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

OBJECTIVE: Postmenopausal women taking hormone replacement therapy (HRT) require secondary intervention following vascular reconstruction more often than women not taking HRT due to increased intimal hyperplasia (IH). Matrix metalloproteinases (MMPs) play a role in 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 were aged to 12 months and ovariectomized (OVX). Four weeks later hormones or placebo were delivered via a 90-day slow-release pellet. Following six weeks of HRT each rat underwent balloon angioplasty of the left common carotid artery. At 14 days post-injury tissue samples were collected and stained with Trichrome elastin and for isoform-specific MMPs.
RESULTS: OVX in the placebo group reduced I:M ratios to 0.94±0.03 compared to 1.25±0.26 in non-OXV controls (n=3). Following OVX, estrogen replacement alone had very little effect (1.07±0.17) compared to placebo while 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 following injury dropped in response to OVX and was increased by hormone therapy (Table 1; n=2-3). Conversely, estrogen resulted in a 5-fold decrease in TIMP-2 compared to placebo (Table 1; n=2-3). At 14 days post-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, then decreases significantly. Assays at earlier time points are needed in hormone groups to examine the role of HRT in the possible exacerbation of this early peak.

CONCLUSION: Here we demonstrate ovariectomy may result in decreased IH development, contrary to the theory endogenous hormones are protective against vascular disease. Furthermore, we show progesterone alone and in combination with estrogen may increase IH, consistent with the theory HRT plays a deleterious role 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 via 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.