Mechanisms of cysteine regulation of liver cholesterol metabolism and adipose tissue thermogenesis

Abstract

Cysteine is a rare and highly conserved amino acid whose biological significance arises from the distinct chemical reactivity of its thiol (–SH) group. This intrinsic reactivity allows cysteine contained within both small molecules and proteins to regulate redox homeostasis, antioxidant defense, and critical metabolic signaling pathways. Moreover, cysteine’s chemical versatility makes it an ideal target for selective covalent modification by electrophilic small molecules. This thesis explores mechanisms that regulate cysteine metabolism and the extent to which they can be targeted for therapeutic benefit in the context of metabolic disease. First, to uncover proteins that regulate cysteine metabolism I deploy a method termed Metabolite-Protein Covariation Architecture (MPCA), a mass spectrometry-based approach to systematically explore metabolite-protein interactions in living tissues. Using MPCA, we uncovered a previously unknown cysteine-sensitive regulatory pathway involving the E3 ligase adaptor protein, leucine-rich repeat-containing 58 (LRRC58), which mediates proteasomal degradation of cysteine dioxygenase 1 (CDO1), the enzyme controlling cysteine catabolism toward taurine synthesis. Stabilization of CDO1 upon LRRC58 depletion reduced the total cysteine pool by shifting metabolism to elevated taurine production, resulting in a significant reduction of hepatic cholesterol in vivo. In the second study, I focus on cysteine residues within proteins that can be therapeutically targeted to promote adipocyte thermogenesis. I discover and characterize ZNL-06-030, a cysteine-targeted covalent small molecule that activates thermogenic adipocyte G protein-coupled receptors (GPCRs). ZNL-06-030 induces thermogenesis via Gs-dependent cyclic AMP signaling, protein kinase A activation, enhanced lipolysis, and mitochondrial respiration. Preliminary findings nominate orphan receptors GPR6 and GPR12 as potential targets, although further validation is necessary to fully elucidate receptor specificity. Together, these findings uncover new mechanisms for regulation of cysteine biology and suggest new avenues for regulation of metabolic physiology.