Key Survival Factor, Mcl-1, Correlates with Sensitivity to Combined Bcl-2/Bcl-xL Blockade

Abstract

An estimated 40,000 deaths will be attributed to breast cancer in 2016, underscoring the need for improved therapies. Evading cell death is a major hallmark of cancer, driving tumor progression and therapeutic resistance. To evade apoptosis, cancers use anti-apoptotic Bcl-2 proteins to bind to and neutralize apoptotic activators, such as Bim. Investigation of anti-apoptotic Bcl-2 family members in clinical breast cancer datasets, revealed greater expression and more frequent gene amplification of MCL1 as compared to BCL2 or BCL2L1 (Bcl-xL) across three major molecular breast cancer subtypes, Luminal (A and B), HER2-enriched, and Basal-like. While Mcl-1 protein expression was elevated in estrogen receptor α (ERα)-positive and ERα-negative tumors as compared to normal breast, Mcl-1 staining was higher in ERα+ tumors. Targeted Mcl-1 blockade using RNAi increased caspase-mediated cell death in ERα+ breast cancer cells, resulting in sustained growth inhibition. In contrast, combined blockade of Bcl-2 and Bcl-xL only transiently induced apoptosis, as cells rapidly acclimated through Mcl-1 upregulation and enhanced Mcl-1 activity, as measured in situ using Mcl-1/Bim proximity ligation assays. Importantly, MCL1 gene expression levels correlated inversely with sensitivity to pharmacological Bcl-2/Bcl-xL inhibition in luminal breast cancer cells, whereas no relationship was seen between gene expression of BCL2 or BCL2L1 and sensitivity to Bcl-2/Bcl-xL inhibition. These results demonstrate that breast cancers rapidly deploy Mcl-1 to promote cell survival, particularly when challenged with blockade of other Bcl-2 family members, warranting the continued development of Mcl-1 selective inhibitors for targeted tumor cell killing.

MCL-1 is a prognostic indicator and drug target in breast cancer

“…The contrasting findings of these studies (notably with smaller patient numbers of 170 and 125, respectively), with no separation by disease subtype could be explained if a prognostic role for MCL-1 occurred only in particular subsets of patients. More recently, MCL-1 was shown to be widely expressed in breast tumours, regardless of subtype or ER status 10 , 24 although these studies did not report patient outcome. Intriguingly, low levels of MCL-1 protein were correlated with poor prognosis in a cohort of Luminal A breast cancer patients 24 .…”

“…In contrast to our findings with MCL-1, BCL-2 has been shown to be a favourable prognostic marker in breast cancer, often associated with slowly proliferating low grade ER-positive tumours 43 – 45 . High BCL-2 protein expression predicts favourable outcome regardless of ER, PR or HER2 status 46 however, BCL2 is only expressed in a small proportion of TN breast cancers 7 and MCL1 mRNA is higher than BCL2 or BCL2L1 (BCL-XL) across all subtypes of breast cancer 10 . We find an inverse relationship between MCL1 and BCL2 mRNA (Fig.…”

“…Due to differential binding affinities, various BH3-mimetics display specificity for particular anti-apoptotic BCL-2 proteins. BH3-mimetics targeting BCL-2/BCL-XL have also shown promise in preclinical studies of solid tumours, including breast, when used in combination with docetaxel or tamoxifen 7 , 8 but resistance can be mediated by MCL-1 9 , 10 . In addition to differential BH3-binding properties, MCL-1 is distinguished by its short protein half-life and ability to regulate mitochondrial metabolism 11 , 12 .…”

“…This oncogenic role for MCL-1 may be widespread as the MCL1 locus is one of the most frequently amplified regions of the human genome across a wide variety of cancers including breast cancer 18 . Recent evidence from in vitro experiments suggests an important role for MCL-1 in breast cancer cell survival 10 , 19 , 20 , particularly in triple-negative (TN) breast cancers 21 – 23 and expression of a mutant form of BIM that specifically interacts with MCL-1 inhibits metastases of TN breast cancer cell lines in xenograft models 24 . TN breast cancers are aggressive with poor patient prognosis and because they lack expression of the ER and the progesterone receptor (PR) and do not have amplification of ERBB2 , they do not respond to current targeted therapies.…”

BCL-2 family deregulation in colorectal cancer: potential for BH3 mimetics in therapy

“…Several tumor types show heterogeneous dependence on MCL-1 including breast, lung, multiple myeloma (MM) and MYC-driven lymphomas [139]. Interestingly, there is evidence of MCL-1 expression levels increasing upon treatment with ABT-737 and this is implicated in resistance to other BH3 mimetics such as Navitoclax and Venetoclax, which do not inhibit MCL-1 [156,157]. MCL-1 is also involved in resistance to various chemotherapeutics, making it a promising target for combination and second-line treatment in resistant tumors [158,159].…”