Matrix metalloproteinase-3 is activated by HtrA2/Omi in dopaminergic cells: relevance to Parkinson's disease

Dopaminergic neurons in the substantia nigra are particularly vulnerable, and their degeneration leads to Parkinson's disease. We have previously reported that matrix metalloproteinase-3 (MMP-3) activity is involved in dopaminergic neurodegeneration by multiple mechanisms and that this requires activation of MMP-3 from proMMP-3 by an intracellular serine protease. HtrA2/Omi is a mitochondrial serine protease that has been shown in non-dopaminergic cells to translocate into the cytosol where it triggers apoptosis. In the present study we sought to determine whether HtrA2/Omi might cause activation of MMP-3 in dopaminergic neuronal cells using CATH.a cell line. Mitochondrial stress induced by rotenone led to MMP-3 activation and HtrA2/Omi translocation into the cytosol. The MMP-3 activation involved HtrA2/Omi, because both pharmacological inhibition and siRNA-induced knockdown of HtrA2/Omi attenuated the activation induced by rotenone or MPP+. Overexpression of mature HtrA2/Omi, but not mutant HtrA2/Omi, resulted in MMP-3 activity increase and cell death. Addition of recombinant and catalytically active HtrA2/Omi to lysate of untreated cells led to activation of the endogenous MMP-3, and incubation of the HtrA2/Omi with recombinant proMMP-3 caused cleavage of proMMP-3 to a 48kD protein, corresponding to the active form, which was accompanied by an increase in MMP-3 activity. Taken together, the data indicate that HtrA2/Omi, which normally exists in the mitochondria, can cause MMP-3 activation in the cytosol under a cell stress condition, which can ultimately lead to demise of dopaminergic neuronal cells.