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Endometrial Cancers With Activating KRas Mutations Have Activated Estrogen Signaling and Paradoxical Response to MEK Inhibition

Objectives The aims of this study were to determine if activating KRas mutation alters estrogen signaling in endometrial cancer (EC) and to explore the potential therapeutic impact of these alterations. Methods The Cancer Genome Atlas was queried for changes in estrogen-regulated genes in EC based on KRas mutation status. In vitro studies were conducted to evaluate estrogen receptor &agr; (ER&agr;) phosphorylation changes and related kinase changes in KRas mutant EC cells. The resulting effect on response to MEK inhibition, using trametinib, was evaluated. Immunohistochemistry was performed on KRas mutant and wild-type EC tumors to test estrogen signaling differences. Results KRas mutant tumors in The Cancer Genome Atlas showed decreased progesterone receptor expression (P = 0.047). Protein analysis in KRas mutant EC cells also showed decreased expression of ER&agr; (P < 0.001) and progesterone receptor (P = 0.001). Although total ER&agr; is decreased in KRas mutant cells, phospho-ER&agr; S118 was increased compared with wild type. Treatment with trametinib in KRas mutant cells increased phospho-ER&agr; S167 and increased expression of estrogen-regulated genes. While MEK inhibition blocked estradiol-stimulated phosphorylation of ERK1/2 and p90RSK in wild-type cells, phospho-ERK1/2 and phospho-p90RSK were substantially increased in KRas mutants. KRas mutant EC tumor specimens showed similar changes, with increased phospho-ER&agr; S118 and phospho-ER&agr; S167 compared with wild-type EC tumors. Conclusions MEK inhibition in KRas mutant cells results in activation of ER signaling and prevents the abrogation of signaling through ERK1/2 and p90RSK that is achieved in KRas wild-type EC cells. Combination therapy with MEK inhibition plus antiestrogen therapy may be necessary to improve response rates in patients with KRas mutant EC.