Skylite: Skyline-Based Lipid Isomer Retention Time Evaluation for Lipidomics in Metabolic Dysfunction-Associated Steatohepatitis

Metabolic dysfunction-associated steatotic liver disease
(MASLD)
is the most prevalent liver disorder worldwide and can progress to
steatohepatitis. Elevated de novo lipogenesis (DNL) is a key contributor
to hepatic steatosis. Fatty acid (FA) desaturation produces several
unsaturated lipid isomers that are structurally very similar but have
diverse biological functions. However, due to their structural similarity,
many conventional mass spectrometry approaches cannot detect such
metabolic alterations. Thus, we introduce the Skylite (Skyline-based
lipid isomer retention time evaluation) workflow using conventional
liquid chromatography–mass spectrometry (LC–MS) to identify
important isomer features. Retention times of isomeric phosphatidylcholines
are compared with the well-characterized human plasma reference standard
NIST 1950. Retention time trends correlate well with fixed-charge
derivatized FA in liquid chromatography and ozone-induced dissociation
mass spectrometry data. The interpretation is supported by double
bond diagnostic fragments in LC–MS/MS experiments of epoxidized
hydrolyzed fatty acids. We investigate hepatic lipid profiles, focusing
on esterified fatty acids in two mouse models of metabolic dysfunction-associated
steatohepatitis (MASH). Out of 37 phosphatidylcholine sum compositions,
the workflow identifies 123 lipid features. Importantly, CCl4-induced and melanocortin-4 receptor knockout mice on a western diet
(WD) have significantly higher levels of mead acid, branched-chain
fatty acid, and n-7 PUFA incorporated into phosphatidylcholines.
While the MASH mouse liver tissues contain notable amounts of n-7 PUFA, no n-10 PUFA were detected, potentially
indicating a unique desaturation pattern. The screening for altered
lipid isomer profiles bridges the gap between high-throughput analyses
and specialized structure-resolved techniques.