Iron Must Be in Haemin to Act as a Pro-Inflammatory Stimulus in Cultured Human THP-1 Monocytes

Cardiovascular diseases due to atherosclerosis remain a dominant medical problem. Macro-phages play a crucial role in both atherosclerosis progression and recycling of body iron. Epidemio-logical data point to elevation of body iron stores as one of non-classical cardiovascular risk factors, and we know that iron must be contained within macrophages to be atherogenic. Presumably, iron already contained within circulating monocytes turns cells to a more pro-inflammatory and hence atherogenic phenotype, but experimental evidence for such relationship remains limited. In this study, human monocytic THP-1 cells were loaded with micromolar iron in the forms of transferrin, ferric-ammonium citrate (FAC) and haemin for 2 and 24 hours. Only haemin was cytotoxic. All kinds of iron elevated the labile iron pool at 2 hours, as well as ferritin expression at 24 hours. Expression of scavenger receptors A and B, pro-inflammatory as well as anti-inflammatory cytokines, haem oxygenase and haptoglobin receptor were measured by quantitative PCR. Very few changes, none pro-inflammatory, were observed in response to transferrin or FAC. Haemin suppressed the expression of scavenger receptors, increased the expression of pro-inflammatory, and variably, anti-inflammatory cytokines, and dramatically induced haem oxygenase. The effects of haemin were not prevented by apotransferrin. We conclude that iron must be in the haemin form to act as a pro-inflammatory stimulus in THP-1 monocytes. Non-haem iron might require the presence of other factors to be atherogenic. Haemin treatment of THP-1 cells may represent a convenient experimental model to study the pro-inflammatory effects of haem that are observed in late stages of atherosclerosis.