Novel role of c-jun N-terminal kinase in regulating the initiation of cap-dependent translation

Our plain-English summary of Novel role of c-jun N-terminal kinase in regulating the initiation of cap-dependent translation, by M.R. Patel, A.A. Sadiq, J. Jay-Dixon, et al (International Journal of Oncology, 2012).

The study

• Authors: M.R. Patel, A.A. Sadiq, J. Jay-Dixon, T. Jirakulaporn, B.A. Jacobson, F. Farassati, P.B. Bitterman, R.A. Kratzke
• Journal: International Journal of Oncology
• Published: 2012;40(2):577-82
• Cited by: 11 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Initiation of protein translation by the 5' mRNA cap is a tightly regulated step in cell growth and proliferation. Aberrant activation of cap-dependent translation is a hallmark of many cancers including non-small cell lung cancer. The canonical signaling mechanisms leading to translation initiation include activation of the Akt/mTOR pathway in response to the presence of nutrients and growth factors. We have previously observed that inhibition of c-jun N-terminal kinase (JNK) leads to inactivation of cap-dependent translation in mesothelioma cells. Since JNK is involved in the genesis of non-small cell lung cancer (NSCLC), we hypothesized that JNK could also be involved in activating cap-dependent translation in NSCLC cells and could represent an alternative pathway regulating translation. In a series of NSCLC cell lines, inhibition of JNK using SP600125 resulted in inhibition of 4E-BP1 phosphorylation and a decrease in formation of the cap-dependent translation complex, eIF4F. Furthermore, we show that JNK-mediated inhibition of translation is independent of mTOR. Our data provide evidence that JNK is involved in the regulation of translation and has potential as a therapeutic target in NSCLC.

In plain English

This laboratory study examined how a signaling enzyme called c-jun N-terminal kinase (JNK) affects the first step of protein production (cap-dependent translation) in non-small cell lung cancer (NSCLC) cells grown in the laboratory. It did not study asbestos, people, or mesothelioma. The authors mention only their own earlier work in mesothelioma cells. JNK activity was at least 1.5-fold higher in 4 of 7 NSCLC cell lines than in normal human bronchial epithelial cells. In those 4 lines, the JNK-blocking drug SP600125 reduced phosphorylation of a protein called 4E-BP1, and the reduction grew with the dose. The drug also increased 4E-BP1 binding to eIF4E and decreased eIF4G binding, which the authors interpreted as repression of the translation-starting complex. Cell growth fell as the dose rose. H522 cells were the most sensitive (IC50 of about 5 µM) and H520 cells the least (IC50 above 25 µM). The authors found no direct binding between JNK and 4E-BP1, and they reported that the effect was independent of mTOR. They concluded that JNK acts, at least in part, to regulate the initiation of cap-dependent translation in NSCLC. They stated that the mechanism remains unclear.

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 (in vitro) study of human cancer cell lines
• Who was studied: Seven non-small cell lung cancer cell lines (H2009, H522, H520, H460, H1299, H2030, H661), with normal human bronchial epithelial cells (NHBE) as the comparison. Later experiments used the 4 lines with increased JNK activity. No human subjects or asbestos-exposed people were studied.
• Exposure: The JNK inhibitor SP600125 at various concentrations (for example 10 and 25 µM), and the mTOR inhibitor rapamycin, alone and in combination. No asbestos or fiber exposure was studied.
• Main results:
  • JNK activity was at least 1.5-fold greater in 4 of 7 NSCLC cell lines than in normal bronchial epithelial cells.
  • In the 4 NSCLC lines with increased JNK activity, SP600125 decreased phosphorylation of 4E-BP1 in a dose-dependent manner.
  • In the cap-binding assay, JNK inhibition increased 4E-BP1 binding to eIF4E and decreased eIF4G binding in all cell lines tested.
  • Proliferation decreased with dose in each of the 4 cell lines. H522 cells were the most sensitive (IC50 about 5 µM) and H520 cells the least sensitive (IC50 above 25 µM).
  • PARP cleavage, a marker of caspase activation, increased in H2009 cells only at the 25 µM dose, and increased with dose in H522 cells.
  • Co-immunoprecipitation did not detect direct binding between JNK and 4E-BP1.
  • JNK inhibition increased, rather than decreased, phosphorylation of p70S6K, an mTOR target. The authors took this to mean the effect on 4E-BP1 did not go through mTOR signaling.
  • Combining rapamycin with SP600125 reduced 4E-BP1 phosphorylation more than rapamycin alone in H2009 cells, but not in H522 cells.
• The authors' conclusion: The authors concluded that JNK activation in NSCLC acts, at least in part, to regulate the initiation of cap-dependent translation, and that JNK-mediated phosphorylation of 4E-BP1 may be a mechanism of resistance to mTOR inhibition.
• Limitations the authors state:
  • The kinase assay result could reflect phosphorylation of c-jun by a different kinase, which the authors called speculative.
  • How JNK regulates 4E-BP1 phosphorylation remains unclear, because no direct interaction was found.
  • SP600125 has been reported to affect other kinases, notably p70S6K. The authors argued this effect is not likely relevant to their experiments.
  • Because the Akt result differed between cell lines, the authors could not conclude that SP600125's effects work by removing the rapamycin feedback mechanism.
  • The idea that JNK-mediated 4E-BP1 phosphorylation causes rapamycin resistance is speculative and needs further study.

Topics

Medical terms in this study, explained in our Lexicon: Non-Small Cell Lung Cancer, Mesothelioma.

Related studies

• 4EGI-1 represses cap-dependent translation and regulates genome-wide translation in malignant pleural mesothelioma — A. De, B.A. Jacobson, et al. (2018)
• Sensitivity of global translation to mTOR inhibition in REN cells depends on the equilibrium between eIF4E and 4E-BP1 — S. Grosso, E. Pesce, et al. (2011)
• AKT/mTOR and c-Jun N-terminal kinase signaling pathways are required for chrysotile asbestos-induced autophagy — Z. Lin, T. Liu, et al. (2014)
• Combined treatment with cisplatin and sirolimus to enhance cell death in human mesothelioma — M.L. Hartman, J.M. Esposito, et al. (2010)
• The c-Jun N-terminal kinase signaling pathway mediates chrysotile asbestos-induced alveolar epithelial cell apoptosis — P. Li, T. Liu, et al. (2015)
• Small-molecule inhibition of oncogenic eukaryotic protein translation in mesothelioma cells — E.Z. Chen, B.A. Jacobson, et al. (2014)

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