Hormone Replacement Therapy Influences Intimal Hyperplasia after Vascular Injury: Role of Matrix Metalloproteinases

Objective: Postmenopausal women taking hormone replacement therapy (HRT) require a secondary intervention after vascular reconstruction due to increased intimal hyperplasia (IH) more often than women not taking HRT. Matrix metalloproteinases (MMPs) affect IH by degradation of components of the basement membrane, allowing excess vascular smooth muscle cell (VSMC) migration and proliferation. The MMP pathway is regulated by a balance between MMPs, membrane type-MMPs (MT-MMPs), and tissue inhibitor of MMPs (TIMPs), and we have recently provided evidence for unbalanced regulation of this pathway in VSMCs exposed to hormones in vitro. Here we studied the role of HRT in the modulation of this regulatory pathway in vivo and on the development of IH in a postmenopausal rodent model of vascular injury. Methods: Female rats aged to 12 months underwent ovariectomy. Hormones or placebo were delivered 4 weeks later through a 90-day slow-release pellet. After 6 weeks of HRT, each rat underwent balloon angioplasty of the left common carotid artery. At 14 days after injury, tissue samples were collected and stained with trichrome elastin and for isoform-specific MMPs. Results: Ovariectomy in the placebo group reduced I/M ratios to 0.94 ± 0.03 compared with 1.25 ± 0.26 in non-ovariectomy controls (n = 3). After ovariectomy, estrogen replacement alone had very little effect (1.07 ± 0.17) compared with placebo, whereas progesterone alone and in combination with estrogen increased I/M ratios to 1.26 ± 0.05 and 1.16 ± 0.21, respectively. Intimal expression of MMP-2 and −9 after injury dropped in response to ovariectomy and was increased by HRT (Table; n = 2 to 3). Conversely, estrogen resulted in a fivefold decrease in TIMP-2 compared with placebo (Table; n = 2 to 3). At 14 days after injury, there was no effect on intimal MT1-MMP in any group. In a time course study of IH development, we have previously shown MT1-MMP is highest at day 2, and then decreases significantly. Assays at earlier times are needed in hormone groups to examine the role of HRT in the possible exacerbation of this early peak. Conclusion: Ovariectomy may result in decreased IH development, contrary to the theory that endogenous hormones are protective against vascular disease. Furthermore, we show progesterone alone and combined with estrogen may increase IH, consistent with the theory that HRT has a deleterious affect in vascular pathology. Larger study groups are needed to delineate the actual role of HRT as a modulator of IH in vivo. We previously reported that hormone exposure results in unbalanced MMP regulation in vitro. Here we demonstrate HRT modulates the MMP regulatory pathway in vivo, specifically by the upregulation of MMP-2 and −9 without a counter-regulatory increase in TIMPs. This unbalanced regulation may be a key factor in increased IH development seen with HRT. In future studies, in vivo manipulation of this unbalanced MMP regulation for prevention of IH should be examined, and timing of HRT initiation with respect to vascular injury should be addressed.