Significance of UBIAD1 in Regulation of HMG CoA Reductase Degradation, Mevalonate Metabolism, and Menaquinone-4 Synthesis

UbiA戊烯基转移酶结构域蛋白-1 （UBIAD1）利用非固醇类异戊二烯类香叶基焦磷酸（GGpp）合成一种名为甲基萘醌-4 （MK-4）的维生素K。人类UBIAD1突变与施耐德角膜营养不良症（SCD）的关联表明，UBIAD1是多功能的。这是一种罕见的常染色体显性眼病，其特征是由于胆固醇的异常积累而导致角膜混浊。我们的研究表明，甾醇导致UBIAD1结合内质网（ER）定位的HMG辅酶a还原酶（HMGCR），这是合成胆固醇和必需的非甾醇类异戊二烯（包括焦磷酸法尼酯、GGpp、MK-4、泛醌和醇）的限速酶。甾醇诱导的泛素化对于HMGCR的er相关降解（ERAD）是必需的，该降解由GGpp增强，并构成了该酶反馈控制的一种机制。GGpp阻断了甾醇诱导的UBIAD1与HMGCR的结合，从而增强了其ERAD，并允许UBIAD1从er转运到高尔基体。SCD相关的UBIAD1突变体抵抗ggpp诱导的HMGCR位移，并在内质网中保持隔离以抑制ERAD，这有助于增强胆固醇的合成和细胞内积累。在小鼠中进行基因敲除研究，试图阐明UBIAD1在MK-4的组织特异性分布中的作用，这仍然是维生素K生物学中一个未解决的主要问题。然而，纯合子种系缺失Ubiad1会导致胚胎致死。我们最近在表达泛素化抗性HMGCR的敲入小鼠中发现了Ubiad1的纯合缺失，这意味着与Ubiad1缺失相关的胚胎致命性是由于HMGCR的ERAD增强和甲羟戊酸代谢物的消耗，这与MK- 4不同。我们将通过以下具体目标阐明UBIAD1在调节HMGCR ERAD和MK-4合成中的组织特异性作用：1)探索UBIAD1介导的GGpp感知在肝脏中调节HMGCR ERAD和甲羟戊酸代谢中的作用；2)确定ubiad1介导的MK-4合成对骨骼肌稳态的贡献；3)确定UBIAD1在胰腺生存和功能中的作用；4)研究UBIAD1在肠道形态和功能中的作用。在胆固醇代谢调节机制（DeBose-Boyd）和维生素K生物学（Booth）方面具有互补专业知识的pi的共同努力将导致发现由MK-4调节的组织特异性途径/过程。此外，这些研究将具有直接的临床意义。HMGCR是他汀类药物的靶标，他汀类药物广泛用于降低血浆ldl -胆固醇和减少心血管疾病（CVD）的发病率。然而，他汀类药物引发的反应会导致HMGCR的积累，从而减弱其临床效果。这种增加的部分原因是ubiad1介导的HMGCR ERAD抑制。我们的研究可能揭示ubiad1介导的GGpp感知作为一种治疗靶点，可以防止他汀类药物诱导的HMGCR积累，提高他汀类药物的疗效，最终降低心血管疾病和心脏病发作的发生率。

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) uses the nonsterol isoprenoid geranylgeranyl pyrophosphate (GGpp) to synthesize a form of vitamin K called menaquinone-4 (MK-4). UBIAD1 is multifunctional as indicated by the association of mutations in human UBIAD1 with Schnyder corneal dystrophy (SCD). This rare autosomal dominant eye disease is characterized by progressive corneal opacification owing to abnormal accumulation of cholesterol. Our studies revealed that sterols cause UBIAD1 to bind endoplasmic reticulum (ER)-localized HMG CoA reductase (HMGCR), the rate-limiting enzyme in synthesis of cholesterol and essential nonsterol isoprenoids including farnesyl pyrophosphate, GGpp, MK-4, ubiquinone, and dolichol. Sterol- induced ubiquitination is obligatory for ER-associated degradation (ERAD) of HMGCR that is augmented by GGpp and constitutes one mechanism for feedback control of the enzyme. GGpp blocks sterol-induced binding of UBIAD1 to HMGCR, which enhances its ERAD and permits ER-to-Golgi transport of UBIAD1. SCD- associated mutants of UBIAD1 resist GGpp-induced displacement from HMGCR and remain sequestered in the ER to inhibit ERAD, which contributes to enhanced synthesis and intracellular accumulation of cholesterol. Gene knockout studies in mice were attempted to elucidate the role of UBIAD1 in tissue-specific distribution of MK-4, which remains a major unresolved question in vitamin K biology. However, homozygous germ-line deletion of Ubiad1 caused embryonic lethality. We recently generated homozygous deletion of Ubiad1 in knock- in mice expressing ubiquitination-resistant HMGCR, which implies embryonic lethality associated with Ubiad1 deficiency results from enhanced ERAD of HMGCR and depletion of mevalonate metabolites distinct from MK- 4. We will now elucidate tissue-specific roles of UBIAD1 in regulating HMGCR ERAD and MK-4 synthesis by pursuing the following Specific Aims: 1) Explore role of UBIAD1-mediated sensing of GGpp in regulation of HMGCR ERAD and mevalonate metabolism in the liver; 2) Determine contribution of UBIAD1-mediated synthesis of MK-4 to skeletal muscle homeostasis; 3) Establish roles for UBIAD1 in pancreatic subsistence and function; and 4) Examine role of UBIAD1 in morphology and function of the intestine. Combined efforts of the PIs, who have complementary expertise in mechanisms underlying regulation of cholesterol metabolism (DeBose-Boyd) and vitamin K biology (Booth), will lead to discovery of tissue-specific pathways/processes modulated by MK-4. In addition, these studies will have immediate clinical implications. HMGCR is the target of statins, widely prescribed drugs that lower plasma LDL-cholesterol and reduce incidence of cardiovascular disease (CVD). However, statins trigger responses that cause accumulation of HMGCR, which blunts their clinical effects. Part of this increase results from UBIAD1-mediated inhibition of HMGCR ERAD. Our studies may expose UBIAD1-mediating sensing of GGpp as a target of therapies that prevent statin-induced accumulation of HMGCR, increasing statin efficacy and ultimately reducing incidence of CVD and heart attacks.