circulating monocytes to vascular endothelial cells is a crucial event in development of vascular inflammatory conditions, including atherosclerosis. We investigated the roles of connexin43 (Cx43) and ATP release on monocyte-endothelial adhesion. Cx43 function and expression were manipulated by connexin channel inhibitors, overexpression and siRNA. Connexin channel inhibitors rapidly decreased ATP release from U937 monocytes and increased adhesion to human umbilical vein endothelial cells (HUVEC). Monocyte ATP release correlated with Cx43 expression, not with Cx37 expression. Exogenous adenosine (ADO) or ATP decreased adhesion, and inhibition of ATP conversion to ADO increased adhesion. We infer that monocyte Cx43 channel activity causes ATP release, likely via Cx43-containing hemichannels, and that ATP decreases adhesion via conversion to ADO. Inhibition of HUVEC connexin channel activity did not affect ATP release or adhesion. In contrast, expression of Cx43 protein in U937 cells enhanced adhesion. Thus, Cx43 channel function and expression have opposite effects: Cx43 channel function in monocytes, but not in HUVEC, rapidly decreases adhesion via ATP release and conversion to ADO, whereas Cx43 expression itself enhances adhesion. These studies suggest that local regulation of monocyte Cx43 activity within the vasculature can dynamically modulate the monocyte-endothelial adhesion that is an initiating event in vascular inflammatory pathologies, with the baseline adhesion set by Cx43 expression levels. This balance of rapid and tonic influences may be crucial in development of vascular pathologies.

Gap Junction Enhancer Potentiates Cytotoxicity of Cisplatin in Breast Cancer Cells

Cisplatin is one of the most widely used anti-cancer drugs due to its ability to damage DNA and induce apoptosis. However, increasing reports of side effects and drug resistance indicate the limitation of cisplatin in cancer therapeutics. Recent studies showed that inhibition of gap junctions diminishes the cytotoxic effect and contributes to drug resistance. Therefore, identification of molecules that counteract gap junctional inhibition without decreasing the anti-cancer effect of cisplatin could be used in combinational treatment, potentiating cisplatin efficacy and preventing resistance. This study investigates the effects of combinational treatment of cisplatin and PQ1, a gap junction enhancer, in T47D breast cancer cells. Our results showed that combinational treatment of PQ1 and cisplatin increased gap junctional intercellular communication (GJIC) as well as expressions of connexins (Cx26, Cx32 and Cx43), and subsequently decreased cell viability. Ki67, a proliferation marker, was decreased by 75% with combinational treatment. Expressions of pro-apoptotic factors (cleaved caspase-3/-8/-9 and bax) were increased by the combinational treatment with PQ1 and cisplatin; whereas, the pro-survival factor, bcl-2, was decreased by the combinational treatment. Our study demonstrates for the first time that the combinational treatment with gap junction enhancers can counteract cisplatin induced inhibition of gap junctional intercellular communication and reduction of connexin expression, thereby increasing the efficacy of cisplatin in cancer cells.

Gap junctions propagate opposite effects in normal and tumor testicular cells in response to cisplatin

Gap junctions propagate toxic effects among tumor cells during chemotherapy, but could also enhance killing of normal cells by the same mechanism. We show that the effect of gap junctional intercellular communication (GJIC) on cisplatin toxicity differs between normal and tumor testicular cells. Downregulation of GJIC by each of several different manipulations (no cell contact, pharmacological inhibition, siRNA suppression) decreased cisplatin cytoxicity in tumor cells but enhanced it in normal cells. Enhanced toxicity due to GJIC downregulation in normal cells correlated with increased DNA interstrand crosslinks. Thus, GJIC protects normal cells from cisplatin toxicity while enhancing it in tumor cells, suggesting that enhancement/maintenance of GJIC increases therapeutic efficacy while decreasing off-target toxicity.

Propofol depresses the cytotoxicity of X-ray irradiation through inhibition of gap junctions

BACKGROUND

General anesthetics (e.g., propofol) influence the therapeutic activity of intraoperative radiotherapy but the mechanism of the effects is largely unknown. It has been reported that propofol inhibits gap junction (GJ) function briefly, and a functional GJ enhances the efficacy of radiotherapy in some cancer cells. Yet the mechanisms underlying the inhibition of GJ function by propofol and the influence of propofol on therapeutic activity of intraoperative radiotherapy are unknown.

METHODS

The role of propofol at clinically relevant concentrations in the modulation of radiograph-induced cytotoxicity in HeLa cells transfected with connexin 32 (Cx32) plasmid was explored by manipulation of connexin expression, GJ presence, and function. GJ function, Cx32 protein level, and Cx32 mRNA expression were determined by "Parachute" dye-coupling assay, Western blotting, and reverse transcriptase-polymerase chain reaction, respectively.

RESULTS

Propofol significantly reduced radiograph-induced cytotoxicity only in the presence of functional GJ. Four-hour propofol exposure inhibited GJ function mainly by diminution of Cx32 protein levels but without influence on Cx32 mRNA expression.

CONCLUSIONS

These results suggest that propofol inhibits the function of the GJ through the reduction of Cx32 protein levels by a transcription-independent mechanism. They further indicate that propofol depresses the cytotoxicity of radiograph irradiation through inhibition of GJ activity.