Small molecule inhibition of group I p21-activated kinases in breast cancer induces apoptosis and potentiates the activity of microtubule stabilizing agents

Abstract

IntroductionBreast cancer, the most common cause of cancer-related deaths worldwide among women, is a molecularly and clinically heterogeneous disease. Extensive genetic and epigenetic profiling of breast tumors has recently revealed novel putative driver genes, including p21-activated kinase (PAK)1. PAK1 is a serine/threonine kinase downstream of small GTP-binding proteins, Rac1 and Cdc42, and is an integral component of growth factor signaling networks and cellular functions fundamental to tumorigenesis.MethodsPAK1 dysregulation (copy number gain, mRNA and protein expression) was evaluated in two cohorts of breast cancer tissues (n = 980 and 1,108). A novel small molecule inhibitor, FRAX1036, and RNA interference were used to examine PAK1 loss of function and combination with docetaxel in vitro. Mechanism of action for the therapeutic combination, both cellular and molecular, was assessed via time-lapse microscopy and immunoblotting.ResultsWe demonstrate that focal genomic amplification and overexpression of PAK1 are associated with poor clinical outcome in the luminal subtype of breast cancer (P = 1.29 × 10−4 and P = 0.015, respectively). Given the role for PAK1 in regulating cytoskeletal organization, we hypothesized that combination of PAK1 inhibition with taxane treatment could be combined to further interfere with microtubule dynamics and cell survival. Consistent with this, administration of docetaxel with either a novel small molecule inhibitor of group I PAKs, FRAX1036, or PAK1 small interfering RNA oligonucleotides dramatically altered signaling to cytoskeletal-associated proteins, such as stathmin, and induced microtubule disorganization and cellular apoptosis. Live-cell imaging revealed that the duration of mitotic arrest mediated by docetaxel was significantly reduced in the presence of FRAX1036, and this was associated with increased kinetics of apoptosis.ConclusionsTaken together, these findings further support PAK1 as a potential target in breast cancer and suggest combination with taxanes as a viable strategy to increase anti-tumor efficacy.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-015-0564-5) contains supplementary material, which is available to authorized users.

Proteogenomics connects somatic mutations to signalling in breast cancer

“…Overall, multiple outlier kinases generate testable therapeutic hypotheses for which enabling inhibitors are in development. For example PAK1 has recently been confirmed to be a therapeutic target and poor prognosis factor in luminal breast cancer 40 .…”

Structure, biochemistry, and biology of PAK kinases

“…It may be because of their undesirable pharmacological properties such as drug efflux (Bradshaw-Pierce, et al, 2013). More recently, a derivative of FRAX-597, the FRAX-1306, has been shown to have high selectivity against PAK4, but may also have off-target receptor tyrosine kinases (Ong, et al, 2015). Similarly, another PAK1 inhibitor IPA3 was found to be not suitable for clinical use, but now serves as a good tool to dissect the intricacies of PAK1 signaling in model systems (Singhal et al, 2012).…”

Targeting PAK1

“…An advanced member of this series, FRAX1036 (PDB ID:5DFP), exhibits high PAK1 potency (PAK1 Ki = 23 nM), refined kinome selectivity [42, 46, 47], and represents a useful tool compound for single and combinatorial experimental therapeutics [42, 46–48]. However, all of these early FRAX compounds were found to have strong adverse inhibition of hERG potassium channels.…”

“…To address this issue, better selection of tumor types may be required. Recent work from our group and others [46, 47, 68] suggests that using PAK1 gene amplification as a biomarker might select for tumor cells that are sensitive to PAK1 inhibition, and it will be instructive to test NVS-PAK1-1 and similar isoform-selective Pak inhibitors in this setting. Similarly, tumors bearing activating mutations in the small GTPase RAC1 ( e.g ., ~5% of melanoma) [69, 70], which encodes a direct activator of group I Paks, might be good candidates for consideration for future anti-Pak agents.…”