The Oncogene MYC Regulates the Branched Chain Amino Acid Metabolism and mTOR Signaling in Diffuse Large B Cell Lymphoma

Abstract

Diffuse large B‐cell Lymphoma (DLBCL) is the most common type of aggressive non‐Hodgkin lymphoma (NHL) worldwide. While therapeutic strategies to combat DLBCL have advanced, these malignancies continue to be challenging to treat due to their heterogeneity and frequently recurring mutations. Aberrations in the oncogene MYC are commonly found in high grade DLBCL and are associated with poor responses to standard therapy. In search of downstream targets of MYC in DLBCL, we found MYC‐binding sites in the promoter regions of BCAT1, BCAT2, and BCKDHA. These genes encode enzymes in the metabolism of the branched chain amino acids (BCAAs), and they have been previously linked to cancer progression. BCAT1 and 2 encode the cytosolic (BCATc) and mitochondrial (BCATm) branched‐chain aminotransferases, while BCKDHA encodes the E1α subunit of the branched‐chain ketoacid dehydrogenase complex. We hypothesized that MYC activates these genes in DLBCL tissues leading to upregulation of BCAA metabolism and lymphoma growth. Human tissues collected from DLBCL patients (n=89) or controls (n=33) were used to analyze the gene expression of MYC, BCAT1, BCAT2 and BCKDHA followed by correlative analysis of the expression of these genes and the survival of DLBCL patients. Next, human DLBCL and B‐lymphoblast (control) cells were treated with the MYC inhibitor 10058‐F4 (0, 25, 50, 100 µM), the BCAA antagonist N‐acetyl leucine amide (NALA, 0, 10, 20, 40 mM) and the BCATc inhibitor gabapentin (0, 5, 10, 20 mM). 24 hours post treatment, the cell viability and cell proliferation, as well as the protein expression of BCATc, BCATm, and E1α, and the total and phosphorylation states of the ribosomal protein S6 (a downstream target of the mammalian target of rapamycin, mTOR) were determined. Results showed that the gene expression of BCAT1, BCAT2and BCKDHA were significantly elevated in tumor tissues from DLBCL patients compared to controls, and there was a positive correlation between their expression and the upregulation of MYC in DLBCL tissues. The high expression of MYC and BCAT2, but not that of BCAT1 and BCKDHA, correlated with poor overall cancer survival. Inhibiting the MYC function by 10058‐F4, the BCAA uptake by NALA, and the BCATc activity by gabapentin led to concentration dependent reductions in the cell viability and proliferation of the DLBCL cells. The protein expression of BCATm and E1α were reduced in the presence of 10058‐F4, which correlated with downregulation of mTOR signaling as measured by the deceased phosphorylation state of S6 in treated DLBCL cells. These results support the hypothesis that MYC is an upstream regulator of genes associated with BCAA metabolism. The results also demonstrate that DLBCL cells need MYC and BCAAs to upregulate mTOR signaling and to survive and may represent a new molecular mechanism via which MYC exerts its effects in DLBCL lymphoma.