Nuclear Translocation of LDHA Promotes the Catabolism of BCAAs to Sustain GBM Cell Proliferation through the TxN Antioxidant Pathway

Glutamate is excitotoxic to neurons. The entry of glutamine or glutamate from the blood into the brain is limited. To overcome this, branched-chain amino acids (BCAAs) catabolism replenishes the glutamate in brain cells. Branched-chain amino acid transaminase 1 (BCAT1) activity is silenced by epigenetic methylation in IDH mutant gliomas. However, glioblastomas (GBMs) express wild type IDH. Here, we investigated how oxidative stress promotes BCAAs' metabolism to maintain intracellular redox balance and, consequently, the rapid progression of GBMs. We found that reactive oxygen species (ROS) accumulation promoted the nuclear translocation of lactate dehydrogenase A (LDHA), which triggered DOT1L (disruptor of telomeric silencing 1-like)-mediated histone H3K79 hypermethylation and enhanced BCAA catabolism in GBM cells. Glutamate derived from BCAAs catabolism participates in antioxidant thioredoxin (TxN) production. The inhibition of BCAT1 decreased the tumorigenicity of GBM cells in orthotopically transplanted nude mice, and prolonged their survival time. In GBM samples, BCAT1 expression was negatively correlated with the overall survival time (OS) of patients. These findings highlight the role of the non-canonical enzyme activity of LDHA on BCAT1 expression, which links the two major metabolic pathways in GBMs. Glutamate produced by the catabolism of BCAAs was involved in complementary antioxidant TxN synthesis to balance the redox state in tumor cells and promote the progression of GBMs.

BCAT1 knockdown-mediated suppression of melanoma cell proliferation and migration is associated with reduced oxidative phosphorylation

Malignant melanoma has a high mutational rate. As a result, resistance to current therapies is common. Consequently, there is an unmet medical need to develop novel therapies. Recent data suggest that branched-chain amino acid transaminase 1 (BCAT1) is overexpressed in multiple cancers, and such overexpressed BCAT1 is necessary for individual cancer progression. Therefore, BCAT1 appears to be a good target in cancer treatment. Additionally, because its expression in healthy tissues is highly restricted in adults and is limited to the brain, ovary, and placenta, BCAT1 is especially an ideal target in cancer therapies. Currently, the function of BCAT1 in malignant melanoma has not been demonstrated. Therefore, we investigated the role of BCAT1 in the proliferation and migration of malignant melanomas using human samples and mouse malignant B16 melanoma cell line. Our data showed that BCAT1 was overexpressed in malignant melanoma tissues both in humans and mice. Besides, BCAT1 knockdown suppressed melanoma cell proliferation and migration, which was associated with reduced oxidative phosphorylation. Collectively, our data indicate that BCAT1 is a promising therapeutic target for the treatment of malignant melanomas.

BCAT1 promotes tumor cell migration and invasion in hepatocellular carcinoma

Branched-chain amino acid transaminase 1 (BCAT1) has been associated with numerous types of tumors; however, few previous studies have evaluated the expression and role of BCAT1 in hepatocellular carcinoma (HCC). In the present study, the expression of BCAT1 was detected by reverse transcription-quantitative polymerase chain reaction and immunoblotting in six HCC cell lines and 74 pairs of HCC and adjacent non-cancerous liver tissues. In addition, the correlation between the expression levels of c-Myc and BCAT1 was analyzed using immunohistochemistry. Furthermore, RNA silencing was performed using c-Myc-specific or BCAT1-specific small interfering RNA, after which wound healing and Transwell cell invasion assays were performed. Finally, the clinicopathological characteristics of BCAT1 in patients with HCC were analyzed. It was shown that the expression of BCAT1 was significantly higher in HCC tissues compared with adjacent non-tumor tissues (P<0.001), and in HCC cell lines compared within the L-02 hepatic cell line (P<0.001). In addition, immunohistochemical analyses indicated that the expression of BCAT1 was positively correlated with c-Myc (r=0.706, P<0.001). BCAT1 expression was shown to be downregulated in c-Myc-knockdown cells, and silencing of BCAT1 expression reduced the invasion and migration of HCC cells. Furthermore, a clinical analysis indicated that BCAT1 expression in HCC tissues was significantly associated with the tumor-node-metastasis stage, tumor number and tumor differentiation (all P<0.05), and that BCAT1 was able to predict the 5-year survival and disease-free survival rates of patients with HCC (both P<0.001). The results of the present study suggested that BCAT1 expression is upregulated in patients with HCC, and that BCAT1 may serve as a potential molecular target for the diagnosis and treatment of HCC.