BCAA (Branched-Chain Amino Acids) Inhibiting the Autophagy System via the Activation of mTORC1, Thereby Upregulating the Tumor Suppressor PDCD4 in Huh7 Hepatoma Cells

Branched-chain amino acids (BCAAs) are essential amino acids in humans, with reported anti-proliferative effects on HepG2 hepatoma cells and the potential to reduce hepatocellular carcinoma (HCC) development in cirrhotic patients. PDCD4, a tumor suppressor that is downregulated in many cancers, is also suppressed by serum, EGF, or TPA treatment. This study examined the effect BCAA has on PDCD4 expression and related cellular pathways in Huh7 hepatoma cells. Cells were treated with different concentrations of BCAA, and analyzed by Western blotting, qRT-PCR, and immunofluorescence staining. Treatment with BCAA upregulated the protein levels of PDCD4, while downregulating its mRNA levels. BCAA enhanced the phosphorylation of mTORC1 substrate 4E-BP1, p70S6K1, and p70S6K1 substrate S6 ribosomal protein. BCAA also elevated the protein levels of autophagy factors p62 and ATG5 while reducing LC3-II particle formation, thus indicating impaired autophagy. ULK1 knockdown also upregulated the protein levels of PDCD4 and p62. Additionally, BCAA upregulated the phosphorylation of ULK1 at serine 757, which was inhibited by rapamycin. These findings suggest that BCAA inhibits autophagy through the mTORC1-mediated phosphorylation of ULK1 at serine 757, thereby impairing autophagosome formation and upregulating the PDCD4 protein levels by inhibiting its degradation via autophagy. Furthermore, FACS analysis showed that BCAA inhibited the proliferation of Huh7 cells. BCAA may have a preventive effect against tumor development through the modulation of autophagy and the tumor suppressor pathways.