Reactive oxygen species are critical in the oleic acid-mediated mitogenic signaling pathway in vascular smooth muscle cells

Obese hypertensive patients with cardiovascular risk factor clustering have increased plasma nonesterified fatty acid levels and are at high risk for atherosclerotic events. Our previous studies demonstrated that oleic acid induces a mitogenic response in rat aortic smooth muscle cells (RASMCs) through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. In the present study we investigated the possibility that the generation of reactive oxygen species (ROS) constitutes a critical component of the oleic acid-induced mitogenic signaling pathway in RASMCs. We studied the effect(s) of oleic acid on the generation of ROS using the oxidant-sensitive fluoroprobe 2',7'-dichlorofluorescin diacetate. Relative fluorescence intensity and fluorescent images were obtained with laser confocal scanning microscopy from 1 to 5 minutes, since preliminary studies demonstrated that the peak fluorescence intensity occurred within 5 minutes. Oleic acid (100 μmol/L) induced a time-dependent increase of cell fluorescence that was >8-fold of that seen in control cells at 5 minutes. This was blocked by catalase, which suggests that H₂O₂ was the principal ROS. The oleic acid-induced increases in H₂O₂ were blocked when PKC was inhibited with the use of bisindolylmaleimide and when PKC activity was downregulated by exposing RASMCs to phorbol 12-myristate 13-acetate for 24 hours. Stearic and elaidic acids, which are weak PKC activators, did not significantly increase H₂O₂ production. The increase of H₂O₂ in response to oleic acid was inhibited by the antioxidant N-acetylcysteine. N- Acetylcysteine also completely blocked ERK activation and the increase of thymidine incorporation in response to oleic acid. The data suggest that generation of H₂O₂ in RASMCs exposed to oleic acid is PKC dependent. Moreover, H₂O₂ production emerges as a critical intermediary event in the oleic acid-mediated mitogenic signaling pathway between the activation of PKC and ERK. These observations raise the possibility that the elevated plasma nonesterified fatty acids, including oleic acid, in obese hypertensive patients contribute to vascular growth and remodeling by a PKC-dependent mechanism to generate ROS that subsequently activate ERK.