Activation of MMP-9 Activity by Acrolein in Saliva from Patients with Primary Sjögren's Syndrome and Its Mechanism

We have recently reported that the altered recognition patterns of immunoglobulins due to acrolein conjugation are at least partially responsible for autoimmune diseases in patients with primary Sjögren's syndrome (pSS). In the current study, it was found that the specific activity (activity/ng protein) of metalloproteinase-9 (MMP-9) in saliva was elevated about 2.4-fold in pSS patients. Accordingly, it was examined whether MMP-9 is activated by acrolein. It was found that the MMP-9 with 92kDa molecular weight was activated by acrolein. Under the conditions studied, Cys99, located in the propeptide, was conjugated with acrolein together with Cys230, 244, 302, 314, 329, 347, 361, 373, 388 and 516, which are located in fibronectin repeats and glycosyl domains, but not on the active site of MMP-9. In addition, 82 and 68kDa constructs of MMP-9s, lacking the NH-terminal domain that contains Cys99, were not activated by acrolein. The results suggest that acrolein conjugation at Cys99 caused the active site of MMP-9 to be exposed. Activation of MMP-9 by acrolein was inhibited by cysteine, and slightly by lysine, because these amino acids inhibited acrolein conjugation with MMP-9. Conversely, MMP-9 activity in the presence of 50μM acrolein was enhanced by 100μM histidine. This was due to the inhibition of acrolein conjugation with His405 and 411 located at the Zn binding site of MMP-9. These results suggest that activation of 92kDa MMP-9 by acrolein is involved in tissue damage in pSS patients and is regulated by cysteine and histidine, and slightly by lysine. Activated 82 and 68kDa MMP-9s were not detected in saliva of pSS patients by Western blotting.