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