Transcriptomic and epigenomic effects of insoluble particles on J774 macrophages.

Here we report epigenomic and transcriptomic changes in a prototypical J774 macrophage after engulfing talc or titanium dioxide particles in presence of estrogen. Macrophages are the first immune cells to engage and clear particles of various nature. A novel paradigm is emerging, that exposure to so-called ‘inert’ particulates that are considered innocuous is not really free of consequences. We hypothesized that especially the insoluble, non-digestible particles that do not release a known hazardous chemical can be underappreciated agents acting to affect the regulation inside macrophages upon phagocytosis. Specifically, there has been a poorly understood link between talc powder exposure and ovarian cancer, as well as a link connecting titanium dioxide particle exposure to airway disease; the mechanistic studies to support these associations are scarce. We performed gene chip microarray profiling and found that talc alone, and especially with estrogen, has induced a substantially more prominent gene expression change than titanium dioxide; the affected genes were involved in pathways of cell proliferation, immune response and regulation, and, unexpectedly, enzymes and proteins of epigenetic regulation. We therefore tested the DNA methylation profiles of these cells via epigenome-wide bisulfite sequencing and found vast epigenetic changes in hundreds of loci, remarkably after a very short (24-hour) exposure to particles; ELISA assay for methylcytosine levels determined the particles induced an overall decrease in DNA methylation. We found a few loci where both the transcriptional changes and epigenetic changes occurred in the pathways involving immune and inflammatory signaling; however the majority of the DNA methylation changes were not immediately linked to genes with the altered expression. Some transcriptomic and epigenomic changes were shared between talc and titanium dioxide, however it is especially interesting that each of the two particles of similar size and insoluble nature has also induced a specific pattern of gene expression and DNA methylation changes which we report here.