Alterations in phosphatidylethanolamine metabolism impacts hepatocellular lipid storage, energy homeostasis, and proliferation

Abstract

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic membranes and is involved in several cellular processes. An important pathway for de novo PE synthesis is the Kennedy Pathway. The rate limiting enzyme in the pathway, CTP:phosphoethanolamine cytidyltransferase, catalyzes the synthesis of CDP-ethanolamine from phosphoethanolamine (pEtn) and CTP. Ethanolamine phosphate phospholyase (ETNPPL) has the potential to breakdown pEtn by catabolizing it to form acetaldehyde, ammonium, and inorganic phosphate. Research on this enzyme is limited and it is unclear how its activity affects PE synthesis; therefore, an investigation into ETNPPL's biological effects is required.

ETNPPL was expressed in human hepatoma cell line (Huh7) by stable transfection, allowing for long-term expression in cells without ETNPPL. We show that ETNPPL reduces cellular pEtn synthesized from ethanolamine, which decreased synthesis of PE and an increased PC:PE ratio, which has been shown to be associated with metabolic dysfunction-associated steatotic liver disease (MASLD) and impaired mitochondrial function. Experiments conducted show increased neutral lipid storage accompanied by decreased ATP production and oxygen consumption; however, no differences in triglyceride secretion were seen, although ApoB100 secretion was reduced. Huh7 cells expressing ETNPPL proliferate at a slower rate than control and have increased mRNA expression of p53 and tumor suppressor genes (CDKN1A, BBC3, BAX, BRCA1), implicating ETNPPL in cell proliferation, cancer development and/or tumor progression. Overall, ETNPPL rewires hepatic lipid metabolism, altering several processes including increasing lipid storage and decreasing proliferation. The impacts observed in this study may create a link between hepatic ETNPPL expression and MASLD/HCC pathophysiology.