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 viabi... More
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 EC50. 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， a more comprehensive index composed of Mcl-1， Bcl-xL， Bim， Bak and Noxa protein or mRNA expression correlates well with Mcl-1 sensitivity in TNBC and can also predict Mcl-1 dependency in non-small cell lung cancer cell lines.
