Macrophage 12/15-lipoxygenase Controls the Development of Experimental Hypertension through PPAR?

Targeted disruption of the 12/15-lipoxygenase (Alox15) gene in mice prevents the development of angiotensin II-, DOCA-salt-, and L-NAME-induced experimental hypertension. Macrophages are the primary source of Alox15. Using Alox15-/- and wild type (WT) mice, we previously demonstrated that macrophage Alox15 is essential for experimental hypertension; however, the underlying mechanism is not known. Since Alox15-derived metabolites of arachidonic and linoleic acids are potent PPARγ agonists, we hypothesized that activation of macrophage PPARγ is the key step in Alox15 mediation of L-NAME-induced hypertension. Thioglycollate (TG) injection, used to elicit peritoneal macrophages, upregulated PPARγ protein expression and expression of its target gene CD36 in peritoneal macrophages, but not in aorta or kidney, of both WT (6.8±0.3 and 9.3±0.5 fold increase for PPARγ and CD36, respectively) and Alox15-/- mice (4.7±0.4 and 11.1±0.6 fold increase for PPARγ and CD36, respectively). Thus, TG bypassed Alox15 to activate PPARγ selectively in macrophages. Upregulation of PPARγ by TG restored the sensitivity to L-NAME hypertension (100 mg/kg/day L-NAME for 7 days) in Alox15-/- mice (129.4±7.3 mmHg for TG vs. 94.8±5.6 mmHg for non-injected controls, p<0.005). A similar hypertensive effect to L-NAME was observed with the adoptive transfer of TG-elicited Alox15-/- macrophages into Alox15-/- recipient mice (122.4±5.5 mmHg for macrophage vs. 96.2±6.2 mmHg for vehicle, p<0.05). The role of PPARγ was further specified by using the selective PPARγ antagonist, GW9662. When WT mice were treated with GW9662, 50 μg/kg/day for 12 days, L-NAME-induced hypertension did not develop (97.6±4.8 mmHg for GW9662 vs. 131.7±6.5 mmHg for vehicle, p<0.01). The PPARγ antagonist also prevented L-NAME-induced hypertension in TG-injected Alox15-/- mice, indicating a PPARγ-mediated effect in macrophages that results in hypertension. These results demonstrate that TG selectively upregulates macrophage PPARγ, bypassing the missing component in Alox15-/- macrophages, and permitting L-NAME-induced hypertension in Alox15-/- mice. Thus, the macrophage Alox15/PPARγ pathway is an important checkpoint in the development of L-NAME-induced hypertension.