No abstract

An epithelial to mesenchymal transition (EMT) has been correlated to malignant tumor progression and metastasis by promoting cancer cell migration and invasion and chemoresistance. Hence, finding druggable EMT effectors is critical to efficiently interfere with metastasis formation and to overcome therapy resistance. We have employed a high-content microscopy screen in combination with a kinome and phosphatome-wide siRNA library to identify signaling pathways underlying an EMT of murine mammary epithelial cells and breast cancer cells. This screen identified the MEK5-ERK5 axis as a critical player in TGFβ-mediated EMT. Suppression of MEK5-ERK5 signaling completely prevented the morphological and molecular changes occurring during a TGFβ-induced EMT and, conversely, forced highly metastatic breast cancer cells into a differentiated epithelial state. Inhibition of MEK5-ERK5 signaling also repressed breast cancer cell migration and invasion and substantially reduced lung metastasis without affecting primary tumor growth. The results suggest that the MEK5-ERK5 signaling axis via activation of MEF2B and other transcription factors plays an important role in the induction and maintenance of breast cancer cell migration and invasion and thus represents an exploitable target for the pharmacological inhibition of cancer cell metastasis.

The hepatocyte growth factor (HGF) also known as scatter factor activates cancer cell invasion and metastasis. We show that in ovarian cancer cells HGF induced the phosphorylation of the small heat shock protein of 27 kDa (HSP27) by activating the p38MAPK. HSP27 is increased in many cancers at advanced stage including ovarian cancer and associated with cancer resistance to therapy and poor patients' survival. The phosphorylation of HSP27 regulates both its chaperone activity and its control of cytoskeletal stability. We show that HSP27 was necessary for the remodeling of actin filaments induced by HGF and that motility in vitro depended on the p38MAPK-MK2 axis. In vivo, HSP27 silencing impaired the ability of the highly metastatic, HGF-secreting ovarian cancer cells to give rise to spontaneous metastases. This was due to defective motility across the vessel wall and reduced growth. Indeed, HSP27 silencing impaired the ability of circulating ovarian cancer cells to home to the lungs and to form experimental hematogenous metastases and the capability of cancer cells to grow as subcutaneous xenografts. Moreover, HSP27 suppression resulted in the sensitization of xenografts to low doses of the chemotherapeutic paclitaxel, likely because HSP27 protected microtubules from bundling caused by the drug. Altogether, these data show that the HSP27 is required for the proinvasive and prometastatic activity of HGF and suggest that HSP27 might be not only a marker of progression of ovarian cancer, but also a suitable target for therapy.

The tyrosine kinase encoded by the MET oncogene is activated by gene mutation or amplification in tumors, which in most instances maintain addiction, i.e., dependency, to MET activation. This makes MET an attractive candidate for targeted therapies. Here we show that, in 3/3 MET-addicted human gastric cancer cell lines, MET kinase inhibition resulted in a 3- to 4-fold increased expression of the antiapoptotic small heat-shock protein of 27 kDa (HSP27, HSPB1). HSP27 increase depended on the inhibition of the MEK/ERK pathway and on heat-shock factor 1 (HSF1) and hypoxia-inducible factor-1α (HIF-1α) regulation. Importantly, HSP27-silenced MET-addicted cells underwent 2- and 3-fold more apoptosis following MET inhibition in vitro and in vivo, respectively. Likewise, in human cancer cells susceptible to epidermal growth factor receptor (EGFR) inhibition, EGFR inhibitors induced HSP27 expression and were strengthened by HSP27 suppression. In control cell lines that were not affected by drugs targeting MET or EGFR, these drugs did not induce HSP27 increase. Therefore, in cancer therapies targeting the MET pathway, the induction of HSP27 might limit the efficacy of anti-MET agents. As HSP27 increase also impairs the effectiveness of EGFR inhibitors and is known to protect cells from chemotherapeutics, the induction of HSP27 by targeted agents might strongly affect the success of combination treatments.