AP was originally described as
AP-1 was originally described as a heterodimer of the bZIP proteins c-Jun and c-Fos. The experiments described here showed that stimulation of TRPV1 channels induced the expression of both of these classical constituents of AP-1. The essential role of both proteins was analyzed in a series of experiments. Based on expression experiments of a dominant-negative mutant of c-Jun, we identified c-Jun as an important regulator of AP-1 activity in capsaicin-treated H2C1 cells. Likewise, capsaicin-induced activation of AP-1 was attenuated in H2C1 cells following expression of a dominant-negative mutant of Elk-1. By this means serum response element-mediated stimulation of c-Fos expression was attenuated. These data confirm previous results showing that the TCF proteins are important regulators of AP-1 , , , , . We conclude, c-Jun and TCFs are regulators of AP-1 that are required to connect TRPV1 stimulation with the activation of AP-1.
In summary, we present here the first study showing that stimulation of TRPV1 channels with natural or plant-derived ligands activates the transcription factor AP-1. These results indicate that TRPV1 controls cellular functions via regulating gene transcription of AP-1-regulated genes. Mutational analysis identified the AP-1 DNA binding site as capsaicin-responsive element. The analysis of the signaling pathway revealed that TRPV1 ligands induced a rise in cytoplasmic Ca2+ and this Ca2+ influx was essential to transduce the signal generated by activation of TRPV1 to the cytoplasm. ERK1/2 was identified as signal transducer within the signaling cascade connecting TRPV1 channels with AP-1, while in the nucleus, the transcription factors c-Jun and TCF were shown to be required for TRPV1-triggered activation of AP-1. In sensory neurons, stimulation of TRPV1 channels has been shown to activate Medroxyprogesterone acetate gene-related peptide expression involving the transcription factor cAMP response element binding protein (CREB) . Future work has to include the regulation of AP-1 by TRPV1 stimulation in these cells, in order to identify tissue-specific delayed response genes of AP-1.
Acknowledgements We thank Libby Guethlein for critical reading of the manuscript. This work was supported by the Saarland University (LOM-T201000492) and by a grant from Deutsche Forschungsgemeinschaft to CG (GR1829/1-1).
Introduction Inflammation is a response to tissue injury or infection, and it is characterized in its acute phase by increase in vascular permeability by plasma extravasations, resulting in an accumulation of fluid, leucocytes and mediators to the inflammatory site (Guo et al., 2012). Inflammation attracts immune cells and induces the local production of several cytokines (e.g. TNF-α) and chemokines (e.g. IL-8) (Rickard and Young, 2009). Expression and over production of pro-inflammatory cytokines promote systemic inflammation, leading to the development of many inflammatory diseases, such as asthma, rheumatoid arthritis and bowel diseases (Choy, 2012). For the past half a century, synthetic glucocorticoids (GCs) have been extensively used to treat chronic inflammatory diseases (Barnes, 2006, Hillier, 2007). Glucocorticoids are among the most widely prescribed and most effective anti-inflammatory medications that are currently available. The anti-inflammatory effect of glucocorticoids are due to their ability to repress the inflammatory gene expression that occurs in all kinds of inflammatory contexts. By reducing the expression of cytokines, chemokines, adhesion molecules and other inflammatory proteins, glucocorticoids prevent the recruitment of inflammatory cells to sites of inflammation (Newton et al., 2010). Furthermore, glucocorticoids promote the death of many inflammatory cells and this may further contribute to reducing the inflammatory cell burden. Glucocorticoids inhibit the expression of inflammatory mediators in macrophages and other cells and are used in the treatment of many immune-mediated inflammatory diseases (Newton, 2000). Immunosuppressive properties and their potent ability to reduce inflammation make synthetic glucocorticoids (GCs) the most prescribed drugs used in the treatment of disorders, such as asthma, arthritis or dermatitis. GCs are also used to treat patients suffering from a wide range of hematological and non-hematological cancers either because of their inhibiting effects on cell cycle progression and apoptosis promotion or for their beneficial properties, e.g. decreasing oedema, pain, nausea and reducing toxicity of the standard chemotherapy regimens in healthy tissues (Rutz, 2002, Rutz and Herr, 2004).