Immunotherapy has revolutionized cancer treatment, yet most patients do not respond. Here, we investigated mechanisms of response by profiling the proteome of clinical samples from advanced stage melanoma patients undergoing either tumor infiltrating lymphocyte (TIL)-based or anti-programmed death 1 (PD1) immunotherapy. Using high-resolution mass spectrometry, we quantified over 10,300 proteins in total and $4,500 proteins across most samples in each dataset. Statistical analyses revealed higher oxidative phosphorylation and lipid metabolism in responders than in non-responders in both treatments. To elucidate the effects of the metabolic state on the immune response, we examined melanoma cells upon metabolic perturbations or CRISPR-Cas9 knockouts. These experiments indicated lipid metabolism as a regulatory mechanism that increases melanoma immunogenicity by elevating antigen presentation, thereby increasing sensitivity to T cell mediated killing both in vitro and in vivo. Altogether, our proteomic analyses revealed association between the melanoma metabolic state and the response to immunotherapy, which can be the basis for future improvement of therapeutic response.

The human multidrug resistance protein 1 (hMRP1) transporter is implicated in cancer multidrug resistance as well as immune responses involving its physiologic substrate, glutathione (GSH)‐conjugated leukotriene C4 (LTC4). LTC4 binds a bipartite site on hMRP1, which a recent cryoelectron microscopy structure of LTC4‐bound bovine Mrp1 depicts as composed of a positively charged pocket and a hydrophobic (H) pocket that binds the GSH moiety and surrounds the fatty acid moiety, respectively, of LTC4. Here, we show that single Ala and Leu substitutions of H‐pocket hMRP1‐Met1093 have no effect on LTC4 binding or transport. Estrone 3‐sulfate transport is also unaffected, but both hMRP1‐Met1093 mutations eliminate estradiol glucuronide transport, demonstrating that these steroid conjugates have binding sites distinct from each other and from LTC4. To eliminate LTC4 transport by hMRP1, mutation of 3 H‐pocket residues was required (W553/M1093/W1246A), indicating that H‐pocket amino acids are key to the vastly different affinities of hMRP1 for LTC4 vs. GSH alone. Unlike organic anion transport, hMRP1‐mediated drug resistance was more diminished by Ala than Leu substitution of Met1093. Although our findings generally support a structure in which H‐pocket residues bind the lipid tail of LTC4, their critical and differential role in the transport of conjugated estrogens and anticancer drugs remains unexplained.—Conseil, G., Arama‐Chayoth, M., Tsfadia, Y., Cole, S. P. C. Structure‐guided probing of the leukotriene C4 binding site in human multidrug resistance protein 1 (MRP1; ABCC1). FA8EB J. 33, 10692–10704 (2019). http://www.fasebj.org

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