42 Chapter 2 Amino-acid starvation reversibly reduces the levels of peroxisomal proteins in hepatic organoids In vivo studies of rodents on a low-protein diet showed a decline of peroxisome numbers and altered mitochondrial morphology and function in the liver29,30. Compromised oxidation of fatty acids by peroxisomes and mitochondria explains hepatic steatosis in these studies. We aimed to determine whether these findings could be recapitulated in amino-acid-deprived hepatic organoids. Peroxisomal proliferator-activated receptor gamma coactivator 1-alpha (PGC1α), an activator of peroxisomal and mitochondrial biogenesis37, was reduced in the starved condition. Also, the protein levels of PMP-70, an ATP-binding cassette transporter and a major component of the peroxisomal membrane38, was strongly reduced upon amino acid deprivation. The same was found for the peroxisomal enzymes acyl-CoA oxidase 1 (Acox-1), the first enzyme of the peroxisomal beta-oxidation and catalase, an integral part of the pathway as it scavenges the produced H2O2 (Figure 4a, b). Peroxisomal fatty acid oxidation was assessed following the metabolism of phytol, a branched-chain fatty acid precursor of phytanic acid39 (Figure 4c). While levels of phytanic acid did not show any regulation, pristanic acid levels were significantly increased in organoids depleted of amino acids (Figure 4d). Pristanoyl CoA is the product of the peroxisomal alpha-oxidation, which is further metabolized in the peroxisomal beta-oxidation (Figure 4c). Phytanic acid is a branched-chain fatty acid and substrate for the peroxisomal α-oxidation, whereas pristanic acid is a substrate for β-oxidation. The increased pristanic acid levels may therefore indicate a reduction in peroxisomal beta-acid oxidation. Very-long chain fatty acids tetrasanoic and hexasanoic acids (C24 and C26, respectively) did not show any regulation (Figure 4e), in line with the absence of long-chain fatty acids from the medium.
RkJQdWJsaXNoZXIy MTk4NDMw