LAUSR

&NA; PIKfyve phosphoinositide kinase produces PtdIns(3,5)P2 and PtdIns5P and governs a myriad of cellular processes including cytoskeleton rearrangements and cell proliferation. The latter entails rigorous investigation since the cytotoxicity of PIKfyve inhibition is a potential therapeutic modality for cancer. Here we report the effects of two PIKfyve‐specific inhibitors on the attachment/spreading and viability of mouse embryonic fibroblasts (MEFs) and C2C12 myoblasts. Importantly, 18‐h treatment of adherent cells with YM201636 (800 nM) and apilimod (20 nM) in serum‐containing culture media did not affect cell viability despite the presence of multiple cytoplasmic vacuoles, a hallmark of PIKfyve inhibition. Strikingly, at the same dose and duration the inhibitors caused excessive cytoplasmic vacuolation, initial suppression of cell attachment/spreading and subsequent marked detachment/death in serum‐deprived cells. The remaining adherent cells under serum‐deprived conditions had smaller surface area, lacked vinculin/actin‐positive focal adhesions and displayed vacuoles occupying the entire cytoplasm. Serum or growth factors protected against PIKfyve inhibitor cytotoxicity. This protection required Akt activation evidenced by the abrogated beneficial effect of serum upon treatment with the clinically‐relevant Akt inhibitor MK‐2206. Moreover, Akt inhibition triggered cell detachment/death even in serum‐fed adherent MEFs treated with apilimod. Intriguingly, BafilomycinA1 (H+‐vacuolar ATPase inhibitor), which prevents the cytoplasmic vacuolation under PIKfyve perturbations, rescued all defects in attaching/spreading as well as in adherent cells under serum‐starved or serum‐fed conditions, respectively. Together, the results indicate that the cytotoxicity of PIKfyve inhibitors in MEFs and C2C12 myoblasts requires Akt suppression and excessive cytoplasmic vacuolation. HighlightsPIKfyve inhibitors trigger cytoplasmic vacuoles in MEFs and C2C12 myoblasts.The vacuoles do not affect cell viability in presence of serum or growth factors.Cytotoxicity is manifested in absence of serum or when AKT kinase is co‐inhibited.Cytotoxicity is associated with excessive vacuolation occupying the whole cytoplasm.Suppressing the vacuolation by BafilomycinA1 rescues PIKfyve inhibitor cytotoxicity.