Introduction
Methamphetamine (Meth) is an illicit drug abused worldwide, posing major public health challenges (1, 2). Meth use increases the spread and replication of Human Immunodeficiency Virus (HIV-1) by fostering risky sexual behaviors, and by facilitating viral infection and rapid progression to AIDS (3–7). Furthermore, Meth abuse is associated with poor adherence to anti-retroviral therapies (8, 9). A clearer understanding of the mechanisms that mediate the molecular effects of Meth on immune defenses should enhance the design of therapies targeted to patients who abuse the drug.
Distinct from HIV-1 infection, Meth has also been shown to alter the expression of inflammatory cytokines in several murine tissues, in the serum of self-administering rats, and in the plasma of human subjects in early recovery from addiction (10–12). Specifically, Meth increased Interleukin-1β (IL-1β) mRNA and protein expression in dendritic cells, and in the rat hypothalamus (13, 14). Chronic increased expression of IL-1β can result in deleterious over-stimulation of the innate immune response, and tissue damage (15). Fine-tuning the innate immune response is crucial for effective immunity. Type I Interferons (IFNs) have been shown to participate in a counter-regulatory antagonistic relationship with IL-1β to maintain the balance necessary for innate immunity (15). Interestingly, Meth suppressed expression of IFNα, a type I IFN, in macrophages (4). Notably, increased IL-1β expression and down regulation of IFN stimulated genes, including TRAF6, are associated with enhanced HIV-1 infection and replication (4, 16, 17). MicroRNAs (miRNAs) participate in transcriptional and translational regulation, cellular homeostasis and feedback regulation, and can serve as indicators of disease states (18–21). For example, miRNA signatures have been established as markers for different cancers, and can act either as oncogenes or tumor suppressors (22). miRNAs contain a highly conserved sequence at their 5′ end, known as a seed region, which binds a target sequence to cause translational repression and mRNA decay (23). Targeted transcripts are highly sensitive to changes in miRNA expression (23–25). In particular, miR-146a has garnered attention recently for its roles in immune regulation (19, 26–28). miR-146a can target TNF Receptor-Associated Factor 6 (TRAF6) and Interleukin-1 Receptor-Associated Kinase 1 (IRAK1) to suppress innate immune responses via negative feedback regulation (19). TRAF6 and IRAK1 serve as fundamental molecules for effective signal transduction resulting from innate immune stimuli (29). Several studies indicate a role for IL-1β in the induction of miR-146a (30, 31). Furthermore, miR-146a plays an important role in T-cell homeostasis, and overexpression of miR-146a in mice results in an autoimmune-like T-cell profile in the periphery (32). These functions of miR-146a merit its exploration it as a target of Meth in HIV-1 pathobiology.
To date, very little is known about the effects of Meth on miRNA expression outside of the central nervous system (CNS). We found that in CD4+ T-cells, miR-146a is up-regulated by Meth exposure in an IL-1β dependent manner. Upon Meth treatment, there was altered expression of miR-146a targets, specifically TRAF6. Furthermore, we observed that Meth activated an IL-1β positive auto-regulatory loop, which resulted in enhanced HIV-1 replication. To our knowledge, this is the first report that Meth induces an IL-1β feedback loop linked to increased HIV-1 replication and dysregulation of key innate immune pathways.