Upregulated iRhom2 in the Hypothalamic Paraventricular Nucleus is Associated with TACE‐Mediated Shedding of Transforming Growth Factor‐α and Sympathetic Excitation in Heart Failure Rats

Abstract

Tumor necrosis factor (TNF)‐α converting enzyme (TACE), also known as a disintegrin and metalloprotease (ADAM)17, is a key mediator of cell signaling by proteolytically cleaving extracellular domains of various cytokines and growth factors. TACE‐mediated shedding of transforming growth factor (TGF)‐α, has been shown to transactivate epidermal growth factor receptor (EGFR) to activate the mitogen‐activated protein kinase signaling pathway in the pathophysiological conditions. We previously reported that both TACE and TGF‐α are upregulated in the hypothalamic paraventricular nucleus (PVN, a critical cardiovascular and autonomic center) and contribute to the sympathetic excitation in heart failure (HF). However, the mechanisms by which TACE and TGF‐α are upregulated in the PVN in HF remain unclear. Recent evidence reveals that inactive Rhomboid proteins (iRhoms) including iRhom1 and iRhom2 are essential upstream regulators of TACE/ADAM17. The present study sought to determine the expression of iRhoms and its relationship with TACE‐mediated shedding of TGF‐α in the PVN in HF. Male rats underwent coronary artery ligation to induce HF or sham surgery (Sham). The left ventricular function and infarction size were assessed by echocardiography. Four weeks later, these animals were euthanized to collect cerebrospinal fluid (CSF), PVN tissues and blood for molecular and biochemical measurements. All values are expressed as the mean ± SD. Real‐time PCR showed abundant mRNA expression of both iRhom1 and iRhom2 in the PVN and brain cortex in Sham or HF rats. Compared with Sham rats, HF rats had significantly (*P<0.05) increased mRNA expression of iRhom2 (2.00 ± 0.57* vs 1.03 ± 0.28, HF vs Sham) but not iRhom1 (1.32 ± 0.41 vs 1.02 ± 0.21) in the PVN. Additionally, HF rats exhibited marked increases in mRNA expression of TACE (2.25 ± 1.15* vs 1.04 ± 0.36) in the PVN and the plasma level of norepinephrine (NE, a marker of sympathetic excitation, 12.39 ± 4.96* vs 5.82 ± 2.76 ng/mL), along with elevated levels of TGF‐α (23.54 ± 6.25* vs 10.08 ± 4.38 pg/mL) in CSF. The levels of epidermal growth factor (9.88 ± 3.59 vs 8.15 ± 4.24 pg/mL) in the CSF did not alter significantly in HF compared with Sham rats. Moreover, the linear regression analysis indicated that the mRNA expression of iRhom2 in the PVN was positively correlated with TACE level in the PVN (R2 = 0.62, P <0.01), TGF‐α level in the CSF (R2 = 0.64, P <0.01) and NE level in the plasma (R2 = 0.63, P <0.01). These results suggest that the upregulated expression of iRhom2 in the PVN may promote ectodomain shedding of TGF‐α by enhancing TACE expression to drive sympathetic activation in HF. Brain iRhom2 is a potential target for therapeutic intervention in HF.