CYCLOOXYGENASE AND TUMORIGENESIS

流行病学研究表明，非甾体类抗炎药（NSAIDs）可以降低结肠癌的发病率。由于阿司匹林和其他非甾体抗炎药的药理作用是抑制环氧化酶[COX，前列腺素（PG）生物合成中的限速酶]，因此可以推断，非甾体抗炎药的有益作用可能是通过抑制PG生物合成来介导的。然而，几条实验观察表明，非甾体抗炎药的有益作用可能通过cox依赖性和cox非依赖性途径介导。因此，本文的具体目的是：1)通过转染组成型表达的COX-1或诱导型COX-2 cDNA，确定COX在结肠癌细胞系或正常肠上皮细胞中的过表达是否会导致致瘤表型的变化，以及非甾体抗炎药抑制COX是否会消除体外（培养细胞）和体内（移植肿瘤到胸腺裸鼠的生长）的变化。这些研究的结果将建立或否定COX与肿瘤细胞生长的直接联系。2)确定非甾体抗炎药的环氧化酶非依赖性作用是否通过丝裂原活化蛋白激酶（MAPK）、NFkappaB或过氧化物酶体增殖物活化受体（PPAR）信号通路介导。最近的证据表明，非甾体抗炎药调节MAPK， NFkappaB和PPAR信号通路。因此，这些研究旨在确定非甾体抗炎药抑制肿瘤发生的cox非依赖性信号通路。3)采用减法杂交和基因表达序列分析的方法，鉴定结肠癌细胞系与过表达COX-2的正常肠上皮细胞的差异表达基因。在癌细胞中鉴定由COX-2过表达或非甾体抗炎药治疗引起的上调或下调基因，将为前列腺素生成增加如何导致致瘤表型的变化提供线索，或为鉴定非甾体抗炎药作用的细胞靶点（COX以外）提供线索。4)确定激活腺苷酸环化酶的前列腺素受体在结肠癌细胞中与正常细胞是否存在差异表达，并确定前列腺素E受体（Ep2）的下游信号通路。了解前列腺素在癌细胞中的作用及其信号传导途径，有助于探索结肠癌和其他癌症的新治疗和/或预防策略，利用药物和/或饮食手段调节前列腺素的生物合成。

Epidemiological studies have demonstrated that nonsteroidal antiinflammatory drugs (NSAIDs) can reduce the incidence of colon cancer. Since the well-documented pharmacological action of aspirin and other NSAIDs is inhibition of cyclooxygenase [COX, the rate limiting enzyme in prostaglandin (PG) biosynthesis], it can be inferred that the beneficial effect of NSAIDs may be mediated through the inhibition of PG biosynthesis. However, several lines of experimental observations imply that the beneficial effects of NSAIDs may be mediated through both COX-dependent and COX-independent pathways. Thus, Specific Aims are: 1) To determine whether the overexpression of COX in colon cancer cell lines or normal intestinal epithelial cells by transfecting with constitutively expressed COX-1 or inducible COX-2 cDNA, results in changes in tumorigenic phenotypes, and whether inhibiting COX by NSAIDs abrogates the changes in vitro (cells in culture) and in vivo (growth of transplanted tumors to athymic nude mice). Results from these studies will establish or negate a direct link of COX to tumor cell growth. 2) To determine whether cyclooxygenase-independent effects of NSAIDs are mediated through mitogen-activated protein kinase (MAPK), NFkappaB or peroxisome proliferator-activated receptor (PPAR) signaling pathways. Recent evidence suggests that NSAIDs modulate MAPK, NFkappaB and PPAR signaling pathways. Thus, these studies are aimed to identify COX-independent signaling pathways through which NSAIDs suppress tumorigenesis. 3) To identify differentially expresssed genes in the colon cancer line and normal intestinal epithelial cells overexpressing COX-2 by subtractive hybridization and Serial Analysis of Gene Expression methods. Identifying up-or-down-regulated genes caused by the overexpression of COX-2 or NSAID treatment in the cancer cells will provide a clue as to how increased prostaglandin production can lead to changes in tumorigenic phenotypes, or a clue to identifying the cellular targets (other than COX) of the NSAID actions. 4) To determine whether prostaglandin receptors that activate adenylate cyclase are differentially expressed in the colon cancer cells as compared with normal cells and to identify downstream signaling pathways of prostaglandin E receptor (Ep2). Understanding the roles of prostaglandins and their signaling pathways in cancer cells could help explore new treatments and/or preventive strategies for colon cancer and perhaps other cancers using pharmacological agents and/or dietary means that modulate prostaglandin biosynthesis.