RAS proto-oncogenes are mutated in ~30% of human cancers, but no mechanism-based treatments exist for reversing the biochemical output of oncogenic Ras proteins, which are exceedingly difficult targets for rational drug discovery. Our long-term goal is to implement mechanistic strategies to selectively inhibit the growth of cancers with somatic RAS mutations. In this project, we will investigate the Ras palmitoylation/depalmitoylation cycle, which regulates the subcellular trafficking of the N-Ras, H-Ras, and K-Ras4a isoforms, as a therapeutic target for selectively inhibiting the growth of malignancies with oncogenic NRAS mutations. Acyl protein thioesterase 1 and 2 (APT1 and APT2) catalyze N-Ras depalmitoylation and we found that first generation chemical inhibitors designed to inhibit these enzymes (Palmostatin B and Palmostatin M) selectively reduced the growth of primary hematopoietic progenitors and leukemia cells expressing oncogenic N-RasG12D. However, our recent studies also infer the existence of additional biochemical targets of these compounds that are essential for the growth of NRAS mutant cancer cells. This project involves a cross-disciplinary collaboration that brings together investigators with extensive expertise in synthetic chemistry (Dr. Howell), enzymology and chemical biology (Dr. Cravatt), and hematologic cancer, Ras signaling, and preclinical therapeutics (Dr. Shannon). We have collaborated to generate extensive preliminary data and novel reagents, which we will use to pursue the goals of: (1) identifying additional biochemical targets of the palmostatins; (2) developing new chemical inhibitors with improved potency and selectivity for palmostatin targets; (3) using these inhibitors combined with genetic methods to discern the relevant enzyme(s) that regulate N-Ras depalmitoylation in cancer cells; and, (4) utilizing human cancer cell lines and a new strain of mice to interrogate the palmitoylation/depalmitoylation cycle as a therapeutic target in early stage and advanced NRAS-mutant cancers. We will address these questions through two highly integrated specific aims.
In Aim 1, we will identify additional biochemical targets of palmostatin M, design and characterize new chemical inhibitors, and evaluate the efficacy of these compounds in cancer cells with NRAS mutations.
In Aim 2, we will utilize a novel strain of NrasG12D,C181S knock in mice to ask if the palmitoylation/depalmitoylation cycle is required for the growth of oncogenic Nras-driven cancers in vivo. These studies will rigorously assess the importance of the palmitoylation/depalmitoylation cycle and inform the development of new therapeutic strategies for cancers with oncogenic RAS mutations.
The NRAS gene is mutated in many human cancers, which results in an over-active N-Ras protein that drives malignant growth. The abnormal N-Ras proteins expressed in cancer cells are extremely difficult biochemical targets for conventional drug discovery. The goals of this project are to test a novel strategy for inhibiting the mutant N-Ras proteins found in cancer cells by blocking a pathway that plays a key role in transporting it within cells.
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