Macrophage GAPDH and Iron Uptake
Mammalian glyceraldehyde-3-phosphate dehydrogenase was identified as a dual receptor for both transferrin and the related iron transport protein lactoferrin on macrophages, as well as several other cell types (Raje et al. 2007; Kumar et al. 2011; Rawat et al. 2012). The finding was somewhat surprising given that GAPDH bears no structural similarity to the two well-characterized receptors for transferrin (i.e., TfR1 and TfR2) or with the known Lf receptors. The GAPDH-Tf pathway differs from the TfR-Tf pathway in several aspects including: membrane localization of the receptor; affinity of binding; internalization pathway; kinetics of uptake; and regulation and fate of endocytosed transferrin (Kumar et al. 2011). In addition, GAPDH carries out its iron delivery functions not only as a membrane-bound receptor but is also secreted into the extracellular milieu, where it functions as a soluble receptor for transferrin effecting autocrine and paracrine delivery of transferrin and iron into cells (Sheokand et al. 2013).