The exosome pathway in K562 cells is regulated by Rab11

During maturation, reticulocytes lose some membrane proteins that are not required on the mature red cell surface. The proteins are released into the extracellular medium associated with vesicles that are formed by budding of the endosomal membrane into the lumen of the compartment; this process results in the formation of multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles, termed exosomes. K562 cells release exosomes with similar characteristics to reticulocyte exosomes, in particular the transferrin receptor (TfR) is found associated with the vesicles. Interestingly, this cell line has been shown to possess high amounts of Rab11 compared with other Rab proteins. To assess the regulation of transferrin receptor release via exosome secretion by Rab11 in this cell type, K562 cells were stably transfected with GFP-Rab11wt or the GTP- and GDP-locked mutants. The distribution of the proteins was assessed by fluorescence microscopy. Transferrin recycling and the number of TfRs present on the surface of the transfected cells were reduced by overexpression of either Rab11wt or the mutants. The amount of released exosomes was analyzed by measuring different molecular markers present on these vesicles either biochemically or by western blot. Overexpression of the dominant-negative mutant Rab11S25N inhibited exosome release, whereas the secretion of exosomes was slightly stimulated in cells transfected with Rab11wt. Taken together, the results demonstrate that in K562 cells Rab11 modulates the exosome pathway although the exact step involved is still not known.