A hepatocyte circular RNA derived from FADS2 exons 2 to 5-heterogeneous nuclear ribonucleoprotein D-purinergic receptor P2Y, subtype 1 axis restrains arachidonic acid output and tunes alloimmune injury in pediatric liver transplantation.

Pediatric liver transplantation is frequently complicated by T cell-mediated rejection (TCMR), yet graft-intrinsic regulators and actionable biomarkers remain limited. Using integrative RNA profiling and mechanistic interrogation, we identified circular RNA derived from fatty acid desaturase 2 (FADS2) exons 2 to 5 (circFADS2) as a hepatocyte-dominant circular RNA that is markedly reduced in TCMR. circFADS2 distinguishes TCMR from nonrejection with an area under the curve value of 0.86, supporting its potential as a candidate biopsy-based molecular adjunct to characterize rejection-associated graft remodeling. Mechanistically, circFADS2 binds purinergic receptor P2Y, subtype 1 (P2RY1) messenger RNA and engages a heterogeneous nuclear ribonucleoprotein D-dependent decay program to promote transcript destabilization, reducing P2RY1 abundance and attenuating P2RY1-linked signaling output as assessed by diminished agonist-evoked Ca2+ mobilization and reduced phosphorylation of downstream effectors, including MAP kinase-interacting serine/threonine-protein kinase 1 and cytosolic phospholipase A2. Consistent with this signaling attenuation, circFADS2 overexpression reduces arachidonic acid (AA) and AA-derived lipid mediators released from hepatocytes. Limiting AA is accompanied by reduced CD4+ T cell activation, proliferation, and effector cytokine production in hepatocyte-conditioned media assays. Finally, pharmacologic P2RY1 stimulation exacerbates, whereas P2RY1 inhibition alleviates, graft injury in a murine liver transplantation model, supporting the in vivo relevance of the P2RY1-linked pathway downstream of circFADS2.