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.

HMG-CoA还原酶（HMGCR）是胆固醇生物合成途径中的限速酶，其催化结构域是降胆固醇药物他汀类药物的众所周知的靶标。HMGCR受到负反馈回路的影响；过量的胆固醇会抑制该基因的转录，而羊毛甾醇和氧固醇会加速HMGCR的降解。已知一类合成的小分子，以SR12813为例的双膦酸酯可诱导HMGCR加速降解并降低血清胆固醇水平。尽管遗传和生化研究表明，加速降解需要HMGCR和Insig（内质网膜上的氧固醇传感器）的膜结构域，但双膦酸酯的直接目标仍不清楚。在这个研究中，我们通过初步的结构-活性关系研究开发了一种有效的双膦酸酯光亲和探针，并证明了双膦酸酯与HMGCR膜结构域的结合。这些结果为理解SR12813介导的HMGCR降解的难以捉摸的机制提供了重要线索，并为开发针对酶非催化膜结构域的更有效的HMGCR降解剂奠定了基础。