Niclosamide, a
drug used to treat
tapeworm infection, possesses anticancer effects by interfering with multiple signaling pathways.
Niclosamide also causes intracellular acidification. We have recently discovered that the
amino acid transporter SLC38A5, an
amino acid-dependent Na+/H+ exchanger, activates macropinocytosis in
cancer cells via
amino acid-induced intracellular alkalinization. Therefore, we asked whether
niclosamide will block basal and SLC38A5-mediated macropinocytosis via intracellular acidification. We monitored macropinocytosis in pancreatic and
breast cancer cells using TMR-
dextran and the function of SLC38A5 by measuring Li+-stimulated
serine uptake. The
peptide transporter activity was measured by the uptake of
glycylsarcosine. Treatment of the
cancer cells with
niclosamide caused intracellular acidification. The
drug blocked basal and
serine-induced macropinocytosis with differential potency, with an EC50 of ~5 μM for the former and ~0.4 μM for the latter. The increased potency for
amino acid-mediated macropinocytosis is due to direct inhibition of SLC38A5 by
niclosamide in addition to the ability of the
drug to cause intracellular acidification. The
drug also inhibited the activity of the H+-coupled
peptide transporter. We conclude that
niclosamide induces nutrient
starvation in
cancer cells by blocking macropinocytosis, SLC38A5 and the
peptide transporter. These studies uncover novel, hitherto unknown, mechanisms for the anticancer efficacy of this antihelminthic.