Deciphering the molecular mechanism of signal transducing adaptor family member 1 (STAP1) in the regulation of cholesterol metabolism

Dr Wong, Chi Ming   (Principal Investigator (PI))

24

2016-04-01

2016-05-03

45980

Deciphering the molecular mechanism of signal transducing adaptor family member 1 (STAP1) in the regulation of cholesterol metabolism

hypercholesterolemia, signal transducing adaptor family member 1 (STAP1)

Diabetes/Metabolism,Endocrinology

201511159162

Seed Fund for PI Research – Basic Research

2015

Completed

Background: Familial hypercholesterolemia (FH) is a very common autosomal dominant inherited disorder. The prevalence is around 1:200 to 1:500. FH is a disorder of cholesterol metabolism which characterized by markedly elevated low-density lipoprotein cholesterol (LDLC) levels. FH patient often have 2 to 3 fold higher LDLC level than normal, which would greatly increase the risk for premature cardiovascular disease and myocardial infarction. As the development of atherosclerosis caused by the excess LDLC could begin in childhood and often not recognized until early middle age. If treatment of FH starts early, it is a treatable disease. The aggressive lipid lowering to reduce LDLC level more than 50% usually required and greater LDL cholesterol reductions would be necessary for FH patients who have other cardiovascular disease risk factors. Therefore, the early and appropriate diagnosis of FH is very important. FH can be inherited in a Mendelian fashion in either in an autosomal dominant manner (Autosomal Dominant Hypercholesterolemia; ADH) or an autosomal recessive manner (Autosomal Recessive Hypercholesterolemia; ARH). Most ADH is caused by genetic variants. Three genes, namely LDL receptor (LDLR), specific domains of apolipoprotein B (APOB), and proprotein convertase subtilisin/kexin 9 (PCSK9), have been shown to cause ADH. A recessive form of FH due to mutations in LDLRAP1 is also known. The frequency of FH due to LDLR, APOB, PCSK9 and ARH mutations is 52%-76%, 2-10%, 2% and 2%, respectively (http://www.snpedia.com/index.php/Familial_hypercholesterolemia). In brief, mutations in the LDL receptor (encoded by LDLR) and in the LDL receptor-binding domain of the apolipoprotein B-100 (encoded by APOB) lead to decreased intracellular uptake of cholesterol. PCSK9 encodes NARC-1 (neural apoptosis regulated convertase) is identified as a human subtilase that is highly expressed in the liver and contributes to cholesterol homeostasis in 2003 [1]. According to current model, PCSK9 induces LDL receptor degradation through binding to its epidermal growth factor repeat A domain, so inhibiting PCSK9 activity should up-regulate receptor level and enhance LDL clearance [2]. Mutations in LDLR, APOB and PCSK9 can only be detected in 80% of definite FH (DFH) patients suggesting that there are likely other genetic causes, located outside of the currently screened regions, which are yet to be identified [3]. Recent studies identified the mutations in Signal transducing adaptor family member 1 [STAP1; known as BRDG1 BCR downstream signaling protein 1) or stem cell adaptor protein 1] are also associated with ADH [4, 5]. Little information is available for the function of STAP1 in cholesterol metabolism. STAP1 was identified as novel substrate of c-kit [also known as SCRF (Mast/stem cell growth factor receptor) tyrosine-protein kinase Kit or CD117) by yeast two-hybrid screening in 2000 [6]. STAP1 is proposed as a docking protein acting downstream of Tec tyrosine kinase in B cell antigen receptor signaling [7]. STAP1 can be directly phosphorylated by Tec family kinases (such as c-kit and c-fms) in vitro which has been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. In line with STAP1, Tec is also preferentially expressed in liver [8] and c-kit regulates systemic cholesterol levels. However, it is necessary to validate the correlation findings by independent study and animal model. It is also very important to further explore the molecular mechanism behind how a B cell expressed adaptor protein STAP1 regulates hepatic cholesterol metabolism. As mouse ortholog, stap1, shares 83% identity with its human counterpart (Figure 1). STAP1 may be new diagnostic and therapeutic target for the hypercholesterolemia in humans. Key issues and problems being addressed: B cells perform various immunological functions that include production of antibody, presentation of antigens, secretion of multiple cytokines and regulation of immune responses. B cell can be rapidly recruited to liver by the stimulation of invariant natural killer T (iNKT) cells [9]. B cells and/or antibodies it generated are protective against atherosclerosis and that this protection may be conferred by B cell–mediated immune regulation. [10, 11]. B-cell tumor development induced hepatic lipid accumulation due to enhanced hepatic fatty acid (FA) uptake and impaired FA oxidation [12]. Interestingly, hepatic B cells have the potential to initiate rather than regulate inflammatory responses was reported. The finding based on fact that the LPS-activated hepatic B cells produce significantly elevated levels of proinflammatory interferon (IFN)-γ, IL-6, tumour necrosis factor (TNF)-α and very low levels of IL-10 compared with activated splenic B cells [13]. In addition, hepatic B cells activation has also been associated with liver fibrosis [14, 15]. In this study, we are going to address whether hepatic B cells in regulating hepatic cholesterol metabolism. Project objectives: 1. To determine the type of cell expressing STAP1. 2. To extensive the metabolic phenotype of mice overexpressing mutated STAP1 in liver by rAAV mediated gene delivery system. 3. To explore the molecular mechanism of STAP1 regulates cholesterol metabolism