In rodent models the loss of estrogens through ovariectomy

In rodent models, the loss of estrogens through ovariectomy has been shown to promote the development of intracranial aneurysms. Conversely, the administration of an p38 inhibitor receptor beta agonist restores its protective effect, but is abolished with the introduction of a nitric oxide synthase inhibitor, suggesting that the downstream regulation of estrogen signaling pathway may play a crucial role in the growth of aneurysms. A number of studies suggest there is a difference in prevalence of aneurysmal subarachnoid hemorrhage in men and women.21, 46 The age of menopause and use of hormone replacement therapy have also been widely studied, and the loss of estrogens is associated with prevalence of cerebral aneurysm,28, 29 further supporting the hypothesis that circulating estrogens may play a significant role in protecting against cerebral aneurysm growth. Although many studies have demonstrated a strong association between estrogens and development of intracranial aneurysms, few studies have analyzed the underlying genetic changes in the estrogen receptor pathway. Prior gene expression studies have identified gene loci associated with many biological pathways, including the extracellular matrix, inflammation, and the immune system.30, 47, 48 Although many candidate genes have been identified, none to date have been associated with the estrogen signaling pathway. Given the existing data supporting the association of estrogen with the development of cerebral aneurysms in both clinical studies and in animal models, we postulated that there are differences in estrogen receptor associated gene signaling between aneurysm and normal arterial tissue. In this study, we performed a meta-analysis of gene expression studies to identify differentially expressed genes associated with the estrogen signaling pathway. We identified 4 differentially expressed genes in cerebral aneurysm tissue (PIK3R1, ADCY9, HBEGF, and ADCY7). Of the 4 genes identified, 2 were found to be downregulated (PIK3R1 and ADCY9). PIK3RI is a regulatory subunit of phosphoinositide-3-kinase that acts as an adapter to mediate the association of the p110 catalytic unit to the plasma membrane, and is involved in suppressing cell growth and development. Its dysfunction and downregulation have been associated with oncologic growth.49, 50 ADCY9 encodes a membrane-bound adenylate cyclase, with the highest concentration found in the brain. In humans, ADCY9 has been found to be associated with obesity in prior genome-wide association studies and with the regulation of lipid metabolism. Expression and variability of lipid metabolism genes have also previously been identified to be associated with intracranial aneurysms, suggesting that the association between ADCY9 and the development of aneurysms may be through metabolic regulation associated with the estrogen receptor pathway. HBEGF and ADCY7 were 2 genes found to be upregulated in aneurysm tissue in our study. HBEGF is an angiogenic factor and an endogenous ligand of EGFR and ERBB4 that is associated with smooth muscle cell proliferation.53, 54 ADCY7 is a membrane-bound adenylyl cyclase that catalyzes the formation of cyclic adenosine monophosphate from adenosine tri-phosphate. It has been identified to be significantly increased in abdominal aortic aneurysm tissue in prior genome-wide association studies. Given hormonal differences between sexes, we performed subgroup analyses for each sex. Interestingly, PIK3R1 expression is significantly reduced in both women and men with similar fold changes. Additional genes were identified that were only significant in women (JUN, ADCY5, and MMP9), suggesting a potential difference between men and women in aneurysm formation. All 3 of these genes have been previously reported to be involved in the pathogenesis of cerebral aneurysms. JUN is a proto-oncogene which encodes the c-JUN protein, involved in the regulation of the extracellular matrix. The JNK/c-JUN pathway is involved in vascular remodeling via apoptotic regulation in the pathogenesis of cerebral aneurysms. Furthermore, increased expression of phosphorylated c-JUN has been reported in human cerebral aneurysms, and the inhibition of c-JUN activity has been demonstrated to regress aortic abdominal aneurysm. ADCY5 encodes a membrane enzyme involved in vascular smooth muscle regulation. The depletion of ADCY5 downregulates myocardin, a key contractility regulator of vascular smooth muscle cells that has also been thought to play a role in the generation of intracranial aneurysms. Matrix metalloproteinases are important elastases and play a significant role in vascular remodeling. Specifically, MMP9 is a member of the matrix metalloproteinase gene family, which encodes a proteolytic enzyme, with elevated levels found in the wall of intracranial aneurysms.48, 60, 61