Connexin 43 enhances Bax activation via JNK activation in sunitinib-induced apoptosis in mesothelioma cells

Our plain-English summary of Connexin 43 enhances Bax activation via JNK activation in sunitinib-induced apoptosis in mesothelioma cells, by M. Uzu, H. Sato, A. Shimizu, et al (Journal of Pharmacological Sciences, 2017).

The study

• Authors: M. Uzu, H. Sato, A. Shimizu, Y. Shibata, K. Ueno, A. Hisaka
• Journal: Journal of Pharmacological Sciences
• Published: 2017;134(2):101-107
• Cited by: 21 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The constituent protein of gap junctions, connexin (Cx), interacts with various proteins via its C-terminus region, including kinases, cell-adhesion proteins, and a pro-apoptotic protein, Bax. This molecular interaction may affect expression and functioning of the interacting proteins and modulate the cellular physiology. In our previous work, Cx43 was found to interact directly with Bax and in the presence of sunitinib, lead to the Bax-mediated apoptosis in mesothelioma cells. In this study, we investigated the mechanism of how Cx43 promotes Bax-mediated apoptosis using the same cell line. Treatment with sunitinib increased the expression of the active conformation of the Bax protein, which was predominantly localized at the mitochondria, only in Cx43-transfected cells. Bax oligomerization and decrease in the mitochondrial membrane potential were also observed. The involvement of c-Jun N-terminal kinase (JNK) in the interaction of Cx43 and Bax was further examined. Treatment with sunitinib increased the expression of phosphorylated (active) form of JNK only in the Cx43-transfected cells. Phosphorylated JNK and active Bax were co-localized, and the co-localization was suppressed by the knockdown of Cx43. Moreover, JNK inhibition clearly suppressed Bax activation. In conclusion, we identified a novel Cx43-JNK-Bax axis regulating the process of apoptosis for the first time.

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In plain English

This laboratory study used a human malignant mesothelioma cell line, H28, grown in culture. The authors compared H28 cells engineered to make the protein connexin 43 (H28-T) with control-vector cells (H28-W), and treated both with the anti-cancer drug sunitinib. In H28-T cells, sunitinib increased the active form of Bax, a protein that promotes cell death, and its build-up at the mitochondria. Active Bax was not detected in H28-W cells. Reducing connexin 43 in H28-T cells lessened this effect. Mitochondrial membrane potential fell significantly in H28-T cells at the higher of two sunitinib concentrations tested, but did not change in H28-W cells at any concentration. Sunitinib raised the active form of the enzyme JNK in H28-T cells, with no change in H28-W cells, and a JNK inhibitor reduced Bax activation in a dose-dependent manner. The authors concluded that connexin 43 is involved in activating JNK and helps JNK activate Bax when sunitinib is present. They noted that the cells carried artificially added connexin 43 and that how connexin 43 activates JNK needs further study. The study did not examine asbestos.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: laboratory experiment on cultured cancer cells (cell-line study)
• Who was studied: The human malignant mesothelioma cell line H28. One version was transfected with connexin 43 (H28-T) and the other with a control vector (H28-W).
• Exposure: The anti-cancer drug sunitinib, compared with DMSO as a control, mostly given for 8 hours. Some experiments also used connexin 43 knockdown by siRNA or the JNK inhibitor SP600125. No asbestos exposure was studied.
• Main results:
  • In connexin 43-transfected H28-T cells, sunitinib caused cell shrinkage, increased active Bax, and caused active Bax to accumulate at the mitochondria. Active Bax was not detected in H28-W cells.
  • In H28-T cells, knocking down connexin 43 significantly decreased the sunitinib-induced increase in active Bax and its movement to the mitochondria.
  • A band thought to be a Bax dimer (about 35 kD) was seen only in H28-T cells, and sunitinib increased it.
  • Two sunitinib concentrations were tested (printed as 5 and 10 mM). Mitochondrial membrane potential did not change in H28-W cells at any sunitinib concentration. In H28-T cells it was significantly decreased at the higher concentration (printed as 10 mM) compared with control cells.
  • In H28-W cells, sunitinib did not change JNK phosphorylation or total JNK.
  • In H28-T cells, sunitinib increased the smaller form of total JNK and increased phosphorylation (activation) of both JNK forms.
  • In H28-T cells, phosphorylated JNK and active Bax partly co-localized. After connexin 43 knockdown, JNK phosphorylation decreased and active Bax was almost not detected.
  • The JNK inhibitor SP600125 suppressed sunitinib-induced Bax activation in a dose-dependent manner, with a slight decrease in mitochondrial localization.
• The authors' conclusion: The authors concluded that connexin 43 is involved in activating JNK and helps Bax to be activated by JNK in the presence of sunitinib.
• Limitations the authors state:
  • The cells carried connexin 43 that was added artificially (exogenously), not the cells' own connexin 43.
  • The mechanism by which connexin 43 activates JNK was not determined.
  • It is not known whether the 'active Bax' detected is the same as Bax phosphorylated at threonine167.
  • The authors' metabolic explanation for why JNK was not activated in H28-W cells rests on preliminary data and lacks direct measurement of NADPH production.

Topics

Medical terms in this study, explained in our Lexicon: Sunitinib (Sutent), Apoptosis, Mesothelioma.

Related studies

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• A phase II study of intermittent sunitinib malate as second-line therapy in progressive malignant pleural mesothelioma — A.K. Nowak, M.J. Millward, et al. (2012)
• Brief report: a phase II study of sunitinib in malignant pleural mesothelioma. the NCIC Clinical Trials Group — S.A. Laurie, A. Gupta, et al. (2011)
• The c-Jun N-terminal kinase signaling pathway mediates chrysotile asbestos-induced alveolar epithelial cell apoptosis — P. Li, T. Liu, et al. (2015)
• Pemetrexed induces apoptosis in malignant mesothelioma and lung cancer cells through activation of reactive oxygen species and inhibition of sirtuin 1 — K.E. Hwang, Y.S. Kim, et al. (2015)
• SU6668, a multiple tyrosine kinase inhibitor, inhibits progression of human malignant pleural mesothelioma in an orthotopic model — T.T. Van, M. Hanibuchi, et al. (2012)

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