Final Report Summary - IMAGEMELTHERAPYLRI (Single cell level intravital imaging of response, tolerance, and resistance to targeted therapies)
Since the discovery of oncogenes that encoded protein kinases it has been hoped that inhibition of the relevant kinases would be an effective chemotherapeutic strategy. This aspiration has become a clinical reality with the development of inhibitors against Abl tyrosine kinase, EGFR family kinases and BRAF. However, agents targeting either EGFR or BRAF typically show good efficacy in tumors with matching oncogenic mutations for a number of months before genetically resistant cells dominate the tumor and the therapy fails In the case of EGFR mutant lung tumors it has been shown that resistant cells may be present even before treatment and that these are at a strong selective advantage during therapy. However, the situation in BRAF mutant melanoma treated with BRAF inhibitors is less clear. There is significant variability in the magnitude of initial response to BRAF inhibition and genetically resistant sub-clones have not been detected prior to treatment, even in tumors that show modest responses. It has been proposed that non-cell autonomous mechanisms involving HGF production by the tumor stroma may drive resistance. However, it is not clear how selective pressure would act on the genetically stable stroma to promote the emergence of resistant disease. Establishing the chronology of biochemical responses to targeted therapy and biological changes elicited within the context of complex tumor microenvironments remains challenging.
We clarified how the tumor microenvironment affects response to BRAF inhibition in the treatment of BRAF mutant melanoma by utilizing intravital imaging techniques and fluorescence resonance energy transfer (FRET) microscopy. Initially melanoma cells respond to a BRAF inhibitor, but rapid reactivation of ERK/MAPK is observed in areas of high stromal density. This is linked to ‘paradoxical’ activation of melanoma-associated fibroblasts by BRAF inhibition and the promotion of matrix production and remodeling leading to elevated integrin β1/FAK/Src signaling in melanoma cells. Fibronectin-rich matrices with 3-12 kPa elastic modulus are sufficient to provide BRAF inhibitor tolerance. Co-inhibition of BRAF and FAK abolished ERK reactivation and led to more effective control of BRAF-mutant melanoma. We propose that paradoxically activated MAFs provide a ‘safe haven’ for melanoma cells to tolerate BRAF inhibition, and that this safe haven enhances the population of cancer cells from which genetically resistance emerges. Our work highlights the need to consider the effects of targeted therapies on the tumor microenvironments and strongly suggests that co-targeting signals in or from these microenvironments should augment the effect of cancer treatment.
We clarified how the tumor microenvironment affects response to BRAF inhibition in the treatment of BRAF mutant melanoma by utilizing intravital imaging techniques and fluorescence resonance energy transfer (FRET) microscopy. Initially melanoma cells respond to a BRAF inhibitor, but rapid reactivation of ERK/MAPK is observed in areas of high stromal density. This is linked to ‘paradoxical’ activation of melanoma-associated fibroblasts by BRAF inhibition and the promotion of matrix production and remodeling leading to elevated integrin β1/FAK/Src signaling in melanoma cells. Fibronectin-rich matrices with 3-12 kPa elastic modulus are sufficient to provide BRAF inhibitor tolerance. Co-inhibition of BRAF and FAK abolished ERK reactivation and led to more effective control of BRAF-mutant melanoma. We propose that paradoxically activated MAFs provide a ‘safe haven’ for melanoma cells to tolerate BRAF inhibition, and that this safe haven enhances the population of cancer cells from which genetically resistance emerges. Our work highlights the need to consider the effects of targeted therapies on the tumor microenvironments and strongly suggests that co-targeting signals in or from these microenvironments should augment the effect of cancer treatment.
