Small Molecule Inhibition of Group I p21-Activated Kinases in Breast Cancer Induces Apoptosis and Potentiates the Activity of Microtubule Stabilizing Agents

Christy C Ong; Sarah Gierke; Cameron Pitt; Meredith Sagolla; Christine K Cheng; Wei Zhou; Adrian M Jubb; Laura Strickland; Maike Schmidt; Sergio G Duron; David A Campbell; Wei Zheng; Seameen Dehdashti; Min Shen; Nora Yang; Mark L Behnke; Wenwei Huang; John C McKew; Jonathan Chernoff; William F Forrest; Peter M Haverty; Suet-Feung Chin; Emad A Rakha; Andrew R Green; Ian O Ellis; Carlos Caldas; Thomas O'Brien; Lori S Friedman; Hartmut Koeppen; Joachim Rudolph; Klaus P Hoeflich

Disclosures

Breast Cancer Res. 2015;17(59) 

In This Article

Discussion

The advent of high-throughput techniques for genetic and epigenetic characterization of tumor specimens has led to an exponential increase in our understanding of molecular events underlying the process of carcinogenesis. This is especially true for breast cancer, an indication in which tumor tissues can be successfully obtained and analyzed with high frequency. Typically, novel putative driver genes for breast cancer have been preliminarily evaluated using genetic and knockdown approaches. However, more comprehensive and rigorous assessment of intracellular targets for therapeutic intervention requires selective, potent and cell-active small molecules with good biochemical and cellular properties. FRAX1036 displays selectivity for PAK1-3 relative to group II PAK members as well as other kinases (Additional file 2: Table S1 http://www.breast-cancer-research.com/content/17/1/59/additional) and can be used as a tool compound for in vitro target validation experiments.

Interestingly, PAK1 genomic amplification or protein overexpression are strongly associated with poor outcome for luminal (or estrogen receptor-positive) breast cancer patients (Figure 1; Additional file 1: Figure S1 http://www.breast-cancer-research.com/content/17/1/59/additional). A subset of breast carcinomas without genomic amplification also display high mRNA and protein expression of PAK1 (Figure 1A; Additional file 1: Figure S1B http://www.breast-cancer-research.com/content/17/1/59/additional). The molecular mechanisms underpinning dysregulated PAK1 expression in the absence of genomic amplification are not well characterized, although regulation by microRNAs[28] and gene translocation (Peter Haverty, unpublished data) have both been observed. PAK1 copy number alterations have also been observed in other tumor indications, such as ovarian cancer and melanoma[16,29] and further validation efforts are necessary to apply the findings reported here to these other indications.

Hormone receptor-positive breast cancer patients with localized disease receive front-line treatment with endocrine therapies, such as tamoxifen or aromatase inhibitors. There is some evidence that PAK1 may directly phosphorylate estrogen receptor-α[30] or components of the estrogen receptor multi-protein complex.[31] However, the potential roles for PAK1 inhibition in combination with later lines of therapy, such as taxanes, have yet to be explored. Given the evolutionarily conserved role of PAK1 in regulating cytoskeletal dynamics and the common use of microtubule inhibitors in later lines of breast cancer treatment, we evaluated the mechanism and potential therapeutic benefit of FRAX1036 combination with docetaxel. We show that signaling changes elicited by FRAX1036 and docetaxel potentiate apoptosis of breast cancer cells and that microtubule morphology is affected by both pathways (Figure 4A). Combination treatment of FRAX1036 significantly diminished time in docetaxel-induced mitotic arrest (Figure 4C; Additional file 5: Figure S5 http://www.breast-cancer-research.com/content/17/1/59/additional), pushed cell fate from mitotic slippage to apoptosis (Figure 4B) and accordingly increased the kinetics of breast tumor cell apoptosis (Figure 3C-F; Additional file 3: Figure S2 http://www.breast-cancer-research.com/content/17/1/59/additional). Given that luminal breast cancer patients generally do not respond durably to chemotherapy, combination of FRAX1036 with taxanes may help address unmet needs for patients with advanced and metastatic disease.

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