Luciferase-based HMG-CoA reductase degradation assay for activity and selectivity profiling of oxy(lano)sterols

Fine-tuning of nitrogen-containing bisphosphonate esters that potently induce degradation of HMG-CoA reductase

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is the rate-limiting enzyme in the cholesterol biosynthetic pathway, and competitive inhibitors targeting the catalytic domain of this enzyme, so-called statins, are widely used for the treatment of hyperlipidemia. The membrane domain mediates the sterol-accelerated degradation, a post-translational negative feedback mechanism, and small molecules triggering such degradation have been studied as an alternative therapeutic option. Such strategies are expected to provide benefits over catalytic site inhibitors, as the inhibition leads to transcriptional and post-translational upregulation of the enzyme, necessitating a higher dose of the inhibitors and concomitantly increasing the risk of serious adverse effects, including myopathies. Through our previous study on SR12813, a synthetic small molecule that induces degradation of HMG-CoA reductase, we identified a nitrogen-containing bisphosphonate ester SRP3042 as a highly potent HMG-CoA reductase degrader. Here, we performed a systematic structure-activity relationship study to optimize its activity and physicochemical properties, specifically focusing on the reduction of lipophilicity. Mono-fluorination of tert-butyl groups on the molecules was found to increase the HMG-CoA reductase degradation activity while reducing lipophilicity, suggesting the mono-fluorination of saturated alkyl groups as a useful strategy to balance potency and lipophilicity of the lead compounds.

Bisphosphonate esters interact with HMG-CoA reductase membrane domain to induce its degradation

HMG-CoA reductase (HMGCR) is a rate-limiting enzyme in the cholesterol biosynthetic pathway, and its catalytic domain is the well-known target of cholesterol-lowering drugs, statins. HMGCR is subject to layers of negative feedback loops; excess cholesterol inhibits transcription of the gene, and lanosterols and oxysterols accelerate degradation of HMGCR. A class of synthetic small molecules, bisphosphonate esters exemplified by SR12813, has been known to induce accelerated degradation of HMGCR and reduce the serum cholesterol level. Although genetic and biochemical studies revealed that the accelerated degradation requires the membrane domain of HMGCR and Insig, an oxysterol sensor on the endoplasmic reticulum membrane, the direct target of the bisphosphonate esters remains unclear. In this study, we developed a potent photoaffinity probe of the bisphosphonate esters through preliminary structure-activity relationship study and demonstrated binding of the bisphosphonate esters to the HMGCR membrane domain. These results provide an important clue to understand the elusive mechanism of the SR12813-mediated HMGCR degradation and serve as a basis to develop more potent HMGCR degraders that target the non-catalytic, membrane domain of the enzyme.