Dysregulation of BMI1 and microRNA-16 collaborate to enhance an anti-apoptotic potential in the side population of refractory mantle cell lymphoma

Abstract

The proto-oncogene BMI1 and its product, Bmi1, is overexpressed in various types of tumors, particularly in aggressive tumors and tumors resistant to conventional chemotherapy. BMI1/Bmi1 is also crucially involved in cancer-initiating cell maintenance, and is recurrently upregulated in mantle cell lymphoma (MCL), especially aggressive variants. Recently, side population (SP) cells were shown to exhibit tumor-initiating characteristics in various types of tumors. In this study, we show that recurrent MCL cases significantly exhibit upregulation of BMI1/Bmi1. We further demonstrate that clonogenic MCL SP shows such tumor-initiating characteristics as high tumorigenicity and self-renewal capability, and that BMI1 was upregulated in the SP from recurrent MCL cases and MCL cell lines. On screening for upstream regulators of BMI1, we found that expression of microRNA-16 (miR-16) was downregulated in MCL SP cells by regulating Bmi1 in the SPs, leading to reductions in tumor size following lymphoma xenografts. Moreover, to investigate downstream targets of BMI1 in MCL, we performed cross-linking/chromatin immunoprecipitation assay against MCL cell lines and demonstrated that Bmi1 directly regulated pro-apoptotic genes such as BCL2L11/Bim and PMAIP1/Noxa, leading to enhance anti-apoptotic potential of MCL. Finally, we found that a proteasome inhibitor bortezomib, which has been recently used for relapsed MCL, effectively induced apoptosis among MCL cells while reducing expression of Bmi1 and increasing miR-16 in MCL SP. These results suggest that upregulation of BMI1 and downregulation of miR-16 in MCL SP has a key role in the disease's progression by reducing MCL cell apoptosis. Our results provide important new insight into the pathogenesis of MCL and strongly suggest that targeting BMI1/Bmi1 might be an effective approach to treating MCL, particularly refractory and recurrent cases.

MicroRNA-150 inhibits tumor invasion and metastasis by targeting the chemokine receptor CCR6, in advanced cutaneous T-cell lymphoma

“…[3][4][5][6] Our earlier work also demonstrated miRNA expression in aggressive lymphomas, suggesting that such alterations may be deeply associated with the progression of malignant lymphomas. 9,18,[42][43][44] This makes it tempting to suggest that miR-150 downregulation is a crucial component of advanced CTCL. That remains to be tested, however.…”

“…Recently, we demonstrated that human lymphoma cell lines could be grafted into immunodeficient nonobese diabetic/NOG mice. 18 To examine if transduction of miR-150 might affect tumor formation, we subcutaneously inoculated miR-150-transfected or control cells (2 3 10 5 ; n 5 5 each) into NOG mice, and found that cells expressing miR-150 produced significantly smaller tumors (My-La, MJ, and HUT78), or at least showed a trend toward smaller tumors (HH and ATN-1) (supplemental Figure 2). Furthermore, NOG mice inoculated with GFP-control MyLa cells died within 31 to 37 days, while mice receiving GFP-control HH cells died within 55 to 70 days.…”

Role of miR-15/16 in CLL

“…For example, it has been shown that miR-15/16 inhibits MCL1 oncogene in CLL BMI1 oncogene in mantle call lymphoma. 29,31 Two other recent reports demonstrated that miR-15/16 directly target critical cell cycle regulator Cyclin D1 in bladder cancer and osteosarcoma. 32,33 Another study showed that miR-15/16 expression is downregulated in malignant pleural mesothelioma (MPM).…”

Role of microRNA in the pathogenesis of malignant lymphoma

“…miR-17-92 is frequently upregulated in MCL, and because miR-17-92 appears to negatively regulate CDKN1A ⁄ p21, (19) increases in its expression could enhance cell cycle progression. miR-16-1 is expressed normally in MCL, as compared to its expression in normal CD5 + B-cells, (42,43) but Chen et al demonstrate that the 3′UTR of CCND1 is frequently truncated or mutated in MCL, which inhibits the interaction of miR-16-1 with the "seed" sequence of the 3′UTR of CCND1, thereby contributing to continuous upregulation of CyclinD1. (43) In addition, some miRNA have been shown to act as regulators of polycomb-group repressive complex proteins.…”