Nitric oxide, derived from inducible nitric oxide synthase, decreases hypoxia inducible factor-1α in macrophages during aspirin-induced mesenteric inflammation

Background and purpose: Nitric oxide (NO) modulates expression of hypoxia inducible factor-1 (HIF-1), a transcription factor regulating function of myeloid cells. Here, we have assessed the role played by NO, formed by inducible NOS (iNOS), in the inflammation induced by aspirin in the gut, by modulating HIF-1 activity. Experimental approach: The role of iNOS-derived NO on leucocyte-endothelial interactions induced by aspirin was evaluated by intravital microscopy in mesenteric venules of rats pretreated with selective iNOS inhibitors, 1400W or l-N6-(1-iminoethyl)-lysine. NO was localized by fluorescence microscopy, using DAF-FM. iNOS, HIF-1 alpha and CD36 were localized by immunohistochemistry. Key results: Leucocyte-endothelial interactions increased at 6 h and returned to normal levels 24 h after aspirin administration. Numbers of migrated leucocytes were similar between 6 and 24 h after aspirin. iNOS expression and iNOS-derived NO synthesis were observed in leucocytes of the mesentery of aspirin-treated rats. Blockade of iNOS activity in aspirin-treated rats: (i) did not modify leucocyte infiltration at 6 h, but reduced the number of polymorphonuclear leucocyte and increased that of macrophages at 24 h; (ii) increased HIF-1 alpha immunostaining in macrophages of the mesentery; and (iii) prevented the decrease in CD36 immunostaining induced by aspirin in these cells. Conclusions and implications: NO, associated with acute gut inflammation induced by aspirin, diminished HIF-1 alpha stabilization in macrophages. Early inhibition of iNOS-derived NO synthesis, by increasing the activity of HIF-1 in these cells, may accelerate the clearance of leucocytes.