Regulated formation of eicosanoids
Regulated formation of eicosanoids
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DOI：
10.1172/jci13241
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发表时间：
2001-06-01
影响因子：
15.9
通讯作者：
Soberman, R
中科院分区：
文献类型：
--
作者：
Fitzpatrick, FA;Soberman, R
通信地址：FA菲茨帕特里克，亨茨曼癌症研究所，2000年希望之圈，犹他州大学，湖城，犹他州84112，美国。电话：（801）581-6204;传真：（801）585-0101;电子邮件：frank。菲茨帕特里克@ hci.犹他州edu.论文，大概从头合成的考克斯酶，有助于这一进程。然而，NIH 3 T3细胞暴露于外源性花生四烯酸而不是PDGF的额外实验表明，这些细胞具有足够的COX催化形成PGE 2的基础能力。此外，稳态水平的考克斯mRNA在NIH 3 T3细胞上升，但这种上升之后，而不是之前，PGE 2合成的增加。最后，考克斯蛋白水平没有明显上升后，PDGF刺激，即使当相应的mRNA水平升高。这些数据引发了两个问题。首先，如果细胞已经有足够的考克斯酶能力将任何可用的花生四烯酸转化为PGE 2，为什么还要依赖于考克斯酶的从头合成？其次，如果考克斯酶的从头合成确实导致PGDF孵育细胞中PGE 2形成增加，那么为什么mRNA积累、蛋白质积累和PGE 2形成之间的时间和化学计量关系如此扭曲？Herschman和同事们对考克斯-2同工酶的发现（12）（参见Smith和Langenbach，这一系列观点，参考文献13）将最终澄清Lin等人观察到的定量和时间扭曲。（十一）、然而，在没有这一发现的情况下，Lin等提出PDGF受体与磷脂酶并最终与考克斯的偶联依赖于未鉴定蛋白质的从头合成。换句话说，PDGF介导的细胞PGE 2合成的增加依赖于蛋白质的从头合成，所述蛋白质在生长因子受体占据期间协调花生四烯酸的可用性以通过考克斯进行其代谢。这一假说今天仍然具有吸引力和积极性。例如，Murakami et al. (14)已经提出存在一种假设的辅助蛋白，其整合磷脂酶-考克斯-2相互作用以解释考克斯-2表达如何使cPLA 2活化。考虑到花生四烯酸级联反应中酶之间的功能偶联的新兴趣（15），观察尚未鉴定的偶联蛋白是否促进组分受体和酶之间的相互作用将是有趣的。在过去的一年中，磷脂酶-COX-PGH异构酶之间偶联的概念从Kudo及其同事的研究中获得了特别的实验支持和清晰度（14，16，17）。通过表达几种形式的磷脂酶A2（Ca 2+依赖性胞质磷脂酶cPLA 2、分泌型磷脂酶sPLA 2或Ca 2+非依赖性磷脂酶iPLA 2）中的任何一种与异位表达的考克斯-1或考克斯-2的组合，这些作者已经建立了酶之间的功能相互作用的层次。首先，在可称为基础条件的条件下（图1，红色文本和箭头），Ca 2+非依赖性iPLA 2是参与花生四烯酸和相关多不饱和脂肪酸从膜磷脂中释放的主要磷脂酶。iPLA 2主要用于细胞膜重塑，不诱导类花生酸生物合成，因为在基础条件下，通过iPLA 2释放花生四烯酸的速率小于或等于其重新掺入细胞膜的速率;因此，
Address correspondence to: FA Fitzpatrick, Huntsman Cancer Institute, 2000 Circle of Hope, University of Utah, Salt Lake City, Utah 84112, USA. Phone:(801) 581-6204; Fax:(801) 585-0101; E-mail: frank. fitzpatrick@ hci. utah. edu. thesis, presumably de novo synthesis of the COX enzyme, contributed to the process. However, additional experiments in which NIH 3T3 cells were exposed to exogenous arachidonic acid, instead of PDGF, showed that these cells had ample basal capacity for COX-catalyzed formation of PGE2. Furthermore, the steady-state level of COX mRNA in NIH 3T3 cells rose, but this rise followed, rather than preceded, the increase in PGE2 synthesis. Finally, COX protein levels did not rise appreciably following PDGF stimulation, even when the corresponding mRNA levels were elevated. These data prompted two questions. First, why would cells rely on de novo synthesis of COX enzyme if they already had sufficient COX enzymatic capacity to convert any available arachidonic acid into PGE2? Second, if de novo synthesis of COX enzyme did account for increased PGE2 formation in cells incubated with PGDF, why were the temporal and stoichiometric relationships between mRNA accumulation, protein accumulation, and PGE2 formation so distorted? Herschman and colleagues’ discovery (12) of the COX-2 isoenzyme (see Smith and Langenbach, this Perspective series, ref. 13) would eventually clarify the quantitative and temporal distortions observed by Lin et al.(11). However, absent this discovery, Lin et al. proposed that coupling of the PDGF receptor to phospholipase and ultimately to COX relied on de novo synthesis of an unidentified protein. In other words, the PDGF-mediated increase in cellular PGE2 synthesis depends on de novo synthesis of proteins that coordinate the availability of arachidonic acid to its metabolism by COX during growth factor receptor occupancy. This hypothesis remains attractive and active today. For instance, Murakami et al.(14) have proposed the existence of a hypothetical accessory protein that integrates phospholipase–COX-2 interactions to explain how COX-2 expression enables cPLA2 activation. Given the renewed interest in functional coupling among the enzymes of the arachidonic acid cascade (15), it will be interesting to see if as-yet unidentified coupling proteins promote interactions among component receptors and enzymes. During the past year the concept of coupling among the phospholipase-COX-PGH isomerase enzymes has gained particular experimental support and clarity from the investigations by Kudo and colleagues (14, 16, 17). By expressing any of several forms of phospholipase A2 (the Ca2+-dependent cytosolic phospholipase cPLA2, the secretory phospholipase sPLA2; or the Ca2+-independent phospholipase iPLA2) in combination with ectopically expressed COX-1 or COX-2, these authors have established a hierarchy of functional interactions among the enzymes. First, under what might be termed basal conditions (Figure 1, red text and arrows), the Ca2+-independent iPLA2 is the dominant phospholipase involved in the liberation of arachidonic acid and related polyunsaturated fatty acids from membrane phospholipids. iPLA2 serves primarily in cell membrane remodeling and does not induce eicosanoid biosynthesis because, under basal conditions, the rate of arachidonic release via iPLA2 is less than or equal to the rate of its reincorporation into cell membranes; thus there is negligible accumulation