PP2A‐mediated dephosphorylation of eNOS and Erk2: A new molecular mechanism of impaired angiogenesis in experimental duodenal ulcer

Angiogenesis is essential for wound/ulcer healing, but our studies demonstrated an impaired angiogenesis despite elevated VEGF levels in duodenal mucosa. We hypothesized that concurrent elevation of anti‐angiogenic factor endostatin might interfere with VEGF‐induced angiogenesis via activating protein phosphatase 2A (PP2A) that causes dephosphorylaton of eNOS and Erk2. Methods Unfasted SD female rats were given saline or duodenal ulcerogen cysteamine‐HCl (85mg/100g, x1) by gavage. Some rats were also treated with saline or PP2A inhibitor okadaic acid (50 nmol/rat, x2, s.c.) 30 min before and 3 days after cysteamine. The animals were euthanized 3 or 7 days after cysteamine. Duodenal ulcers were measured at autopsy and scrapings of the 2.5 cm proximal duodenal mucosa were harvested to measure phosphorylation of PP2A, eNOS, and Erk2 in duodenal mucosa by ELISA and Western blotting. Angiogenesis was analyzed by CD34 immunostaining in duodenal tissues. Results Phosphorylation of eNOS and Erk2 was significantly decreased in duodenal mucosa after cysteamine in rats. Expression and enzymatic activity of PP2A were markedly increased (3‐ to 4‐folds) during cysteamine‐induced ulceration. Duodenal ulcers were significantly reduced in the rats treated with okadaic acid compared to the control (5.3±0.8 vs. 11.3±2.2 mm 2 , p<0.05). Duodenal microvascular density was significantly increased in the rats treated with okadaic acid (p < 0.05). Conclusions The two major mediators eNOS and Erk2, involved in VEGF angiogenic pathway, were blocked by the increased PP2A during duodenal ulceration, resulting in impaired angiogenesis and delayed ulcer healing. Thus, inhibition of PP2A may be a new therapeutic option for duodenal ulce healing. (The present study was supported by a grant from Southern California Institute of Research and Education)