Molecularly targeted drug therapies have revolutionized cancer treatment; however, resistance remains a major limitation to their overall efficacy. Epithelial-to-mesenchymal transition (EMT) has been linked to acquired resistance to tyrosine kinase inhibitors (TKI), independent of mutational resistance mechanisms. AXL is a receptor tyrosine kinase associated with EMT that has been implicated in drug resistance and has emerged as a candidate therapeutic target. Across 643 human cancer cell lines that were analyzed, elevated AXL was strongly associated with a mesenchymal phenotype, particularly in triple-negative breast cancer and non-small cell lung cancer. In an unbiased screen of small-molecule inhibitors of cancer-relevant processes, we discovered that AXL inhibition was specifically synergistic with antimitotic agents in killing cancer cells that had undergone EMT and demonstrated associated TKI resistance. However, we did not find that AXL inhibition alone could overcome acquired resistance to EGFR TKIs in the EMT setting, as previously reported. These findings reveal a novel cotreatment strategy for tumors displaying mesenchymal features that otherwise render them treatment refractory. Cancer Res; 74(20); 5878-90. Ó2014 AACR.

Melanoma inhibitor of apoptosis (ML-IAP) is a potent inhibitor of apoptosis, which is highly expressed in melanomas and likely contributes to their resistance to chemotherapeutic treatments. Herein, we show that the lineage survival oncogene microphthalmia-associated transcription factor (MITF) is a critical regulator of ML-IAP transcription in melanoma cells. The ML-IAP promoter contains two MITF consensus sites, and analysis of MITF and ML-IAP mRNA levels revealed a high correlation in melanoma tumor samples and cell lines. In reporter assays, MITF promoted a strong stimulation of transcriptional activity from the ML-IAP promoter, and MITF bound the endogenous ML-IAP promoter in melanoma cells by chromatin immunoprecipitation and electrophoretic mobility shift assay. Strikingly, small interfering RNA (siRNA)-mediated knockdown of MITF in melanoma cells led to a dramatic decrease in ML-IAP mRNA and protein levels, establishing that ML-IAP expression in melanoma cells is MITF dependent. Additionally, cyclic AMP-mediated induction of MITF expression in melanocytes resulted in increased ML-IAP expression, suggesting that melanocytes can express ML-IAP when MITF levels are heightened. Disruption of MITF by siRNA led to a decrease in melanoma cell viability, which could be rescued by ectopic expression of ML-IAP. Collectively, these findings implicate MITF as a major transcriptional regulator of ML-IAP expression in melanomas, and suggest that ML-IAP contributes to the prosurvival activity of MITF in melanoma progression. [Cancer Res 2008;68(9):3124-32]

The HER2 oncogene is overexpressed or amplified in 20% of breast cancers. HER2-positive cancer historically portends a poor prognosis, but the HER2-targeted therapy trastuzumab mitigates this otherwise ominous distinction. Nevertheless, some patients suffer disease recurrence despite trastuzumab, and metastatic disease remains largely incurable due to innate and acquired resistance. Thus, understanding trastuzumab resistance remains an unmet medical need. Through RNA interference screening, we discovered that knockdown of the serine/threonine phosphatase PPM1H confers trastuzumab resistance via reduction in protein levels of the tumor suppressor p27. PPM1H dephosphorylates p27 at threonine 187, thus removing a signal for proteasomal degradation. We further determined that patients whose tumors express low levels of PPM1H trend towards worse clinical outcome on trastuzumab. Identifying PPM1H as a novel p27 phosphatase reveals new insight into how cancer cells destabilize a well-recognized tumor suppressor. Furthermore, low PPM1H expression may identify a subset of HER2-positive tumors that are harder to treat. Significance: PPM1H is identified as a phosphatase impacting p27 stability. Low expression of PPM1H may be associated with poor outcome in breast cancer. Cancer Discovery; 1(4); 326–337. ©2011 AACR. Read the Commentary on this article by Aceto and Bentires-Alj, p. 285 This article is highlighted in the In This Issue feature, p. 275