Introduction

Diffuse large B-cell lymphoma (DLBCL) is one of the most common subtype of non-Hodgkin lymphoma with considerable clinical and biological heterogeneity.1 Although a durable complete remission can be achieved by using rituximab combined with chemotherapy in a substantial proportion of patients, up to 30~40% of DLBCL cases are either relapsed or refractory to current standard treatment.2 Therefore, the identification of actionable biomarkers will be helpful in improving the clinical outcome of high-risk DLBCL patients.3

Initially uncovered as the target of t(8;14)(q24;q32) chromosome translocation in Burkitt lymphoma,4 MYC is a master regulator in DLBCL pathogenesis5 and renders lymphoma cell resistance to chemotherapy.6 Clinically, MYC overexpression is related to increased risk of disease relapse and indicates poor disease outcome in DLBCL patients.7 As direct targeting of MYC appears difficult,8 alternative therapeutic strategies to specifically modulate MYC-driven downstream effectors warrants further investigation.9, 10

Correlating with genomics, transcriptomics and proteomics, metabolomics is the end-point of ‘multi-omics’ cascades and provides a ‘real-world’ assessment of cancer cell physiology.11 Generally involved in genetic transcription, MYC alters multiple tumor metabolic processes such as glycolysis, nucleotide and lipid synthesis.12 Previous works indicated that MYC regulates lipid metabolism during lymphomagenesis.13 It has recently been reported that, as an indispensible component of lipid synthesis, choline metabolism is dysregulated in lymphoma.11 C-Choline micro-positron emission tomography/computed tomography was then carried out to visualize choline metabolism of tumors a week following the treatment. Compared with those in untreated mice, standardized uptake value intensity of tumors was significantly reduced in the BBR-treated mice (Figure 5b). As in vitro study, PCYT1A expression was decreased in the tumors of the BBR group, without any obvious change in the MYC protein (Figure 5c). To search for more evidence of in situ tumor cell necroptosis, ultrastructural study was performed on mice tumor. Typical cell necroptosis (swelling mitochondria and intact nuclei membrane) and mitophagy (increased mitochondrial density and accumulation of mitochondria within the double-membrane autophagy lysosomes)14, 25 were frequently observed in BBR-treated tumors (Figure 5d). Accordingly, serum levels of choline, as well as its phosphorylated derivatives PC(16:0/22:6), LPC(16:0) and LPC(18:0) were also restored by BBR treatment (Figure 5e).

Figure 5
figure 5

In vivo activity of BBR on murine xenograft B-lymphoma model. (a) Tumor size of xenograft B-cell lymphoma with subcutaneous injection of DB cells transfected with PCYT1A shRNA or control shRNA (CON shRNA, upper panel, N=5 for each group). Tumor size was measured in BBR group and the untreated group in xenograft B-cell lymphoma with subcutaneous injection of DB cells (lower panel, N=10 for each group). ***P<0.001, **P<0.01 and *P<0.05 comparing with the untreated group. (b) 11C-Choline micro-positron emission tomography/computed tomography was performed one week after BBR treatment and standardized uptake value (SUV) intensity was observed. (c) PCYT1A and MYC expression were detected by immunohistochemistry. (d) Necroptosis and mitophagy were observed under transmission electron microscopy. (e) Serum levels of choline and its phosphorylated derivatives were measured by UPLC-triple quadrupole mass spectrometry. Data in a and e were represented as mean±s.e.m. Assay in e was set up in five mice.

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These results provided in vivo evidence that progression of MYC-driven lymphoma can be tackled by altering downstream effector of choline metabolism with lipid-lowering agents.

Discussion

Growing evidence suggest that the activation of oncogenes can reprogram tumor cell metabolism.28 MYC is a key oncogene and critically involved in lipid synthesis, including those of cholesterol, fatty acid and glycerophospholipid.12, 29, 30 Here we support a direct link between MYC overexpression and dysregulated choline metabolism (a major component of glycerophospholipids). These data in turn highlight the pivotal role of oncogenic MYC on lipid metabolism.

PCYT1A is the major isoform for the key enzyme CTP (choline phosphate cytidylyltransferase), essential for PC synthesis during lipid metabolism. In neuroblastoma, MYC modulates lipid synthesis by coordinating with MondoA.29 As for lung cancer, MYC upregulates cytosolic phospholipase A2 and increases phosphatidylinositols and arachidonate-containing phospholipids, which are required for tumor cell survival and proliferation.31 In the present study, PCYT1A was upregulated by MYC and contributed to dysregulated choline metabolic pathways, suggesting an alternative mechanism involved in MYC-mediated lymphoma cell metabolism and tumor progression. This is consistent with experimental findings in intestinal epithelial, which shows that PCYT1A determines malignant transformation of their normal counterparts.32

Choline and its phosphorylated derivatives are implicated in the initiation and execution of necroptosis, an alternative caspase-independent cell death by modulating RIP1/RIP3 complex, also known as necrosome.33 Alterations in lipid metabolism may damage cellular and subcellular membrane, cause imbalance between mitochondrial fusion and fission, initiate mitophagy and lead to necroptosis. Therefore, PCYT1A may not only act as a downstream effector of MYC on regulation of choline metabolism, but also a biomarker of MYC-mediated lymphoma cell necroptosis and lymphoma progression in DLBCL.

It is promising to treat oncogene-driven tumors through targeting downstream cell metabolism. For example, MYC-mediated glutamine metabolism could be modulated by glutaminase-specific inhibitor, which diminishes tumorigenesis and prolongs the survival of the mice with MYC-associated hepatocellular carcinoma.34 Furthermore, inhibition of fatty acid oxidation has recently been proposed as a potential treatment for triple-negative breast cancer with MYC overexpression.35 BBR is a quartenary ammonium salt belonging to the proto-berberine group of isoquinolone alkaloids and extracted from Chinese herbs known for its diverse pharmacological properties, notably the lipid-lowering effect.16 The effect of BBR on mitochondrial fragmentation and depolarization has also been revealed36 as contributing to necroptotic cell death in cancer cells.37 We showed that BBR inhibits B-lymphoma cell growth by accelerating mRNA degradation of PCYT1A and inducing mitophagy-dependent necroptosis, suggesting an alternative therapeutic relevance of BBR on MYC-High lymphoma. In addition, necroptosis may arise from chemotherapy treatment, accounting for the cell death observed in apoptosis-defective tumor cells.38 Induction of necroptosis to bypass apoptosis and to overcome chemoresistance has been achieved in hematological malignancies, such as acute myeloid leukemia and MLL-rearranged acute lymphoblastic leukemia.39, 40, 41 Consistently, our data further opened up promising treatment avenues for BBR to reverse the chemoresistant effect of MYC by affecting MYC-downstream effectors.

In conclusion, MYC positively regulated PCYT1A expression and was responsible for the dysregulation of choline metabolism in DLBCL. These metabolic alterations could be reversed by lipid-lowering agent BBR, providing a clinical rationale of lipid-lowering strategy in treating MYC-high lymphoma.