A decrease of hepatocellular phosphatidylcholine (PC) is associated with hepatic injury, e.g., in nonalcoholic steatohepatitis (NASH). Therefore, we evaluated the hepatoprotective effect of a PC-precursor lipid specifically targeted to the liver. We synthesized the bile acid-phospholipid conjugate ursodeoxycholyl lysophosphatidylethanolamide (UDCA-LPE), which was designed to target PC to hepatocytes by way of bile-acid transport systems. We synthesized a fluorescently labeled analogue UDCA-6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]hexanoyl PE (UDCA-NBDPE) for uptake and metabolism studies. Unexpectedly, the majority of UDCA-NBDPE was still intact and not hydrolyzed efficiently in HepG2 cells. For targeting in vivo, NBD fluorescence from UDCA-NBDPE-injected mice was recovered in the liver the most, whereas injection of NBDPE alone resulted in an even distribution in liver, kidneys, and intestine. Cytoprotection by UDCA-LPE was tested in starvation and tumor necrosis factor alpha (TNF-alpha) apoptosis models using HepG2 cells. Only the intact UDCA-LPE was able to persistently stimulate growth after 36 to 120-hour starvation, and significantly inhibited TNF-alpha-induced apoptosis. In both models, LPC, LPE, UDCA, or UDCA added with LPE exhibited weak to no cytoprotection. UDCA-LPE stabilized mitochondrial membranes by lowering mitochondrial membrane potential. Western blot analyses of phosphorylated Akt and glycogen synthase kinase-3 (GSK-3)alpha/beta revealed that UDCA-LPE activated phosphatidyl inositol 3-kinase (PI3K)/Akt signaling pathways. The PI3K inhibitor LY294002 or Akt small interfering (si)RNA consistently inhibited the proproliferative effects of UDCA-LPE during starvation. The TNF-alpha death-receptor extrinsic pathway involves caspase 8 activation, which is inhibited by cellular FLICE-inhibitory protein (cFLIP); thus, cFLIP siRNA was employed in our studies. cFLIP siRNA was able to reverse the cytoprotective effects of UDCA-LPE during TNF-alpha-induced apoptosis, and UDCA-LPE concomitantly upregulated protein expression of cFLIP(L).
Conclusion: UDCA-LPE, which targeted the liver in vivo, elicited potent biological activities in vitro by stimulating hepatocyte growth and by inhibiting TNF-alpha-induced apoptosis. Thus, UDCA-LPE may be suitable for evaluation of treatment efficacy in NASH.