Role of connexin 32 in acetaminophen toxicity in a knockout mice model

Connexin-Based Channels in the Liver

Connexin proteins oligomerize in hexameric structures called connexin hemichannels, which then dock to form gap junctions. Gap junctions direct cell-cell communication by allowing the exchange of small molecules and ions between neighboring cells. In this way, hepatic gap junctions support liver homeostasis. Besides serving as building blocks for gap junctions, connexin hemichannels provide a pathway between the intracellular and the extracellular environment. The activation of connexin hemichannels is associated with acute and chronic liver pathologies. This article discusses the role of gap junctions and connexin hemichannels in the liver. © 2022 American Physiological Society. Compr Physiol 12:1-17, 2022.

Intercellular Transmission of Hepatic ER Stress in Obesity Disrupts Systemic Metabolism

Endoplasmic reticulum stress (ERS) has a pathophysiological role in obesity-associated insulin resistance. Yet, the coordinated tissue response to ERS remains unclear. Increased connexin 43 (Cx43)-mediated intercellular communication has been implicated in tissue-adaptive and -maladaptive response to various chronic stresses. Here, we demonstrate that in hepatocytes, ERS results in increased Cx43 expression and cell-cell coupling. Co-culture of ER-stressed "donor" cells resulted in intercellular transmission of ERS and dysfunction to ERS-naive "recipient" cells ("bystander response"), which could be prevented by genetic or pharmacologic suppression of Cx43. Hepatocytes from obese mice were able to transmit ERS to hepatocytes from lean mice, and mice lacking liver Cx43 were protected from diet-induced ERS, insulin resistance, and hepatosteatosis. Taken together, our results indicate that in obesity, the increased Cx43-mediated cell-cell coupling allows intercellular propagation of ERS. This novel maladaptive response to over-nutrition exacerbates the tissue ERS burden, promoting hepatosteatosis and impairing whole-body glucose metabolism.

Gap junctions in liver disease: Implications for pathogenesis and therapy

In the normal liver, cells interact closely through gap junctions. By providing a pathway for the trafficking of low molecular mass molecules, these channels contribute to tissue homeostasis and maintenance of hepatic function. Thus, dysfunction of gap junctions affects a wide variety of liver processes, such as differentiation, cell death, inflammation and fibrosis. In fact, dysfunctional gap junctions have been implicated, for more than a decade, in cholestatic disease, hepatic cancer and cirrhosis. Additionally, in recent years there is an increasing body of evidence that these channels are also involved in other relevant and prevalent liver pathological processes, such as non-alcoholic fatty liver disease, acute liver injury and portal hypertension. In parallel to these new clinical implications the available data include controversial observations. Thus, a comprehensive overview is required to better understand the functional complexity of these pores. This paper will review the most recent knowledge concerning gap junction dysfunction, with a special focus on the role of these channels in the pathogenesis of relevant clinical entities and on potential therapeutic targets that are amenable to modification by drugs.

Connexin and Pannexin (Hemi)Channels: Emerging Targets in the Treatment of Liver Disease

Connexin proteins are the building blocks of hemichannels, which dock further between adjacent cells to form gap junctions. Gap junctions control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions control virtually all aspects of the hepatic life cycle. In the last decade, it has become clear that connexin hemichannels also provide a pathway for cellular communication on their own independent of their role as structural precursors of gap junctions, namely between the cytosol of an individual cell and its extracellular environment. In contrast to gap junctions, connexin hemichannels become particularly active in liver disease by facilitating inflammation and cell death. This equally holds true for cellular channels composed of pannexins, being connexin-like proteins recently identified in the liver that gather in structures reminiscent of hemichannels. This paper gives an overview of the involvement of connexin-based and pannexin-based channels in noncancerous liver disease.

Connexin hemichannel inhibition reduces acetaminophen-induced liver injury in mice

Historically, connexin hemichannels have been considered as structural precursors of gap junctions. However, accumulating evidence points to independent roles for connexin hemichannels in cellular signaling by connecting the intracellular compartment with the extracellular environment. Unlike gap junctions, connexin hemichannels seem to be mainly activated in pathological processes. The present study was set up to test the potential involvement of hemichannels composed of connexin32 and connexin43 in acute hepatotoxicity induced by acetaminophen. Prior to this, in vitro testing was performed to confirm the specificity and efficacy of TAT-Gap24 and TAT-Gap19 in blocking connexin32 and connexin43 hemichannels, respectively. Subsequently, mice were overdosed with acetaminophen followed by treatment with TAT-Gap24 or TAT-Gap19 or a combination of both after 1.5h. Sampling was performed 3, 6, 24 and 48h following acetaminophen administration. Evaluation of the effects of connexin hemichannel inhibition was based on a series of clinically relevant read-outs, measurement of inflammatory cytokines and oxidative stress. Subsequent treatment of acetaminophen-overdosed mice with TAT-Gap19 only marginally affected liver injury. In contrast, a significant reduction in serum alanine aminotransferase activity was found upon administration of TAT-Gap24 to intoxicated animals. Furthermore, co-treatment of acetaminophen-overdosed mice with both peptides revealed an additive effect as even lower serum alanine aminotransferase activity was observed. Blocking of connexin32 or connexin43 hemichannels individually was found to decrease serum quantities of pro-inflammatory cytokines, while no effects were observed on the occurrence of hepatic oxidative stress. This study shows for the first time a role for connexin hemichannels in acetaminophen-induced acute liver failure.

The role of gap junctions in inflammatory and neoplastic disorders (Review)

Gap junctions are intercellular channels made of connexin proteins, mediating both electrical and biochemical signals between cells. The ability of gap junction proteins to regulate immune responses, cell proliferation, migration, apoptosis and carcinogenesis makes them attractive therapeutic targets for treating inflammatory and neoplastic disorders in different organ systems. Alterations in gap junction profile and expression levels are observed in hyperproliferative skin disorders, lymphatic vessel diseases, inflammatory lung diseases, liver injury and neoplastic disorders. It is now recognized that the therapeutic effects mediated by traditional pharmacological agents are dependent upon gap junction communication and may even act by influencing gap junction expression or function. Novel strategies for modulating the function or expression of connexins, such as the use of synthetic mimetic peptides and siRNA technology are considered.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

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.

Connexins and their channels in inflammation

Inflammation may be caused by a variety of factors and is a hallmark of a plethora of acute and chronic diseases. The purpose of inflammation is to eliminate the initial cell injury trigger, to clear out dead cells from damaged tissue and to initiate tissue regeneration. Despite the wealth of knowledge regarding the involvement of cellular communication in inflammation, studies on the role of connexin-based channels in this process have only begun to emerge in the last few years. In this paper, a state-of-the-art overview of the effects of inflammation on connexin signaling is provided. Vice versa, the involvement of connexins and their channels in inflammation will be discussed by relying on studies that use a variety of experimental tools, such as genetically modified animals, small interfering RNA and connexin-based channel blockers. A better understanding of the importance of connexin signaling in inflammation may open up towards clinical perspectives.

Connexin32 deficiency exacerbates carbon tetrachloride-induced hepatocellular injury and liver fibrosis in mice

OBJECTIVE

Liver fibrosis results from the perpetuation of the normal wound healing response to several types of injury. Despite the wealth of knowledge regarding the involvement of intracellular and extracellular signaling pathways in liver fibrogenesis, information about the role of intercellular communication mediated by gap junctions is scarce.

METHODS

In this study, liver fibrosis was chemically induced by carbon tetrachloride in mice lacking connexin32, the major liver gap junction constituent. The manifestation of liver fibrosis was evaluated based on a series of read-outs, including collagen morphometric and mRNA analysis, oxidative stress, apoptotic, proliferative and inflammatory markers.

RESULTS

More pronounced liver damage and enhanced collagen deposition were observed in connexin32 knockout mice compared to wild-type animals in experimentally triggered induced liver fibrosis. No differences between both groups were noticed in apoptotic signaling nor in inflammation markers. However, connexin32 deficient mice displayed decreased catalase activity and increased malondialdehyde levels.

CONCLUSION

These findings could suggest that connexin32-based signaling mediates tissue resistance against liver damage by the modulation of the antioxidant capacity. In turn, this could point to a role for connexin32 signaling as a therapeutic target in the treatment of liver fibrosis.

Connexins and pannexins in liver damage

Connexins and pannexins are key players in the control of cellular communication and thus in the maintenance of tissue homeostasis. Inherent to this function these proteins are frequently involved in pathological processes. The present paper reviews the role of connexins and pannexins in liver toxicity and disease. As they act both as sensors and effectors in these deleterious events connexins and pannexins could represent a set of novel clinical diagnostic biomarkers and drug targets.

Connexin32: a mediator of acetaminophen-induced liver injury?

Connexin32 is the building block of hepatocellular gap junctions, which control direct intercellular communication and thereby act as goalkeepers of liver homeostasis. This study was set up to investigate whether connexin32 is involved in hepatotoxicity induced by the analgesic and antipyretic drug acetaminophen. To this end, whole body connexin32 knock-out mice were overdosed with acetaminophen followed by sampling at different time points within a 24-h time frame. Evaluation was done based upon a series of clinically and mechanistically relevant read-outs, including protein adduct formation, histopathological examination, measurement of alanine aminotransferase activity, cytokine production, levels of reduced and oxidized glutathione and hepatic protein amounts of proliferating cell nuclear antigen. In essence, it was found that genetic ablation of connexin32 has no influence on several key events in acetaminophen-induced hepatotoxicity, including cell death, inflammation or oxidative stress, yet it does affect production of protein adducts as well as proliferating cell nuclear antigen steady-state protein levels. This outcome is not in line with previous studies, which are contradicting on their own, as both amplification and alleviation of this toxicological process by connexin32 have been described. This could question the suitability of the currently available models and tools to investigate the role of connexin32 in acetaminophen-triggered hepatotoxicity.

Connexin and pannexin signaling in gastrointestinal and liver disease

Gap junctions, which mediate intercellular communication, are key players in digestive homeostasis. They are also frequently involved in gastrointestinal and liver pathology. This equally holds true for connexin (Cx) hemichannels, the structural precursors of gap junctions, and pannexin (Panx) channels, Cx-like proteins assembled in a hemichannel configuration. Both Cx hemichannels and Panx channels facilitate extracellular communication and drive a number of deteriorative processes, such as cell death and inflammation. Cxs, Panxs, and their channels underlie a wide spectrum of gastrointestinal and liver diseases, including gastritis and peptic ulcer disease, inflammatory intestinal conditions, acute liver failure, cholestasis, hepatitis and steatosis, liver fibrosis and cirrhosis, infectious gastrointestinal pathologies, and gastrointestinal and liver cancer. This could open promising perspectives for the characterization of new targets and biomarkers for therapeutic and diagnostic clinical purposes in the area of gastroenterology and hepatology.

Simvastatin protects Sertoli cells against cisplatin cytotoxicity through enhanced gap junction intercellular communication

Cisplatin, an important chemotherapeutic agent against testicular germ cell cancer, induces testicular toxicity on Leydig and Sertoli cells, leading to serious side-effects such as azoospermia and infertility. In a previous study, it was found that simvastatin enhanced the sensitivity of Leydig tumor cells to chemotherapeutic toxicity through the enhancement of gap junction functions. In the present study, the effect of simvastatin on the sensitivity of normal Sertoli cells to cisplatin and the role of gap junctions in such effects was investigated. The results showed that, simvastatin attenuated cisplatin toxicity only when cells exhibited high-density culture where gap junctional formation was possible. When gap junction function was decreased by the gap junction inhibitor or by siRNA targeting connexin 43, the protective effect of simvastatin to cisplatin toxicity was substantially attenuated. Simvastatin also enhanced gap junction functions between Sertoli cells. This effect was mediated by the reduction of PKC-mediated connexin phosphorylation, thereby increasing connexin 43 membrane localization. Thus, simvastatin-induced enhancement of gap junction‑mediated intercellular communication attenuated cisplatin toxicity on Sertoli cells. This result indicated that enhancement of gap junction function by simvastatin may have bilateral beneficial effects on cisplatin‑based chemotherapy, enhancing cisplatin killing on cancer while ameliorating the reproduction toxicity.

Galangin Prevents Acute Hepatorenal Toxicity in Novel Propacetamol-Induced Acetaminophen-Overdosed Mice

Acetaminophen (APAP) overdose causes severe liver and kidney damage. APAP-induced liver injury (AILI) represents the most frequent cause of drug-induced liver failure. APAP is relatively insoluble and can only be taken orally; however, its prodrug, propacetamol, is water soluble and usually injected directly. In this study, we examined the time-dependent effects of AILI after propacetamol injection in mice. After analyses of alanine aminotransferase and aspartate aminotransferase activities and liver histopathology, we demonstrated that a novel AILI mouse model can be established by single propacetamol injection. Furthermore, we compared the protective and therapeutic effects of galangin with a known liver protective extract, silymarin, and the only clinical agent for treating APAP toxicity, N-acetylcysteine (NAC), at the same dose in the model mice. We observed that galangin and silymarin were more effective than NAC for protecting against AILI. However, only NAC greatly improved both the survival time and rate consequent to a lethal dose of propacetamol. To decipher the hepatic protective mechanism(s) of galangin, galangin pretreatment significantly decreased the hepatic oxidative stress, increased hepatic glutathione level, and decreased hepatic microsomal CYP2E1 levels induced by propacetamol injection. In addition, propacetamol injection also reproduced the probability of APAP-induced kidney injury (AIKI), appearing similar to a clinical APAP overdose. Only galangin pretreatment showed the protective effect of AIKI. Thus, we have established a novel mouse model for AILI and AIKI using a single propacetamol injection. We also demonstrated that galangin provides significant protection against AILI and AIKI in this mouse model.

Pathways involved in acetaminophen hepatotoxicity with specific targets for inhibition/downregulation

Insights on molecular/immunological mechanisms involve in APAP hepatotoxicity and pave way for researchers/clinicians/pharma bodies to identify novel biomarkers, effective bioactive candidates and fruitful therapy against APAP hepatotoxicity.

Tetramethylpyrazine suppresses transient oxygen-glucose deprivation-induced connexin32 expression and cell apoptosis via the ERK1/2 and p38 MAPK pathway in cultured hippocampal neurons

Tetramethylpyrazine (TMP) has been widely used in China as a drug for the treatment of various diseases. Recent studies have suggested that TMP has a protective effect on ischemic neuronal damage. However, the exact mechanism is still unclear. This study aims to investigate the mechanism of TMP mediated ischemic hippocampal neurons injury induced by oxygen-glucose deprivation (OGD). The effect of TMP on hippocampal neurons viability was detected by MTT assay, LDH release assay and apoptosis rate was measured by flow cytometry. TMP significantly suppressed neuron apoptosis in a concentration-dependent manner. TMP could significantly reduce the elevated levels of connexin32 (Cx32) induced by OGD. Knockdown of Cx32 by siRNA attenuated OGD injury. Moreover, our study showed that viability was increased in siRNA-Cx32-treated-neurons, and neuron apoptosis was suppressed by activating Bcl-2 expression and inhibiting Bax expression. Over expression of Cx32 could decrease neurons viability and increase LDH release. Furthermore, OGD increased phosphorylation of ERK1/2 and p38, whose inhibitors relieved the neuron injury and Cx32 up-regulation. Taken together, TMP can reverse the OGD-induced Cx32 expression and cell apoptosis via the ERK1/2 and p38 MAPK pathways.

Synchronized necrotic death of attached hepatocytes mediated via gap junctions

Extensive studies have unveiled the intracellular molecular signaling pathways of cell death. To better understand cell death in tissues, it is important to investigate the influence of neighboring cells on the response to death stimuli. By time-lapse microscopy, we found that cells in couplets (two hepatocytes attached to each other) died independently when stimulated with anti-Fas antibody and staurosporine, whereas acetaminophen (APAP) and aryl alcohol caused synchronized cell death although its timing varied among different couplets. Synchronized death of couplets was not caused by APAP when hepatocytes were deficient in both Connexin26 and Connexin32, indicating a crucial role of gap junctions in the synchronized death process. We also demonstrated that APAP-sensitive male hepatocytes were protected by attachment to APAP-insensitive female hepatocytes, with this protection being dependent on gap junctions. These findings indicate that APAP-induced and aryl alcohol-induced necrotic death of hepatocytes is modulated by attached neighboring cells via gap junctions.