Transferrin (Tf) endocytosis and recycling are essential for
iron uptake and the regulation of cell proliferation. Tf and Tf
receptor (TfR) complexes are internalized via clathrin-coated
pits composed of a variety of proteins and lipids and pass
through early endosomes to recycling endosomes. We investigated
the role of sphingomyelin (SM) synthases (SMS1 and
SMS2) in clathrin-dependent trafficking of Tf and cell proliferation.
We employed SM-deficient lymphoma cells that lacked
SMSs and that failed to proliferate in response to Tf. Transfection
of SMS1, but not SMS2, enabled these cells to incorporate
SM into the plasma membrane, restoring Tf-mediated proliferation.
SM-deficient cells showed a significant reduction in
clathrin-dependent Tf uptake compared with the parental SMproducing
cells. Both SMS1 gene transfection and exogenous
short-chain SM treatment increased clathrin-dependent Tf
uptake in SM-deficient cells, with the Tf being subsequently sorted to Rab11-positive recycling endosomes. We observed
trafficking of the internalized Tf to late/endolysosomal compartments,
and this was not dependent on the clathrin pathway
in SM-deficient cells. Thus, SMS1-mediated SM synthesis
directs Tf-TfR to undergo clathrin-dependent endocytosis and
recycling, promoting the proliferation of lymphoma cells.