Background
Apolipoprotein A-I (ApoA-I), the major protein of high density lipoprotein (HDL) particles, plays a crucial role in reverse cholesterol transport (RCT). A higher concentration of ApoA-I is associated with both an increased high density lipoprotein functionality and RCT. A promising strategy to prevent cardiovascular diseases can be to improve RCT by increasing de novo ApoA-I production. We here examined the effects of different amino acids on hepatic ApoA-I mRNA transcription and pro-ApoA-I secretion as experimental animal models have indicated that amino acids effects the hepatic lipoprotein metabolism.

Methods
Human hepatocytes (HepG2) were exposed to amino acids (glutamine, glutamic acid, histidine, leucine, proline or tryptophan)  for 48 hours. ApoA-I mRNA expression and pro-ApoA-I protein secretions were analyzed using quantitative polymerase chain reaction (qPCR) and enzyme-linked immunosorbent assays (ELISA), respectively. To study the underlying molecular pathways we analyzed  CPT1 and KEAP1 mRNA expression, peroxisome proliferator-activated receptor alpha (PPARα) transactivation, and mechanistic target of rapamycin complex 1 (mTORC1) phosphorylation.

Results
Leucine, glutamic acid, and tryptophan increased ApoA-I and CPT1 mRNA expression. Additionally, tryptophan strongly increased PPARα transactivation. Glutamine, proline, and histidine significantly increased pro-ApoA-I protein concentrations in a dose-dependent manner. mTORC1 phosphorylation remained unchanged after amino acid treatment of the HepG2 cells.

Conclusion
Individual amino acids have different dose-dependent effects on ApoA-I mRNA expression and pro-ApoA-I production. These effects are in line with specific effects on PPARα transactivation and activity. This indicates a clear role for the PPARs as potential mechanism while there is no indication for involvement of the mTORC1 pathway.