Why do BCL-2 inhibitors work and where should we use them in the clinic?

Intrinsic apoptosis is controlled by the BCL-2 family of proteins but the complexity of intra-family interactions makes it challenging to predict cell fate via standard molecular biology techniques. We discuss BCL-2 family regulation and how to determine cells' readiness for apoptosis and anti-apoptotic dependence. Cancer cells often adopt anti-apoptotic defense mechanisms in response to oncogenic stress or anti-cancer therapy. However, by determining their anti-apoptotic addiction, we can use novel BH3 mimetics to overwhelm this apoptotic blockade. We outline the development and uses of these unique anti-apoptotic inhibitors and how to possibly combine them with other anti-cancer agents using dynamic BH3 profiling (DBP) to improve personalized cancer treatment.

Inhibition of MAPKinase pathway sensitizes thyroid cancer cells to ABT-737 induced apoptosis.

Bcl2 family proteins play an important role in the resistance of thyroid cancer cells to apoptosis induced by chemotherapeutic drugs and targeted therapies. BH3-profiling of seven fresh primary papillary thyroid cancer (PTC) tumors showed dependence for survival on Bcl-xL (2/7), Bcl2 (2/7), and Mcl-1 (2/7), while the majority of thyroid cell lines were mainly dependent on Bcl-xL. Targeting Bcl2 family proteins with the BH3 mimetic, ABT-737, while simultaneously inhibiting ERK pathway proteins with PLX4720 and PD325901 was shown to induce significantly high apoptosis in the majority of cell lines (8505c, SW1736, HTh7, BCPAP) and moderate apoptosis in the TPC-1 cell line. In orthotopic thyroid cancer mouse models of 8505c and BCPAP, treatment with the triple drug combination reduced the size of the tumors and showed significantly higher numbers of cells undergoing apoptosis. This treatment increased the expression of pro-apoptotic protein Bim, while decreasing anti-apoptotic protein Mcl-1. Our results suggest that analyzing the results of BH3-profiling along with the mutational status of tumor can reveal an effective therapy for targeted, personalized treatment of aggressive thyroid cancer.

Spatial Proximity to Fibroblasts Impacts Molecular Features and Therapeutic Sensitivity of Breast Cancer Cells Influencing Clinical Outcomes.

Using a three-dimensional coculture model, we identified significant subtype-specific changes in gene expression, metabolic, and therapeutic sensitivity profiles of breast cancer cells in contact with cancer-associated fibroblasts (CAF). CAF-induced gene expression signatures predicted clinical outcome and immune-related differences in the microenvironment. We found that fibroblasts strongly protect carcinoma cells from lapatinib, attributable to its reduced accumulation in carcinoma cells and an elevated apoptotic threshold. Fibroblasts from normal breast tissues and stromal cultures of brain metastases of breast cancer had similar effects as CAFs. Using synthetic lethality approaches, we identified molecular pathways whose inhibition sensitizes HER2+ breast cancer cells to lapatinib both in vitro and in vivo, including JAK2/STAT3 and hyaluronic acid. Neoadjuvant lapatinib therapy in HER2+ breast tumors lead to a significant increase of phospho-STAT3+ cancer cells and a decrease in the spatial proximity of proliferating (Ki67+) cells to CAFs impacting therapeutic responses. Our studies identify CAF-induced physiologically and clinically relevant changes in cancer cells and offer novel approaches for overcoming microenvironment-mediated therapeutic resistance. Cancer Res; 76(22); 6495-506. ©2016 AACR.

Response and resistance to BET bromodomain inhibitors in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is a heterogeneous and clinically aggressive disease for which there is no targeted therapy. BET bromodomain inhibitors, which have shown efficacy in several models of cancer, have not been evaluated in TNBC. These inhibitors displace BET bromodomain proteins such as BRD4 from chromatin by competing with their acetyl-lysine recognition modules, leading to inhibition of oncogenic transcriptional programs. Here we report the preferential sensitivity of TNBCs to BET bromodomain inhibition in vitro and in vivo, establishing a rationale for clinical investigation and further motivation to understand mechanisms of resistance. In paired cell lines selected for acquired resistance to BET inhibition from previously sensitive TNBCs, we failed to identify gatekeeper mutations, new driver events or drug pump activation. BET-resistant TNBC cells remain dependent on wild-type BRD4, which supports transcription and cell proliferation in a bromodomain-independent manner. Proteomic studies of resistant TNBC identify strong association with MED1 and hyper-phosphorylation of BRD4 attributable to decreased activity of PP2A, identified here as a principal BRD4 serine phosphatase. Together, these studies provide a rationale for BET inhibition in TNBC and present mechanism-based combination strategies to anticipate clinical drug resistance.

Targeting the miR-221-222/PUMA/BAK/BAX Pathway Abrogates Dexamethasone Resistance in Multiple Myeloma.

Despite recent therapeutic advances that have doubled the median survival time of patients with multiple myeloma, intratumor genetic heterogeneity contributes to disease progression and emergence of drug resistance. miRNAs are noncoding small RNAs that play important roles in the regulation of gene expression and have been implicated in cancer progression and drug resistance. We investigated the role of the miR-221-222 family in dexamethasone-induced drug resistance in multiple myeloma using the isogenic cell lines MM1R and MM1S, which represent models of resistance and sensitivity, respectively. Analysis of array comparative genome hybridization data revealed gain of chromosome X regions at band p11.3, wherein the miR-221-222 resides, in resistant MM1R cells but not in sensitive MM1S cells. DNA copy number gains in MM1R cells were associated with increased miR-221-222 expression and downregulation of p53-upregulated modulator of apoptosis (PUMA) as a likely proapoptotic target. We confirmed PUMA mRNA as a direct target of miR-221-222 in MM1S and MM1R cells by both gain-of-function and loss-of-function studies. In addition, miR-221-222 treatment rendered MM1S cells resistant to dexamethasone, whereas anti-miR-221-222 partially restored the dexamethasone sensitivity of MM1R cells. These studies have uncovered a role for miR-221-222 in multiple myeloma drug resistance and suggest a potential therapeutic role for inhibitors of miR-221-222 binding to PUMA mRNA as a means of overcoming dexamethasone resistance in patients. The clinical utility of this approach is predicated on the ability of antisense miR-221-222 to increase survival while reducing tumor burden and is strongly supported by the metastatic propensity of MM1R cells in preclinical mouse xenograft models of multiple myeloma. Moreover, our observation of increased levels of miR-221-222 with decreased PUMA expression in multiple myeloma cells from patients at relapse versus untreated controls suggests an even broader role for miR-221-222 in drug resistance and provides a rationale for the targeting of miR-221-222 as a means of improving patient outcomes.