Abstract B24: A phosphoproteomic comparison of oncogenic BRAF and MKK1/2 inhibition in human melanoma

Abstract

Abstract The purpose of this study was to use phosphoproteomics to gain insight into the different clinical responses to BRAF inhibition, MKK1/2 inhibition, and combined BRAF/MKK inhibition. Experiments were designed to test whether BRAF inhibition may affect a distinct subset of targets that MKK1/2 inhibition does not and whether BRAF and MKK1/2 inhibitors work additively or synergistically when used in combination. To this end, we employed a SILAC-based method to quantify changes in the phosphoproteome of WM239A melanoma cells treated with a clinically relevant BRAF inhibitor (PLX4032; 10μM) or MEK1/2 inhibitor (AZD6244; 10μM) inhibitor. Phosphopeptides were enriched by titanium dioxide prior to fractionation on an electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) column. In a set of three biological replicate experiments, we quantified changes at 23,986 phosphosites on 4,722 proteins. We identified 1,356 phosphosites that decreased in abundance, and 341 that increased in abundance, in response to treatment with either PLX4032 or AZD6244. The overwhelming majority of phosphosites were responsive to both inhibitors. Of the phosphosites that decreased in abundance, 13 responded only to PLX4032, suggesting that certain targets are regulated by BRAF but not MKK1/2. In contrast, we failed to identify any phosphosites that responded only to AZD6244. To determine whether the therapeutic benefit of combined BRAF and MKK1/2 inhibition may be a result of drug synergy, we treated cells either with a single inhibitor or with a combination of AZD6244 and PLX4032, each at submaximal doses (30 nM). We then compared the magnitude of the responses observed with the combination treatment to those observed for each drug alone. Comparison of observed versus predicted changes at 6,463 phosphosites demonstrated that the primary effect of the two inhibitors is additive. Taken together, we show that the combination of BRAF and MKK1/2 inhibitor leads to predominantly additive interactions, and that the BRAF inhibitor has a potential for regulating phosphorylation events that are not targeted by MKK1/2 inhibitor. Citation Format: Scott Stuart, Stephane Houel, Nan Wang, William Old, Natalie Ahn. A phosphoproteomic comparison of oncogenic BRAF and MKK1/2 inhibition in human melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B24.