Histone deacetylases (HDACs) are important histone modifying proteins that are generally involved in gene repression due to their deacetylating activity (Kuo and Allis, 1998
). Removal of acetyl groups from histones by HDACs restores the positive charge of the histones and allows for their tighter interaction with DNA, possibly preventing access of transcriptional factors to promoter binding sites. HDAC2 has been previously shown to be involved in MMP-9 inhibition by metastasis associated gene 1 (MTA-1) and HDAC1 and 3 and to play a role in suppression of MMP-9 in HeLa cells (Yan et al. 2003
Modification of HDACs by ROS and/or RNS results in reduced HDAC protein and activity levels, thus altering their function in transcriptional repression. This reduction is caused by nitrotyrosine modifications and aldehyde-adduct formation in HDACs in response to ROS or RNS. Exposure of the human macrophage cell line MonoMac6 to cigarette smoke, which itself is a potent generator of ROS and RNS, stimulates an imflammatory response resulting from a decline in HDAC1, HDAC2 and HDAC3 activity and protein, and a concomitant increase in the expression and release of proinflammatory cytokines (Yang et al. 2006
). In human alveolar cells A549, both H2
and cigarette smoke result in HDAC2 nitration and a decrease in protein levels and activity accompanied by increase in histone H4 acetylation (Moodie et al. 2004
). Ito et al
estabished that H2
significantly enhances cytokine production in BEAS-2B cells as a result of increased tyrosine nitration and decreased activity of HDAC2 (Ito et al. 2004
HDACs 1 and 2 have been shown to play an important role in cell proliferation as well as mediating repression of pro-inflammatory cytokine expression along with corticosteroids. In COPD patients, the oxidative stress caused by cigarette smoke modifies HDAC2 and renders corticosteroid treatments ineffective in treating inflammation (Moodie et al. 2004
). Moodie et a
l have demonstrated that increasing concentrations of cigarette smoke can reduce HDAC2 levels as well as activity leading to increased histone H4 acetylation. Moreover H2
as well as TNF-α treatment also decreased HDAC2 levels and activity (Moodie et al. 2004
Treatment of A549 cells with the histone deacetylase inhibitor Trichostatin-A (TSA) increased expression of IL-8, the pro-inflammatory cytokine repressed by HDAC2. In addition, TSA treatment also decreased HDAC2 levels suggesting that inactivation of HDAC2 targets it for degradation. In addition to HDAC2, HDAC1 and 3 were also shown to be oxidatively modified by tyrosine nitration, which results in an increase in IL-8 expression in an NFκB dependent manner in MonoMac-6 cells (Yang et al. 2006
). Concomitantly, upon TSA treatment, NFκB was activated to a greater extent which again suggests that HDAC inactivation decreases its protein levels.
Tyrosine nitration, in addition to inactivating an enzyme and targeting it for degradation may also prevent other tyrosine modifications such as phosphorylation that are required for optimal function (Turko and Murad,2002
). Exposure of endothelial cells to peroxynitrite was shown to decrease the amount of tyrosine phosphorylated proteins and increase nitrotyrosine-containing proteins and target those proteins for degradation (Gow et al. 1996
). However peroxynitrite mediated nitration affects signaling pathways in a biphasic manner; low levels of peroxynitrite ranging from 2 μM to 500 μM (depending on the cell type) deplete intracellular glutathione and other reducing agents leading to a more pro-oxidant environment that can inactivate protein phosphatases leading to enhanced signaling. At higher peroxynitrite concentrations tyrosine nitration predominates leading to complete inhibition of tyrosine phosphorylation (Monteiro et al. 2008
). The presence of a mitochondrial nitric oxide synthase, which could potentially produce NO (Parihar et al. 2008
;Ghafourifar and Richter,1997
;Tatoyan and Giulivi,1998
;Elfering et al. 2002
) might mediate nitration as observed for mitochondrial proteins under ischemia/reoxygenation (Koeck et al. 2004
). The amount of peroxynitrite produced in any cellular system and, therefore, the amount of tyrosine nitration depends on the flux between the two substrates, NO and superoxide, which form peroxynitrite at near diffusion rates. Increases or decreases in either of the two affect the rate of the reaction. Therefore, activities of NOS, SOD and NADPH oxidases as well as other superoxide producing processes need to be considered in a cell as a source of peroxynitrite generation.
Tyrosine nitration to date is still considered to be a dead end reaction resulting in the protein being targeted for degradation. However, there is evidence pointing towards a possible mechanism for reversing this reaction. Work done by Murad and co-workers has shown that homogenates from rat liver and spleen could modify nitrotyrosine-containing BSA (Kamisaki et al. 1998
). Incubation with the homogenates resulted in a loss of the nitro-tyrosine epitope that is recognized by a specific monoclonal antibody. This activity of the homogenates was heat labile and sensitive to proteolytic degradation and was identified as a `denitrase'. More recently an in vivo
substrate for the `denitrase' of histone H1.2 was identified in RAW 64.7 cells (Irie et al. 2003
). In addition, various heme containing proteins in the presence of cellular thiols can non-enzymatically convert nitro tyrosines into amino tyrosine (Balabanli et al. 1999
). The amino tyrosine modification can then be potentially removed by the action of nitro reductases in the cell. However, the role of nitro reductases in this process has not been clearly established. Whether either or both mechanisms exist to reverse tyrosine modifications still needs to be extensively characterized.
Overall nitration of HDAC's may result in their ubiquitination and degradation by the proteosome. depicts a proposed model for how oxidants might impact the MMP-1 chromatin remodeling complex. Oxidants promote an increase in the recruitment of specific transcription factors (Ets-1 and AP-1) and histone modifying proteins (P/CAF) to the MMP-1 promoter. Nitration and subsequent degradation of HDAC2 allows for a more accessible chromatin. In addition, oxidants promote increases in JNK and ERK signaling which upregulate Ets-1, c-Jun and c-Fos expression in and play a critical role in driving high level MMP-1 transcription.
Fig. 4 Schematic representation of the role of oxidants in modulating MMP-1 transcription by redox-sensitive chromatin remodeling proteins. Redox-shifts potentially lead to increases in HDAC2 nitration leading to its degradation. Loss of deacetylase activity (more ...)