Post-transcriptional modulation of reversible adaptive resistance to targeted therapies in melanoma: a therapeutic option?

Compelling evidence indicates that the immediate heterogeneous adaptive response of BRAF-mutated melanoma cells plays a major role in the resistance to MAPK inhibition. A large number of compensatory mechanisms of adaptive response to BRAF inhibition rely on variations in gene expression level, many of which occur at the post-transcriptional and translational steps of gene expression and are being regulated by regulatory mRNA-binding proteins (mRBPs). HuR (ELAVL1), which is a ubiquitously expressed stress-response nuclear mRBP, has been shown to be involved in cell proliferation as well as cell differentiation. HuR's extensively studied repertoire includes many of the important nodes of the signaling network involved in the adaptive response to MAPK inhibition. We initiate our work in this thesis by experimentally demonstrating that the BRAF inhibitor-induced immediate paradoxical proliferation of a subpopulation of BRAF-mutated melanoma cells is prevented in HuR-overexpressing melanoma cells. We then conduct a series of observational studies ex vivo and in vivo in patients' metastatic tumors, and show that a larger proportion of HuRLow expressing cells (HuRLow cells) is associated with a higher propensity of the whole malignant cell population for adaptive response. In our subsequent experimental work, we demonstrate that a reversible increase in the proportion of HuRLow cells, which is equivalent to increasing HuR expression fluctuation, enhances the adaptive response to BRAF inhibition. Importantly, HuRLow cells display MAPK independency and a highly “plastic”, slow or fast proliferating expression profile. Conversely, lithium salts therapy suppresses the HuRLow cell subpopulation, restores partially the growth of adapted cells and attenuates the adaptive response of the whole malignant cell population both ex vivo and in vivo in a xenograft model of adaptive response. This effect is associated at the whole cell population level with a less resistant expression profile. Our work has immediate clinical consequences for the therapy of metastatic melanoma considering that we describe a therapeutically actionable mechanism to reduce the proportion of highly plastic and partly slowly dividing cells in which the adaptive proliferation emerges under BRAF inhibition. Indeed, we are already working in initiating a multicentric phase I/II trial to evaluate the combination of lithium salts with BRAF/MEK inhibitors in BRAF-mutated metastatic melanoma. In this trial, we will use the IHC-based automated quantification approach developed in this thesis to monitor the expression distribution of HuR. Our work has broad implications for the targeted therapies of solid malignancies and opens many research perspectives, which include the precise understanding of the mechanisms that differentiate malignant cells that tightly regulate the expression of RBPs, such as HuR, from those in which their expression is loosely regulated. Such studies will likely improve our therapeutic ability to reduce the plasticity of malignant cells and their adaptive response to targeted therapies.