The problem we encountered
Lymphoma is a type of malignant tumor originating from lymphocytes and is divided into Hodgkin's lymphoma (HL) and non-Hodgkin's lymphoma (NHL). Diffuse large B-cell lymphoma (DLBCL) is the most common subtype of lymphoma, accounting for 30-35% of NHL(1, 2) . In recent years, 6-8 cycles of R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone) have become the standard treatment for DLBCL (3-7), and a good efficacy of the R-CHOP regimen has been achieved. However, some DLBCL patients with poor prognosis features remain resistant to therapy or relapse after short-term remission. It is therefore important to identify markers associated with the prognosis of DLBCL patients and the efficacy of the R-CHOP regimen, which will ultimately help in the exploration of new targeted therapies for rituximab/chemotherapy resistant DLBCL.
How we start
Previous studies have demonstrated that the ubiquitin-proteasome system plays an important role in the development of rituximab-chemotherapy resistance in B-cell lymphoma. Moreover, targeting the proteasome system with pharmacological inhibitors has resulted in significant anti-tumor activity in relapsed/refractory DLBCL (8, 9). USP1, a deubiquitinating enzyme, is a member of the ubiquitin-specific processing (USP) family of proteases (10) and implicated in cancer progression (11). As a deubiquitylation enzyme, USP1 binds to the target proteins and maintains their stability by removing the ubiquitylation chain. USP1 can deubiquitinate ID1 (inhibitor of DNA binding 1), a transcription regulator, which was identified to control leukemogenesis by us previously, and protected ID1 from proteasome-mediated degradation (12). Furthermore, it has been reported that USP1 can bind to ID proteins (13, 14), RPS16 (15), KDM4A/SIX1 (16, 17) and KPNA2 (18) in osteosarcoma, gastric cancer, hepatocellular carcinoma, prostate cancer and breast cancer respectively and maintain the stability of these proteins, which suggests that the target of USP1 could be cancer type-specific. To investigate whether the deubiquitination enzyme USP1 plays an important role in diffuse large B-cell lymphoma, we conducted our study.
What we find
To elucidate the potential role of USP1 in DLBCL, we detected the expression of USP1 by using immunohistochemical (IHC) analysis in 106 newly diagnosed DLBCL samples and 16 normal lymph node tissue samples. Our clinical analysis showed that Ubiquitin-specific protease 1 (USP1) was highly expressed in diffuse large B-cell lymphoma (DLBCL) patients, and its high expression predicted poor prognosis (Figure 1).
Figure 1. USP1 is highly expressed in DLBCL and associated with poor prognosis.
Until now, the function and underlying mechanism of USP1 in DLBCL has been unknown. Therefore, we hypothesized that USP1 could be a potential therapeutic target of the therapy for the relapsed/refractory DLBCL patients and investigated the role of USP1 in DLBCL. In this study, knocking down USP1 in DLBCL cells inhibited cell proliferation, induced cell cycle arrest, stimulated autophagy, and demonstrated therapeutic effects in a xenograft mouse model of the relapsed/refractory DLBCL. Interestingly, we find that USP1 interacts with MAX and maintains the stability of the MAX/MYC heterodimer, thereby regulating the transcription of MYC target genes. Pimozide, a specific inhibitor of USP1, could significantly suppress cell proliferation, block cell cycle, increase cell autophagy and retard the growth of lymphoma in the RL-4RH cells or patient-derived xenograft (PDX) mouse model. In addition, pimozide is synergetic with chemotherapeutics such as etoposide, both in vitro and in vivo.
Our study suggests that Inhibition of USP1 reverses the chemotherapy resistance in the relapsed/refractory B Cell Lymphoma by destabilizing MAX and MYC. These findings suggest that USP1 has an important function in the relapsed/refractory DLBCL and may be a potential target for clinical therapy (Figure 2).
Figure 2. Schematic diagram of functions and molecular mechanisms of USP1 in DLBCL.
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