Bmi-1 collaborates with c-Myc in tumorigenesis by inhibiting c-Myc-induced apoptosis via INK4a/ARF

Abstract

The bmi-1 and myc oncogenes collaborate strongly in murine lymphomagenesis, but the basis for this collaboration was not understood. We recently identified the ink4a-ARF tumor suppressor locus as a critical downstream target of the Polycomb-group transcriptional repressor Bmi-1. Others have shown that part of Myc's ability to induce apoptosis depends on induction of p19arf. Here we demonstrate that down-regulation of ink4a-ARF by Bmi-1 underlies its ability to cooperate with Myc in tumorigenesis. Heterozygosity for bmi-1 inhibits lymphomagenesis in Eµ-myc mice by enhancing c-Myc-induced apoptosis. We observe increased apoptosis in bmi-1 −/− lymphoid organs, which can be rescued by deletion of ink4a-ARF or overexpression of bcl2. Furthermore, Bmi-1 collaborates with Myc in enhancing proliferation and transformation of primary embryo fibroblasts (MEFs) in an ink4a-ARF dependent manner, by prohibiting Myc-mediated induction of p19arf and apoptosis. We observe strong collaboration between the Eµ-myc transgene and heterozygosity for ink4a-ARF, which is accompanied by loss of the wild-type ink4a-ARF allele and formation of highly aggressive B-cell lymphomas. Together, these results reinforce the critical role of Bmi-1 as a dose-dependent regulator of ink4a-ARF, which on its turn acts to prevent tumorigenesis on activation of oncogenes such as c-myc.

MIF loss impairs Myc-induced lymphomagenesis

“…Consistent with previous studies on tumor distribution in EmMyc mice, [31][32][33][34] MIF-KO lymphomas involved many of the peripheral lymph nodes, including branchial, cervical, inguinal, and mesenteric sites. Flow cytometric analysis revealed that MIF-KO tumors were B220 þ IgM À pre-B-cell lymphomas and B220 þ IgM þ mature B-cell lymphomas (Figure 1b), again similar to tumors from Em-Myc mice.…”

“…Flow cytometric analysis revealed that MIF-KO tumors were B220 þ IgM À pre-B-cell lymphomas and B220 þ IgM þ mature B-cell lymphomas (Figure 1b), again similar to tumors from Em-Myc mice. [31][32][33][34] Despite these similarities, MIF-deficient lymphomas displayed more pronounced 'starry sky' morphology, indicative of the extensive apoptosis (Figure 1c, d, and Supplementary Figure 1a). In situ TUNEL analysis of representative tumor sections confirmed that apoptosis levels were higher in lymphomas derived from MIF-KO Em-Myc mice (Supplementary Figure 1b).…”

“…Previous studies demonstrated that the majority of tumors (50-60%) arising in Em-Myc transgenic mice sustain p53 mutations or biallelic ARF deletion, almost always in a mutually exclusive fashion. [31][32][33][34][35] The exact mechanism of acquisition of such mutations is presently unknown, although it likely involves Myc-induced genomic instability. 37 Immunoblot analysis revealed that seven out of 24 examined MIF-KO Em-Myc tumors (29%) contained elevated levels of p53, indicative of p53 mutations (examples in Figure 4a and Supplementary Figure 4a).…”

“…[31][32][33][34][35] Moreover, ARF and p53 loss of function contribute to the majority of the Mycinduced lymphomas arising in MIF-KO animals. However, MIF-KO tumors exhibit a significantly higher frequency of ARF mutations (approximately threefold) than MIF-WT tumors.…”

Distinctive expression of the polycomb group proteins Bmi1 polycomb ring finger oncogene and enhancer of zeste homolog 2 in nonsmall cell lung cancers and their clinical and clinicopathologic significance

“…5,6 The Bmi1 polycomb ring finger oncogene (Bmi1), a key component of the PRC1 complex, was identified initially as an oncogene that cooperates with c-myc in the generation of B-cell lymphoma. 7,8 The oncogenic potential of Bmi1 is caused in part by its negative regulation of the Ink4a/Arf locus, which encodes 2 proteins (p16 INK4a and p19 ARF ) that suppress proliferation and promote apoptosis, 9,10 and its role in stem cell maintenance. 11,12 Elevated expression of Bmi1 has been reported in multiple types of cancers, including oral cancer, 13 lymphoma, 14,15 and breast cancer.…”