Hyaluronan fragmentation and MMP‐12 expression during pulmonary ischemia

MMP‐12 has been shown to be important during neovascularization, however, specific mechanisms of action are controversial. Others have shown increased MMP‐12 activity in macrophages after challenge with low molecular wt hyaluronan (LMW HA). In a mouse model where the magnitude of lung angiogenesis is correlated with the number of lavaged macrophages, we measured total HA (ELISA), HA fragments (110–330kDA; agarose gel) and MMP‐12 gene expression (real‐time RT‐PCR). We studied the left lung of mice after the induction of complete pulmonary ischemia with/without antioxidant pretreatment (N‐acetylcysteine; NAC) since reactive oxygen species provide the initiating stimulus for angiogenesis (Nijmeh, 2010). Total HA did not differ between untreated and NAC‐treated mice 4 hrs after the induction of ischemia. However, LMW HA fragments increased on average, 33‐fold greater than control (0 hr lungs). Pretreatment of mice with NAC reduced LMW HA fragments by 88%. Contrary to expectation, MMP‐12 expression increased 4‐fold in NAC‐treated mice compared to ischemia alone. Additionally, MMP‐12 deficient mice showed increased angiogenesis after ischemic insult compared to wild‐type mice. We conclude that accumulation of LMW HA does not appear to stimulate resident lung cells to increase MMP‐12 expression during ischemia. Our results demonstrate that MMP‐12 is angiostatic in this model.