The BMI1 polycomb protein represses cyclin G2-induced autophagy to support proliferation in chronic myeloid leukemia cells

Abstract

The BMI1 polycomb protein regulates self-renewal, proliferation and survival of cancer-initiating cells essentially through epigenetic repression of the CDKN2A tumor suppressor locus. We demonstrate here for the first time that BMI1 also prevents autophagy in chronic myeloid leukemia (CML) cell lines, to support their proliferation and clonogenic activity. Using chromatin immunoprecipitation, we identified CCNG2/cyclin G2 (CCNG2) as a direct BMI1 target. BMI1 downregulation in CD34+ CML cells by PTC-209 pharmacological treatment or shBMI1 transduction triggered CCNG2 expression and decreased clonogenic activity. Also, ectopic expression of CCNG2 in CD34+ CML cells strongly decreased their clonogenicity. CCNG2 was shown to act by disrupting the phosphatase 2A complex, which activates a PKCζ-AMPK-JNK-ERK pathway that engages autophagy. We observed that BMI1 and CCNG2 levels evolved inversely during the progression of CML towards an acute deadly phase, and therefore hypothesized that BMI1 could support acute transformation of CML through the silencing of a CCNG2-mediated tumor-suppressive autophagy response.

Inhibition of BMI1 induces autophagy-mediated necroptosis

“…Interestingly in chronic myeloid leukemia cells, PTC-209 treatment induces CCNG2 expression and CCNG2-mediated autophagy. 9 However, PTC-209 or BMI1 siRNA did not induce CCNG2 indicating absence of such regulation in OvCa cells. In colorectal cancer cells, a 4 d treatment with PTC-209 reduces the frequency of sphere initiation from viable cells.…”

“…These include derepression of CDKN2A/ INK4/ARF in normal neural stem cells, induction of CCNG2-mediated autophagy in leukemic cells and induction of apoptosis in colorectal cancer-initiating cells. 4,8,9 We have previously demonstrated that BMI1 is overexpressed in high-grade serous OvCa patient samples 5 and targeting BMI1 decreases clonal ) and MMP which leads to reduced intracellular ATP. Depletion of ATP stimulates the autophagic stress signal and also activates AMPK.…”

“…Interestingly, in chronic myeloid leukemia cells, treatment with PTC-209 induces CCNG2 expression and CCNG2-mediated autophagy. 9 However, PTC-209 or BMI1 siRNA did not induce CCNG2 indicating absence of such regulation in OvCa cells (Fig. S2).…”

“…3,4 Targeting BMI1 significantly decreases clonal growth, the mechanisms though differ and include derepression of CDKN2A/INK4/ARF in normal neural stem cells, induction of CCNG2 in leukemic cells and induction of apoptosis in colorectal cancer cells. 4,8,9 While decreased self-renewal of neural stem cells has been attributed to the derepression of the CDKN2A/INK4/ARF locus, 10,11 dual deletion of CDKN2A/INK4/ARF in the bmi1 ¡/¡ background only partially rescues neural development indicating regulation of additional pathways by BMI1. 8 Accordingly, in a CDKN2A/INK4/ARF-independent manner, BMI1 regulates mitochondrial function as evidenced by interruption of the electron transport chain and decreased ATP levels in thymocytes from mice lacking BMI1.…”

CML patients with deep molecular responses to TKI have restored immune effectors and decreased PD-1 and immune suppressors

“…[20][21][22] A phase 1 trial (NCT02011945) of dasatinib and nivolumab, a PD-1 inhibitor, is recruiting, with MMR and DMR incidence as planned outcome measures. LAAs such as WT1, PRAME, PR3, and BMI-1 in CML patients represent attractive targets for immunotherapy, [23][24][25][26][27][28][29] with efficacy demonstrated in vaccination, including dendritic cell vaccination, [30][31][32][33] and, more recently, T-cell receptor-mimic antibodies. [34][35][36] In support of these studies, we observed maximal restoration of immune recovery, as demonstrated by increased effector NK cell and T-cell immune responses, reduced PD-1 inhibitory molecule expression on CD4 1 and CD8 1 T cells, and reduced numbers of Mo-MDSC in MR 4.5 only, suggesting DMR may be the preferred threshold for future TFR studies to benefit from the improved effector immune responses.…”