Summary
Matrix metalloproteinases (MMPs) are associated with tissue remodelling and repair.
In non-vascular tissues, NR4A receptors have been involved in the regulation of MMPs
by transcriptional repression mechanisms. Here, we analyse alternative mechanisms
involving NR4A receptors in the modulation of MMP activity in vascular smooth muscle
cells (VSMC). Lentiviral overexpression of NR4A receptors (NOR-1, Nurr1 and Nur77)
in human VSMC strongly decreased MMP-2 and MMP-9 activities (analysed by zymography
and DQ-gelatin assays) and protein levels. NR4A receptors also down-regulated MMP-2
mRNA levels. Real-time PCR analysis evidenced that alpha-2-macroglobulin (A2M), but
not other MMP inhibitors (TIMP-1 and TIMP-2) were up-regulated in NR4A-transduced
cells. Interestingly, A2M was expressed in human vascular tissues including the smooth
muscle media layer. While NR4A receptors increased A2M expression and secretion in
VSMC, NR4A knockdown significantly reduced basal A2M expression in these cells. The
direct transcriptional regulation of the human A2M promoter by NR4A receptors was
characterised in luciferase reporter assays, electrophoretic mobility shift assays
and by chromatin immunoprecipitation, identifying a NGFI-B response element (NBRE-71/-64)
essential for the NR4A-mediated induction. The blockade of A2M partially prevented
the reduction of MMPs activity observed in NR4A-transduced cells. Although mouse A2M
promoter was unresponsive to NR4A receptors, vascular MMP expression was attenuated
in transgenic mice over-expressing human NOR-1 in VSMC challenged with lipopolysaccharide.
Our results show that the panproteinase inhibitor A2M is expressed in the vasculature
and that NR4A receptors modulate VSMC MMP activity by several mechanisms including
the up-regulation of A2M.
Keywords
Matrix metalloproteinases - vascular remodelling - gene expression - transcription
factors - smooth muscle cells