Hepatocyte Serine Palmitoyltransferase 3 is essential for mitochondrial ceramide homeostasis and electron transport...

Sphingolipids constitute essential membrane lipids that also function as signaling molecules mediating fundamental cellular processes. Disruption of constitutive sphingolipid metabolism contributes to a range of metabolic diseases. Sphingolipid biosynthesis is initiated by serine palmitoyltransferase (SPT), a heteromeric enzyme composed of Sptlc1 and either Sptlc2 or the less-characterized subunit Sptlc3. While the canonical Sptlc1/Sptlc2 complex generates most sphingolipids, Sptlc1/Sptlc3 produces non-canonical sphingolipids that remain poorly understood. Sptlc3 expression increases in liver disease in both mouse models and humans, yet its biological role remains unclear. To address this, we generated a liver-specific SPTLC3 knockout (SPT3-hKO) mouse model. Loss of SPTLC3 resulted in severe mitochondrial dysfunction in hepatocytes, characterized by reduced oxygen consumption, decreased ATP production, and elevated NADH/NAD⁺ ratios with a concomitant increase in glycolytic activity. Mechanistically, we found a defect in the electron transport chain (ETC) which stemmed not from intrinsic impairment of the ETC complexes but rather from hindered electron flow between complex I and complex III likely due to reduced availability of Coenzyme Q. While atypical ceramides constituted (∼4% of total ceramides) when measured in liver homogenates, there measurements in mitochondria-enriched fractions revealed enrichment in this cell compartment. Together, our findings identify SPTLC3 as a critical modulator of mitochondrial ceramide composition and ETC function, revealing a specialized role for non-canonical sphingolipids in hepatocyte energy metabolism.