Proliferation of C6 glioma cells is blunted by the increase in gap junction communication caused by tolbutamide

Complex role of connexin 43 in astrocytic tumors and possible promotion of glioma‑associated epileptic discharge (Review)

Connexin (Cx)43 is a multifunction protein which forms gap junction channels and hemi‑channels. It also contains abundant binding domains which possess the ability to interact with certain Cx43‑associated proteins and therefore serve a fundamental role in various physiological and pathological functions. However, the understanding of the association between cancer and Cx43 along with Cx43‑gap junctions (GJ) remains unclear. All available data illustrate that Cx43 and its associated GJ serve important functions in cancers. The expression levels of Cx43 demonstrate a downward trend and an increase in the levels of malignancy, particularly in astrocytomas. The GJ intercellular communication activity in glioma cells can be adjusted via Cx43 phosphorylation and through the combination of Cx43 and its associated protein. Available evidence reveals Cx43 as a tumor‑inhibiting factor that suppresses glioma growth and proliferation. However, its mechanism is also regarded as complicated and ambiguous. Furthermore, it is apparent that Cx43‑GJ and the carboxyl tail may contribute to glioma growth and proliferation too. However, this valuable role could be weakened by its effects on migration and invasiveness. The detailed mechanism remains unclear and full of controversies. Cx43 can enhance the motor ability and invasiveness of astrocytic glioma cells. It is also able to influence glioma cells to detach from the tumor core to the peritumoral neocortex. This peritumoral region has recently been regarded as the basic focus of glioma‑associated seizure. Thus, Cx43 may take part in the onset and development of glioma‑associated epileptic discharge. In addition, change and increase of Cx43 expression in GJs has been observed in seizure perilesional tissue, which is associated with brain tumors. Cx43 or GJ/hemi‑channels exert enduring effects in the promotion of glioma‑associated epileptic release through direct mass effects and change of the tumor microenvironment. However, there are still a number of issues concerning this aspect that require further exploration. Cx43, as a potential treatment target against this incurable disease and its common symptom of epilepsy, requires further investigation.

Subsets of ATP-sensitive potassium channel (KATP) inhibitors increase gap junctional intercellular communication in metastatic cancer cell lines independent of SUR expression

Gap junctional intercellular communication (GJIC) regulates cellular homeostasis by propagating signaling molecules, exchanging cellular metabolites, and coupling electrical signals. In cancer, cells exhibit altered rates of GJIC which may play a role in neoplastic progression. K(ATP) channels help maintain membrane polarity and linkages between K(ATP) channel activity and rates of GJIC have been established. The mechanistic relationship has not been fully elucidated. We report the effects of treatment with multiple K(ATP) antagonist compounds on GJIC in metastatic cell lines demonstrating an increase in communication rates following treatment with compounds possessing specificities towards the SUR2 subunit of K(ATP). These effects remained consistent using cell lines with different expression levels of SUR1 and SUR2, suggesting possible off target effects on GJIC by these compounds.

Inhibition of ATP-sensitive potassium channels increases HSV-tk/GCV bystander effect in U373 human glioma cells by enhancing gap junctional intercellular communication

It is well known that the efficiency of Herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) therapy is improved by the bystander effect, which mainly relies on gap junctional intercellular communication (GJIC). Malignant gliomas communicate poorly through gap junctions, consequently, agents with the ability to increase GJIC are good candidates to improve the efficiency of this therapy. Since we previously showed that the inhibition of ATP-sensitive potassium (KATP) channels promoted by tolbutamide increased GJIC in rat C6 glioma cells, we have investigated whether tolbutamide could increase the bystander effect in HSV-tk/GCV therapy against human glioma cells. We found that tolbutamide increased GJIC in U373 human glioma cells, an effect that was due to the up-regulation of connexin43, a protein that forms gap junctions channels. More interestingly, our results show that tolbutamide increased the efficiency of HSV-tk/GCV in co-cultures containing U373 cells and U373 cells transfected with HSV-tk. This effect was impaired in the presence of carbenoxolone, an inhibitor of GJIC. Furthermore, tolbutamide did not enhance the bystander effect in connexin43-silenced co-cultures. Together our results reveal that the inhibition of KATP channels promoted by tolbutamide enhances the bystander effect in HSV-tk/GCV therapy by increasing connexin43-mediated gap junctional intercellular communication in U373 human glioma cells.

Downregulation of connexin 43 in nasopharyngeal carcinoma cells is related to promoter methylation

Down-regulation of Cx43 expression had been shown to occur in nasopharyngeal carcinoma cells. The present study was undertaken to estimate if methylation of the promoter region in Cx43 gene was responsible for the repression of Cx43 expression in the CNE-1 nasopharyngeal carcinoma cells. Calcein transfer and lucifer yellow transfer were detected to evaluate gap junction intercellular communication (GJIC) in CNE-1 cells. It was found that the control CNE-1 cells showed no fluorescent dye transfer. After treatment with DNA methyltransferase inhibitor 5-aza-CdR, fluorescent dye transfer between cells became obvious. RT-PCR and Western blot were performed to determine the expression of Cx43 gene. The control CNE-1 cells showed a low expression level of Cx43, whereas 5-aza-CdR-treated CNE-1 cells showed an enhanced level of Cx43 expression. Methylation-sensitive restriction enzyme and PCR analysis showed that the methylation of the Cx43 gene promoter region occurred in CNE-1 cells. In addition, treatment with 5-aza-CdR inhibited the growth (including anchorage-independent growth) of CNE-1 cells, and resulted in an accumulation of cells in G0/G1 phase. These results indicate the promoter methylation as an important role in inactivation of Cx43 in CNE-1 cells.

Glucose metabolism and proliferation in glia: role of astrocytic gap junctions

Astrocytes play a well-established role in brain metabolism, being a key element in the capture of energetic compounds from the circulation and in their delivery to active neurons. Their metabolic status is affected in many pathological situations, such as gliomas, which are the most common brain tumors. This proliferative dysfunction is associated with changes in gap junctional communication, a property strongly developed in normal astrocytes studied both in vitro and in vivo. Here, we summarize and discuss the findings that have lead to the identification of a link between gap junctions, glucose uptake, and proliferation. Indeed, the inhibition of gap junctional communication is associated with an increase in glucose uptake due to a rapid change in the localization of both GLUT-1 and type I hexokinase. This effect persists due to the up-regulation of GLUT-1 and type I hexokinase and to the induction of GLUT-3 and type II hexokinase. In addition, cyclins D1 and D3 have been found to act as sensors of the inhibition of gap junctions and have been proposed to play the role of mediators in the mitogenic effect observed. Conversely, in C6 glioma cells, characterized by a low level of intercellular communication, an increase in gap junctional communication reduces glucose uptake by releasing type I and type II hexokinases from the mitochondria and decreases the exacerbated rate of proliferation due to the up-regulation of the Cdk inhibitors p21 and p27. Identification of the molecular actors involved in these pathways should allow the determination of potential therapeutic targets that could lead to the testing of alternative strategies to prevent, or at least slow down, the proliferation of glioma cells.

Tolbutamide reduces glioma cell proliferation by increasing connexin43, which promotes the up-regulation of p21 and p27 and subsequent changes in retinoblastoma phosphorylation

Our previous work has shown that tolbutamide increases gap junctional permeability in poorly coupled C6 glioma cells and that this effect is similar and additive to that found with dbcAMP, a well-known activator of gap junctional communication. Furthermore, the increase in gap junctional communication promoted by tolbutamide or dbcAMP is concurrent with the inhibition of proliferation of C6 glioma cells. In the present work, we show that tolbutamide and dbcAMP increase the synthesis of the tumor suppressor protein Cx43 and that they decrease the level of Ki-67, a protein expressed when cells are proliferating. These effects were accompanied by a reduction in the phosphorylation of pRb, mainly on Ser-795, a residue critical for the control of cell proliferation. The decrease in the phosphorylation of pRb is not likely to be mediated by a reduction in the levels of D-type cyclins, since instead of decreasing the expression of cyclins, D1 and D3 increased slightly after treatment with tolbutamide or dbcAMP. However, the Cdk inhibitors p21 and p27 were up-regulated after treatment with tolbutamide and dbcAMP, suggesting that they would be involved in the decrease in pRb phosphorylation. When Cx43 was silenced by siRNA, neither tolbutamide nor dbcAMP were able to up-regulate p21 and consequently to reduce glioma cell proliferation, as judged by Ki-67 expression. In conclusion, tolbutamide and dbcAMP inhibit C6-glioma cell proliferation by increasing Cx43, which correlates with a reduction in pRb phosphorylation due to the up-regulation of the Cdk inhibitors p21 and p27.

Increased levels of cyclins D1 and D3 after inhibition of gap junctional communication in astrocytes

We showed previously that the inhibition of gap junctional communication in astrocytes increased bromodeoxyuridine (BrdU) incorporation and promoted changes in the metabolic phenotype destined to fulfil the requirements of cell proliferation. In the present study we investigated the changes in the cell cycle of astrocytes promoted by the inhibition of intercellular communication through gap junctions. Thus, the presence of endothelin-1 and carbenoxolone, two gap junction uncouplers, promoted an increase in the percentage of astrocytes found in the S, G2 and M phases of the cell cycle, with a concomitant decrease in G0 and G1 phases. In addition, the levels of Ki-67, a protein present during all active phases of the cell cycle but absent from resting cells, increased after the inhibition of gap junctional communication. These effects were not observed when the inhibition of gap junctions was prevented with tolbutamide, indicating that the inhibition of gap junctional communication promotes the entry of astrocytes into the cell cycle. The passage of the cells from a quiescent state to the cell cycle is ultimately regulated by the degree of retinoblastoma phosphorylation. Inhibition of gap junctions increased the phosphorylation of retinoblastoma at Ser 780 but not at Ser 795 or Ser 807/811. In addition, the levels of cyclins D1 and D3 increased, whereas those of p21 and p27 were not significantly modified. Because D-type cyclins are key regulators of retinoblastoma protein phosphorylation, it is suggested that the phosphorylation of retinoblastoma protein at Ser 780, observed under our experimental conditions, is a consequence of the increase in the levels of cyclins D1 and D3. Our work provides evidence for the involvement of cyclins D1 and D3 as sensors of the inhibition of gap junctional communication in astrocytes.

Kinetics of endothelin-induced inhibition and glucose permeability of astrocyte gap junctions

Gap junctions contribute to important functions of communicating glial cells in brain physiology and pathology. Endothelins (ETs), a vasoactive family of peptides present in the brain, have been described as potent inhibitors of astrocyte gap junctional communication. Through dye-coupling studies we demonstrate here that this inhibition occurs rapidly and then successively reverses and returns to control levels after 90 min of continuous ET1 or ET3 exposure. In addition, long-term exposure of cells to ET3, which acts mainly on ETB receptors, also desensitized the acute action of ET1, which was previously shown to act through either ETA or ETB receptor sites, or both. The gap junction blocker carbenoxolone did not show any time-dependent desensitization and was fully effective also in cultures treated with ETs for prolonged times. The ETs inhibitory effects were partially prevented when blocking pertussis toxin-sensitive G-proteins, chelating intracellular Ca2+, or omitting extracellular Ca2+. We further show that ETs modulate gap junction-mediated transfer of 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-Y1)amino]-2-deoxyglucose (2-NBDG), a fluorescent glucose molecule, indicating a role of astrocyte gap junction coupling in metabolic trafficking and suggesting the importance of these peptides in the control of intercellular diffusion of energetic compounds. These findings might have particular relevance in early tissue reactions after various cerebral injuries, which commonly involve increased cerebral ET levels.

Tellimagrandin I enhances gap junctional communication and attenuates the tumor phenotype of human cervical carcinoma HeLa cells in vitro

Tellimagrandin I and chebulinic acid, two hydrolysable tannins, have been shown to exert anti-tumor properties. Dysfunctional gap junctional communication (GJIC) has been recognized as being involved in carcinogenesis. The human cervical carcinoma HeLa cells have been reported to be deficient in functional GJIC. In present study, we investigated whether tellimagrandin I and chebulinic acid might restore functional GJIC in HeLa cells. Both compounds could inhibit the growth of HeLa cells. Either Lucifer yellow transfer assay or calcein transfer assay demonstrated that tellimagrandin I improved GJIC in HeLa cells while chebulinic acid showed no effect on GJIC. The GJIC enhancement by tellimagrandin I occurred along with an increase of Cx43 gene expression at mRNA and protein levels. Exposure to tellimagrandin I also led to inhibition of proliferation and anchorage-independent growth of HeLa cells. In addition, tellimagrandin I decreased the percentage of cells in the G0/G1 and G2/M phases coinciding with an increase in the percentage of cells in the S phase. The accumulation of cells in S phase was coupled with a decreased expression of cyclin A that was critical to the progression of S phase. These results suggested that restoring GJIC might be one explanation for tellimagrandin I antitumor effects, whereas chebulinic acid exerted antitumor action through other pathways.

The increase in gap junctional communication decreases the rate of glucose uptake in C6 glioma cells by releasing hexokinase from mitochondria

We have previously shown that the enhancement of glucose uptake caused by the inhibition of gap junctional communication is a consequence of the increase in astrocyte proliferation. Since C6 glioma cells are highly proliferative and are poorly coupled through gap junctions, we used these cells to investigate the effect of increasing gap junctional communication on the rate of glucose uptake. Previous work by us had shown that tolbutamide increases gap junctional communication in C6 glioma cells, as does dbcAMP, a classical activator of gap junctional communication. In this work, our results show that both tolbutamide and dbcAMP reduce the rate of glucose uptake in C6 glioma cells and that their effects are additive. The main glucose transporters expressed in C6 glioma cells are GLUT-1 and GLUT-3. Neither the expression nor the cellular localization of either GLUT-1 or GLUT-3 were modified by increasing gap junctional communication. The estimation of glucose uptake with 2-deoxyglucose includes not only glucose transport but also glucose phosphorylation, which in C6 glioma cells is mainly catalyzed by type I and type II hexokinase. Our results reveal that the increase in gap junctional communication caused by tolbutamide and dbcAMP is associated with a decrease in the activity of hexokinase. In agreement with this, tolbutamide and dbcAMP caused a rapid change in the localization of both type I and type II hexokinase, which were detached from the mitochondria to the cytosol.

Cellular sublocalization of Cx43 and the establishment of functional coupling in IMR-32 neuroblastoma cells

Neuroblastoma (NB) is the most common solid pediatric tumor. IMR-32 cells are a highly malignant human NB cell line with uncontrolled proliferation but with the potential to be differentiated under specific conditions. Preliminary research indicated that connexin 43 (Cx43), the most widespread of the Cx family, is aberrantly located in IMR-32 cells, which renders these cells incapable of gap junction (GJ) intercellular communication. Functioning GJ intercellular communication has been strongly associated with growth control and a decrease in tumorigenicity. 8-br-cAMP, known to initiate the differentiation process in cancer cells, was used to examine changes in Cx43 localization and expression via immunocytochemistry, Western blot analysis, and flow cytometry. Exposure of IMR-32 cells to 8-br-cAMP decreased cell proliferation, restored the abnormally localized Cx43 from around the nucleus to the cell membrane, increased de novo Cx43 protein expression, and appeared to phosphorylate Cx43 on serine (Ser) 255 and Ser262. Forskolin, an activator of cAMP dependent protein kinase (PKA), produced identical results to 8-br-cAMP demonstrating the effect that was not unique to a cAMP analog. The use of a PKA inhibitor further confirmed the specificity of 8-br-cAMP and forskolin's effect on Cx43. The cellular relocation of Cx43, combined with the increased protein expression, established first ever GJ intercellular communication between IMR-32 cells as revealed by scrape loading. These results suggest that the GJ-mediated return of growth control, as a prerequisite for further differentiation, offers a new therapeutic avenue in the treatment of NB.

Endothelin-1 stimulates the translocation and upregulation of both glucose transporter and hexokinase in astrocytes: relationship with gap junctional communication

We have previously shown that endothelin-1 increases glucose uptake in astrocytes. In the present work we investigate the mechanism through which endothelin-1 (ET-1) increases glucose uptake. Our results show that ET-1 activates a short-term and a long-term mechanism. Thus, ET-1 induced a rapid change in the localization of both GLUT-1 and type I hexokinase. These changes are probably aimed at rapidly increasing the entry and phosphorylation of glucose. In addition, ET-1 upregulated GLUT-1 and type I hexokinase and induced the expression of isoforms not normally expressed in astrocytes, such as GLUT-3 and type II hexokinase. These changes provide astrocytes with the machinery required to sustain a high rate of glucose uptake for a longer period of time. Our previous work had suggested that the effect of ET-1 on glucose uptake was associated with the inhibition of gap junctions. In this work, we compare the effect of ET-1 with that of carbenoxolone, a classical inhibitor of gap junction communication. Carbenoxolone increased glucose uptake to the same extent as ET-1 following the same mechanisms. Thus, carbenoxolone induced a rapid change in the localization of both GLUT-1 and type I hexokinase, upregulated GLUT-1 and type I hexokinase and induced the expression of GLUT-3 and type II hexokinase. When the inhibition of gap junction was prevented by tolbutamide, neither ET-1 nor carbenoxolone were able to increase the levels of GLUT-1, GLUT-3, type I hexokinase or type II hexokinase, indicating that these events are closely related to gap junctions.

Effect of endothelin-1 on astrocytic protein content

The astrocytic endothelin (ET) receptors, ET(A) and ET(B), modulate calcium signaling and the astrocytic gap junctional network. The nonselective ET receptor ligand ET-1 inhibits gap junction permeability, an effect that can be blocked by tolbutamide. This mechanism may play a role in pathophysiological conditions such as ischemic stroke, characterized by elevated tissue ET-1 levels and hypertrophic-appearing reactive astrocytes. Therefore, the effect of ET-1 on cellular protein content was investigated in confluent once-passaged rat astrocyte cultures under serum-free conditions, by the Lowry method. Gap junction permeability was determined by the dye transfer technique. ET-1 prevented the decrease in astrocytic protein content observed in controls. The effect of ET-1 on cellular protein content was most pronounced in cultures seeded at high density, but it was attenuated in ET(B)-deficient (sl/sl) astrocytes. This effect could be blocked by the nonselective ET antagonist LU 302872 (10 micro M), as well as by the protein synthesis inhibitor cycloheximide (10 micro M). This increase in astrocytic protein content was inhibited by the ATP-sensitive K(+) channel blocker tolbutamide, which also antagonized the ET-1-induced reduction of gap junction permeability and reversed the morphological changes observed in astrocytes upon ET-1 treatment. Cytosine arabinoside (10 micro M), a DNA synthesis blocker, inhibited the ET-1-induced BrdU uptake without affecting the ET-1-induced increase in astrocytic protein content. To conclude, ET-1 induces an increase in astrocytic protein content as well as changes in astrocyte morphology in vitro. This hypertrophic response involves uncoupling of the astrocytic gap junctional network and is not dependent on DNA synthesis.