The Two-Way Valve
A genuine switch: more of it means less ceramide but more steatosis; less of it means the reverse. Which way you want it depends on which endpoint you are treating. Mixed inbound signs are correct here.
In depth
Sphingomyelin synthase — converts ceramide into sphingomyelin, using phosphocholine taken from phosphatidylcholine.
What drives it
Membrane lipid signaling increases it.
Phosphatidylcholine is the obligate phosphocholine donor that sphingomyelin synthase transfers onto ceramide. doi 10.1016/j.bbalip.2013.06.006 · PMC12132366
What it drives
It reduces Ceramide load.
Sphingomyelin synthase (SMS1/SMS2) consumes ceramide - it transfers a phosphocholine head group from phosphatidylcholine onto ceramide, producing sphingomyelin and diacylglycerol. Verbatim from the literature: 'phosphatidylcholine is required for converting pro-apoptotic ceramides to sphingomyelin' and 'PC is substrate of sphingomyelin synthase (EC 2.7.8.27) for the formation of SPH from (pro-)apoptotic ceramides'. This is the exact reverse of the SMASE arm: sphingomyelinase makes ceramide from sphingomyelin; sphingomyelin synthase makes sphingomyelin from ceramide. They are a reversible pair around one node, and dietary choline supplies the head group. Scope, stated: this is sequestration, not destruction - the ceramide is converted, not cleared, and sphingomyelin is itself the substrate the hydrolysis arm draws on. And the equilibrium is species-specific: total SMS knockout in mouse liver 'had a different effect on ceramide subspecies and some ceramide levels are increased and some are decreased' (PMC8022321). No human interventional evidence that feeding choline lowers a plasma ceramide has been retrieved; the CARDIA cohort (PMID 41824420) associates betaine with lower MASLD risk but does not link it to a ceramide. PMID 41824420 · PMC8022321
See it on the map
Open the causal map to follow this pathway upstream and downstream, arrow by arrow, each with its study.