Morphine Induces Redox-Based Changes in Global DNA Methylation and Retrotransposon Transcription by Inhibition of Excitatory Amino Acid Transporter Type 3–Mediated Cysteine Uptake

Abstract

Canonically, opioids influence cells by binding to a G proteincoupled opioid receptor, initiating intracellular signaling cascades, such as protein kinase, phosphatidylinositol 3-kinase, and extracellular receptor kinase pathways. This results in several downstream effects, including decreased levels of the reduced form of glutathione (GSH) and elevated oxidative stress, as well as epigenetic changes, especially in retrotransposons and heterochromatin, although the mechanism and consequences of these actions are unclear. We characterized the acute and long-term influence of morphine on redox and methylation status (including DNA methylation levels) in cultured neuronal SH-SY5Y cells. Acting via m-opioid receptors, morphine inhibits excitatory amino acid transporter type 3-mediated cysteine uptake via multiple signaling pathways, involving different G proteins and protein kinases in a temporal manner. Decreased cysteine uptake was associated with decreases in both the redox and methylation status of neuronal cells, as defined by the ratios of GSH to oxidized forms of glutathione and S-adenosylmethionine to S-adenosylhomocysteine levels, respectively. Further, morphine induced global DNA methylation changes, including CpG sites in long interspersed nuclear elements (LINE-1) retrotransposons, resulting in increased LINE-1 mRNA. Together, these findings illuminate the mechanism by which morphine, and potentially other opioids, can influence neuronal-cell redox and methylation status including DNA methylation. Since epigenetic changes are implicated in drug addiction and tolerance phenomenon, this study could potentially extrapolate to elucidate a novel mechanism of action for other drugs of abuse.

Food-derived opioid peptides inhibit cysteine uptake with redox and epigenetic consequences

“…While the mechanism underlying inhibition of cysteine uptake by food-derived opiate peptides is not fully established, we previously found that protein kinase A (PKA) and/or Mitogen activated protein kinase kinase (MEKK) might be involved in mediating the effects of morphine on EAAT3 (50). The inhibitory effects on cysteine uptake are also observed at the levels of redox status and methylation capacity as well as at the DNA methylation status and the transcription levels of genes involved in maintaining the redox methylation homeostasis.…”

Multigenerational and transgenerational inheritance of drug exposure: The effects of alcohol, opiates, cocaine, marijuana, and nicotine

“…In addition to methylation and acetylation modifications of histones, morphine exposure induces methylation changes on DNA directly (Nielsen et al, 2008; Chorbov et al, 2011; Doehring et al, 2013; Trivedi et al, 2014). Morphine modulates cellular oxidative stress and methyl group donation through the antioxidant glutathione and the methyl donor SAM, respectively (Trivedi et al, 2014).…”

“…Morphine modulates cellular oxidative stress and methyl group donation through the antioxidant glutathione and the methyl donor SAM, respectively (Trivedi et al, 2014). Consequently, as the redox state of the cell impacts methylation events, DNA methylation changes can occur following morphine exposure.…”

“…Just as LINE-1 seems to be vulnerable to histone methylation changes following morphine exposure, the transposable element exhibits decreases in methylation after morphine administration. Hypomethylation at LINE-1 retrotransposons has been identified in leukocytes of chronic heroin users (Doehring et al, 2013) and in neuronal cell lines (Trivedi et al, 2014); consequently, LINE-1 mRNA expression increases (Trivedi et al, 2014). In contrast, increased CpG methylation has been reported in lymphocytes, blood, and sperm of chronic heroin users in the promoter region of the OPRM1 gene (Chorbov et al, 2011; Nielsen et al, 2008).…”

Binding of LINE-1 RNA to PSF transcriptionally promotes GAGE6 and regulates cell proliferation and tumor formation in vitro

“…It was observed that increasing the concentration of L1 RNA increased its inhibitory effect on PSF/DNA binding affinity. L1 RNA is transcribed from endogenous genomic retrotransposons and the level varies depending on the cell type and the level of global hypomethylation (25). Thus, the key parameters that determine whether GAGE6 expression is repressed by PSF or induced by L1 RNA are the type of cell and the global hypomethylation status.…”