Regulation of somatostatin receptor signaling

somatostatin （SS）受体2A （sstr2A）是正常组织和人类肿瘤中表达最广泛的SS受体亚型。它在调节垂体、胰腺和胃肠激素分泌等诸多生物作用中起着重要的生理作用。它也是用于治疗垂体和胃肠胰腺神经内分泌肿瘤患者的SS类似物的治疗靶点，以抑制激素过度分泌，减少肿瘤生长。不幸的是，很大一部分患者从一开始就对SS模拟治疗产生耐药性，而其他患者在治疗过程中产生耐药性。其原因尚不清楚。为了增加我们对SS作用的理解和改善其治疗应用，我们的长期目标是阐明SS受体信号传导的分子事件和调节受体功能产生靶细胞抗性的机制。我们之前的研究表明，激素结合刺激了sstr2A在多个Ser和Thr上的快速和特异性磷酸化，我们已经确定了这一点。受体磷酸化改变了sstr2A信号传导和亚细胞运输，特定的磷酸化位点对受体功能有不同的影响。出乎意料的是，受体磷酸化的模式随着不同的配体而变化很大，包括目前在临床试验中的配体。本提案的目的是确定如何控制sstr2A的磷酸化，以及它如何影响受体信号传导和运输。这些研究的组织假设是，sstr2A的磷酸化和去磷酸化受到严格调控，以产生具有不同磷酸化残基模式的受体——即所谓的磷酸化“条形码”。我们认为这些不同磷酸化的受体在不同的细胞区室中产生，并确定受体与特定细胞质蛋白的相互作用，然后产生对SS和SS模拟刺激的细胞反应。这一假设将在四个特定目标中得到验证：(1)确定特定Ser/Thr残基的磷酸化如何影响sstr2A的运输和信号传导

DESCRIPTION (provided by applicant): Regulation of somatostatin receptor signaling (Schonbrunn) Somatostatin (SS) receptor 2A (sstr2A) is the most widely expressed SS receptor subtype in normal tissues and human tumors. It plays a critical physiological role in regulating pituitary, pancreatic and gastrointestinal hormone secretion, among many biological actions. It is also the therapeutic target for the SS analogs used to treat patients with pituitary and gastro-enteropancreatic neuroendocrine tumors in order to inhibit hormonal hyper-secretion and to reduce tumor growth. Unfortunately, a large fraction of patients are resistant to SS analog therapy from the start and others develop resistance during the course of treatment. The reasons for this are unknown. To increase our understanding of SS action and improve its therapeutic applications our long term goal is to elucidate the molecular events involved in SS receptor signaling and the mechanisms which regulate receptor function to produce target cell resistance. Our previous studies showed that hormone binding stimulates the rapid and specific phosphorylation of sstr2A at multiple Ser and Thr, which we have identified. Receptor phosphorylation alters both sstr2A signaling and subcellular trafficking, with specific phosphorylation sites exerting distinct effects on receptor function. Unexpectedly, the pattern of receptor phosphorylation varies dramatically with different ligands, including those currently in clinical trials. The objective of this proposal is to determine how the phosphorylation of sstr2A i controlled and how it affects receptor signaling and trafficking. The organizing hypothesis for these studies is that phosphorylation and dephosphorylation of sstr2A are stringently regulated in order to produce receptors with distinct patterns of phosphorylated residues - a so-called phosphorylation "bar code". We propose that these differentially phosphorylated receptors are generated in different cellular compartments and determine the interactions of the receptor with specific cytosolic proteins that then produce the cellular response to SS and SS analog stimulation. This hypothesis will be tested in four specific aims: (1) Determine how phosphorylation of specific Ser/Thr residues affects sstr2A trafficking and signaling, (2) Identify the enzymes that regulate the phosphorylation state of the receptor. We will complete the identification of the kinases that phosphorylate sstr2A and will characterize the enzyme complexes that catalyze sstr2A dephosphorylation in a phosphosite- specific manner. We will also determine how specific phosphatases are directed to act on the receptor in particular subcellular compartments, (3) Elucidate the mechanisms that produce cell-type specific differences in sstr2A phosphorylation, trafficking, and signaling in pituitary and gastroenteropancreatic tumor cells, and (4) Elucidate the mechanisms responsible for agonist-specific sstr2A trafficking and desensitization. These studies will provide novel insights into the biochemical and cellular processes which regulate sstr2A, as well as other G protein coupled receptors, and the manner in which different agonists and the cellular environment impact those processes.