Macrophages are unique in controlling iron recycling and immunity. They scavenge aged and damaged red blood cells (RBCs) and extract iron using heme oxygenase-1 (HO-1). They then export iron into circulation by ferroportin (Fpn) to bone marrow for new RBCs to be formed [1], [2], [3]. During inflammation hepatocyte-derived hepcidin degrades ferroportin (Fpn) to prevent iron release from macrophages [4]. A decreased level of Fpn due to repression of transcription during inflammation has also been reported [3], [5]. Together they cause retention of iron in macrophages to deny this essential micronutrient to extracellular pathogens. During extracellular infection macrophages also release pro-inflammatory cytokines to affect pathogens [6]. Lipopolysaccharides (LPS) present in the outer membrane of Gram-negative bacteria act as a prototype of extracellular infection and inflammation to block iron release [3]. It can activate monocytes as well as macrophages to promote various immune responses during response to severe infections like septic shock and systemic inflammatory response syndrome (SIRS) [7], [8]. LPS induced inflammatory process promotes generation of cytokines including tumor necrosis factor- α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6). The generation of these molecules determine the eventual survival of the host.

In mammalian cells, iron from intracellular labile iron pool (LIP) is stored in to ferritin by chaperone poly(rC)-binding protein 1 (PCBP1) [9]. PCBP1 also binds to cytosine-rich motifs in RNA and single-stranded DNA. Thus, it can influence mRNA processing, stability, and translation, resulting alteration of gene expression [10]. Poly (rC)-binding proteins (PCBPs) are multifunctional and belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) family. Among five members of hnRNP family, PCBP1 is ubiquitously expressed in all the mammalian tissues and it is the first recognized iron chaperone [11]. Ferritin is comprised of twenty four subunits containing ferritin-H (Ft-H) and ferritin-L (Ft-L) [12]. Iron bound PCBP1 forms a complex with Ft-H in cytosol for loading iron in to ferritin [9], [11]. PCBP2 is also abundantly and ubiquitously expressed in all the mammalian tissues [10], [11], [13] and may act as iron chaperone like PCBP1. These two are 83% identical to each other [14]. We have recently reported that intracellular pathogen Leishmania can cleave both PCBP1 and PCBP2 to deny iron loading into ferritin in host macrophages as part of their iron acquisition strategy [15]. So far the fate of PCBP1 during extracellular infection or inflammation has not been reported.

Pro-inflammatory macrophages have iron retention property and higher level of ferritin [3]. Thus, an increased interaction of PCBPs with ferritin is expected to load elevated level of intracellular iron from the cytosolic pool. On the contrary, here we report that PCBP1 protein level is decreased in LPS treated macrophages thus iron loading into ferritin is affected. We found that PCBP1 is regulated at the translational level in LPS-induced macrophages. We also found that silencing of PCBP1 exacerbates while overexpression of PCBP1 inhibits LPS-induced generation of pro-inflammatory cytokines. This is the first report of translational silencing of iron chaperone PCBP1 implicated in LPS induced pro-inflammatory cytokine generation in macrophages.