“…Comparing the proteome of HEK293 cells treated with 400 μM of fluidizing oleic acid versus 400 μM of rigidifying palmitic acid shows that ADI-POR2 is one of the most strikingly regulated proteins, being elevated in the presence of palmitic acid because of reduced interaction with, and ubiquitination by, the ER-resident ubiquitin ligase RNF145. [61] Separately, ADIPOR2 emerged as one of the top hits in three unbiased CRISPR/Cas9 screens in human cells designed to identify genes essential for the response to membrane-rigidifying challenges: one screen identified ADIPOR2 mutants as intolerant of palmitic acid and other SFAs, [62] another screen identified ADIPOR2 overexpression as a potent mechanism to increase resistance to palmitic acid, [63] and a third identified ADIPOR2 as essential to combat hypoxia-induced desaturase inhibition. [11] siRNA silencing of ADIPOR2 in the presence of 200 μM palmitic acid, of which the plasma concentration is often >1 mM, [64] results in excess SFA content in phospholipids and membrane rigidification in all mammalian cell types examined so far: HEK293 (embryonic kidney-derived), [65] HepG2 (hepatocyte-derived), [65] 131N1 (astrocyte-like), [65] INS-1E (rat beta cell-derived), [66] Jurkat E6-1 (T lymphocyte-derived), [66] U2 O-S (bone sarcoma-derived), [66] and 2) by homology with the C. elegans PAQR-2, ADIPOR2 may multimerize with an activator ("X" in the model) in a membrane rigidity-dependent manner; (3) once activated, the ADIPOR2-intrinsic ceramidase hydrolyzes ceramides, producing sphingosine that, once phosphorylated, becomes sphingosine 1-phosphate (S1P), a signaling molecule; (4) S1P is secreted then bind to the S1P receptor S1PR3, a GPCR that signals to activate SREBPs; (5) separately, S1P acts as an intracellular ligand for PPARγ; (6) the SREBPs and PPARγ promote transcription of the Δ9 desaturase SCD and other genes, contributing to the conversion of dietary SFAs to UFAs; (7) an ER-associated fatty acid elongation complex (the interaction with HACD3 is thoroughly verified while the interaction with HSD17B12 and ELOVL3/6 is less solidly established) and ACSL4 are recruited by ADIPOR2, allowing local production of long-chain PUFAs (LCPUFAs) and their channelling toward incorporation into phospholipids; (8) once membrane fluidity is restored, RNF145 is stabilized and ubiquitinates ADIPOR2 that is then degraded; and (9) fluidizing lipids are shared among cells and tissues such that membrane homeostasis is cell non-autonomous.…”