Drug-induced expression of the RNA-binding protein HuR attenuates the adaptive response to BRAF inhibition in...

Strategies that aim to limit the adaptive response to pathway inhibition in BRAF-mutated melanoma face the inherent limit of signaling redundancy and multiplicity of possible bypass mechanisms. Drug-induced expression of selected RNA-binding proteins, like the ubiquitously expressed HuR, has the potential to differentially stabilize the expression of many genes involved in the compensatory mechanisms of adaptive response. Here, we detect in BRAF-mutated melanoma cell lines having a higher propensity for adaptive response and in non-responding melanoma tumors, a larger proportion of HuR^Low cells in the expression distribution of HuR. Using knockdown experiments, we demonstrate, through expression profiling and phenotypic assays, that increasing the proportion of HuR^Low cells favors the adaptive response to BRAF inhibition, provided that the HuR^Low state stays reversible. The MAPK dependency of melanoma cells appears to be diminished as the proportion of HuR^Low cells increases. In single-cell assays, we demonstrate that the HuR^Low cells display plasticity in their growth expression profile. Importantly, the adaptive over-proliferating cells emerge in the subpopulation containing the HuR^Low cells. Therapeutic concentrations of lithium salts, although they moderately increase the global expression of HuR, are sufficient to suppress the HuR^Low cells, induce an overall less resistant expression profile and attenuate in a HuR-dependent manner the adaptive response of melanoma cells in ex vivo assays. The therapeutic effectiveness of this approach is also demonstrated in vivo in mice xenografts. This study has immediate clinical relevance for melanoma therapy and opens a new avenue of strategies to prevent the adaptive response to targeted cancer therapy.