Myeloid cell leukemia-1 is an important apoptotic survival factor in triple-negative breast cancer

Abstract

Breast cancer is the second-most frequently diagnosed malignancy in US women. The triple-negative breast cancer (TNBC) subtype, which lacks expression of the estrogen receptor, progesterone receptor and human epidermal growth factor receptor-2, afflicts 15% of patients and is refractory to current targeted therapies. Like many cancers, TNBC cells often deregulate programmed cell death by upregulating anti-apoptotic proteins of the B-cell CLL/lymphoma 2 (Bcl-2) family. One family member, myeloid cell leukemia-1 (Mcl-1), is commonly amplified in TNBC and correlates with a poor clinical prognosis. Here we show the effect of silencing Mcl-1 and Bcl-2-like protein 1 isoform 1 (Bcl-xL) expression on viability in a panel of seventeen TNBC cell lines. Cell death was observed in a subset upon Mcl-1 knockdown. In contrast, Bcl-xL knockdown only modestly reduced viability, indicating that Mcl-1 is a more important survival factor. However, dual silencing of both Mcl-1 and Bcl-xL reduced viability in most cell lines tested. These proliferation results were recapitulated by BH3 profiling experiments. Treatment with a Bcl-xL and Bcl-2 peptide had only a moderate effect on any of the TNBC cell lines, however, co-dosing an Mcl-1-selective peptide with a peptide that inhibits Bcl-xL and Bcl-2 was effective in each line tested. Similarly, the selective Bcl-xL inhibitor WEHI-539 was only weakly cytotoxic across the panel, but sensitization by Mcl-1 knockdown markedly improved its EC 50 . ABT-199, which selectively inhibits Bcl-2, did not synergize with Mcl-1 knockdown, indicating the relatively low importance of Bcl-2 in these lines. Mcl-1 sensitivity is not predicted by mRNA or protein levels of a single Bcl-2 family member, except for only a weak correlation for Bak and Bax protein expression. However Breast cancer is the second-most frequently diagnosed malignancy in US women with 230 000 new cases and 40 000 deaths in 2011. The triple-negative breast carcinoma (TNBC) subtype, which does not express the estrogen receptor (ER) and progesterone receptor (PR) and lacks overexpression of human epidermal growth factor receptor-2 (HER2), afflicts nearly 15% of all breast cancer patients and remains refractory to currently available endocrine and HER2-directed therapies.1,2 The current standard of care for TNBC is radiation and neoadjuvant cytotoxic chemotherapy, and carries a poor clinical prognosis. [3][4][5] As with most cancers, TNBC cells are under metabolic and oncogenic stress and require inhibition of the intrinsic apoptotic pathway for survival. 6 Under normal physiological conditions, this pathway is tightly regulated by both pro-and anti-apoptotic members of the B-cell CLL/lymphoma 2 (Bcl-2) family. Stressors such as DNA damage, hypoxia or oncogenic signaling, cause increased expression or translocation of proapoptotic Bcl-2 family members, such as Bim, Bad and Noxa, to the mitochondria. 7 These proteins subsequently trigger pore formation in the mitochondrial outer membrane via induced multimerization of Bak or Bax, a proc...

Human Cytomegalovirus Promotes Survival of Infected Monocytes via a Distinct Temporal Regulation of Cellular Bcl-2 Family Proteins

“…Indeed, we speculate that Mcl-1 functions as a molecular clock in this cell type to ensure a controlled population of unstimulated monocytes (49). This idea is consistent with what is known about the antiapoptotic role of Mcl-1 in hematopoietic cells (77) and the way in which dysregulation of Mcl-1 contributes to the development of myeloid (78)(79)(80) and other cancers (81)(82)(83)(84)(85)(86). Mcl-1 was first identified as a result of its overexpression during TPA (12-O-tetradecanoyl phorbol 13-acetate)-induced differentiation of a human myeloid leukemia cell line (ML-1) (87).…”

“…Subsequent studies of Mcl-1 have classified the protein as a member of the Bcl-2 family of mitochondrial membrane permeability regulators and defined a dual role for Mcl-1 in myeloid cells: regulating both cell survival and differentiation (77,88). It is the prosurvival function of Mcl-1 that appears to drive the development of myeloid leukemias and other cancers (79,(81)(82)(83)(84)(85)89) in addition to contributing to their drug resistance (89)(90)(91). We have shown that HCMV upregulates Mcl-1 in a transient manner, effectively usurping its prosurvival function to promote early survival of HCMV-infected monocytes (49).…”

BH3 mimetics: Their action and efficacy in cancer chemotherapy

“…Moreover, it has been demonstrated that the survival and proliferation of several cancer cell lines and experimental cancers depend on MCL-1 [32][33][34][35]. In contrast to the importance of MCL-1 expression in cancers, clinically-effective BH3 mimetics ABT-263 and ABT-199 cannot affect MCL-1 (Table 1).…”

“…MS1 (MCL-1-specific peptide) [67] MCL-1 MS1 induced apoptosis in MCL-1-dependent triple-negative breast cancer cells [33] Data on the specificity of MS1 have been published only at high concentrations (100 µM) in triple-negative breast cancer cells [33].…”

Myeloid cell leukemia 1 (MCL-1), an unexpected modulator of protein kinase signaling during invasion

“…5 MCL-1 protein levels correlate with outcome, tumor grade and therapeutic resistance in many cancers including those of the hematopoietic system, breast, lung and pancreas. [6][7][8][9][10][11][12] MCL-1 has been validated to participate in neoplastic progression of B-cell lymphomas 13 as well as haematopoietic progenitor/stem cell tumours 14 and accelerate Myc induced myeloid leukaemia. 15 Several studies have also shown that MCL-1 is a barrier to therapeutic sensitivity, including those that target BCL-2 (reviewed in 16 ).…”