Homeostatic role of acid sphingomyelinase in mtor signaling and autophagy

Abstract

Key regulatory decisions of protein synthesis and autophagy are controlled by the lysosomal nutrient sensing complex (LYNUS). To engage protein synthesis signaling, LYNUS requires cellular availability of amino acids, adenosine triphosphate (ATP), growth factors, and docking at the lysosomal membrane. The molecular determinants of LYNUS signaling and docking are not completely elucidated and may involve regulators of the lipid membrane structure and function of the lysosome. Since ceramides are both bioactive second messengers and determinants of lipid membrane stiffness, we investigated the role of the ceramide-producing lysosomal acid sphingomyelinase (ASM) in the homeostatic function of mammalian target of rapamycin (mTOR) signaling and autophagy. Using ASM inhibition with either imipramine or siRNA against SMPD1, in primary human lung cells or Smpd1+/- mice, we demonstrated that ASM is an endogenous inhibitor of autophagy. ASM was necessary for physiological mTOR signaling and maintenance of sphingosine levels. Whereas overstimulation of ASM has been shown to trigger autophagy with impaired flux, inhibition of ASM activity during homeostatic, non-stressed conditions triggered autophagy with degradative potential, associated with enhanced transcription factor EB (TFEB), a master regulator of autophagy and lysosomal biogenesis genes, translocation to the nucleus and decreased sphingosine levels. These findings suggest LYNUS signaling and autophagy are partially regulated by ASM.