It is now recognized that epithelial cells are critical cell population in the initiation, regulation and resolution of innate and adaptive immune responses at mucosal sites. Thus, the intestinal epithelium forms a regulated and selectively permeable barrier that allows passage of nutrients, but restricts the access of potential harmful substances. These events are consequence, at least in part, of a highly dynamic continuously renewed/repair processes involving cell proliferation and migration.
Arachidonic acid (AA), a common n−6 polyunsaturated fatty acid (PUFA), is found esterified at the sn-2 position of membrane phospholipids. When AA is released, it is oxidized by cyclooxygenases (COX) to produce prostaglandins (PG) such as PGE2. Moreover, AA is also metabolized by lipoxygenases (LOX) producing leukotrienes (LT) such as LTB4 and hydroxyeicosatetraenoic acids (HETEs). Eicosapentaenoic acid (EPA), an n–3 PUFA found mainly in fish oil, can also function as a substrate for COX-2 and LOXs resulting in the synthesis of 3-series PG, 5-series LT and hydroxyeicosapentaenoic acids (HEPEs)(Reference Needleman1, Reference Kobayashi2). Recently, we observed the role of AA metabolites produced by COX on Caco-2 cell growth(Reference Sánchez and Moreno3, Reference Martín-Venegas4). Moreover, AA metabolites of LOX pathway are also involved in epithelial cell proliferation(Reference Moreno5). Taking into account the above-mentioned facts, we sought to investigate the effect of n–3 and n−6 eicosanoids on Caco-2 cell proliferation.
Cell growth was determined by microscopic assay using ethidium bromide/acridine orange staining in preconfluent Caco-2 cell cultures in the presence of eicosanoids (48 h). The data (n=6–9 for each condition) were compared by Student's t test and in all cases, *P<0.05 was considered to denote significance.
Our results show that PGE2 and PGE3 (0.1–10 nM) significantly induce Caco-2 growth in a concentration-dependent manner, reaching an enhancement of almost 100% respect to control condition (Fig. 1). Interestingly, the effect of PGE3 was slight higher than PGE2 and both were blocked by EP1 (SC19220, 60 nM) and EP4 (AH23848, 20 nM) antagonist, but not by EP2 (ONO240, 2 nM) antagonist. LTB4 (1–100 nM) was also able to significantly increase Caco-2 proliferation (50%), whereas LTB5 was without effect (Figure 2). This mitogenic effect of LTB4 (10 nM) was completely reverted by a BLT1 antagonist (U75032, 1 μM). Finally, we observed that 12-HETE and 12-HEPE (0.1 μM) present a significant proliferative action on intestinal epithelial cells (Fig. 3). Thus, n–3 and n−6 eicosanoids synthetised by COX-2 and 12-LOX might be involved in the control of renewed/repair processes in the intestinal epithelium, whereas n–3 but not n−6 metabolites from 5-LOX could also participate in these events.
This work was supported by the Ministerio de Ciencia y Tecnología (BFU2007-61727/BFI) and the Generalitat de Catalunya (2005SGR0269).
