Adult aortic valve interstitial cells have greater responses to toll-like receptor 4 stimulation

Self-eating and Heart: The Emerging Roles of Autophagy in Calcific Aortic Valve Disease

Autophagy is a self-degradative pathway by which subcellular elements are broken down intracellularly to maintain cellular homeostasis. Cardiac autophagy commonly decreases with aging and is accompanied by the accumulation of misfolded proteins and dysfunctional organelles, which are undesirable to the cell. Reduction of autophagy over time leads to aging-related cardiac dysfunction and is inversely related to longevity. However, despite the increasing interest in autophagy in cardiac diseases and aging, the process remains an undervalued and disregarded object in calcific valvular disease. Neither the nature through which autophagy is triggered nor the interplay between autophagic machinery and targeted molecules during aortic valve calcification are fully understood. Recently, the upregulation of autophagy has been shown to result in cardioprotective effects against cell death as well as its origin. Here, we review the evidence that shows how autophagy can be both beneficial and detrimental as it pertains to aortic valve calcification in the heart.

Collagen fiber regulation in human pediatric aortic valve development and disease

Congenital aortic valve stenosis (CAVS) affects up to 10% of the world population without medical therapies to treat the disease. New molecular targets are continually being sought that can halt CAVS progression. Collagen deregulation is a hallmark of CAVS yet remains mostly undefined. Here, histological studies were paired with high resolution accurate mass (HRAM) collagen-targeting proteomics to investigate collagen fiber production with collagen regulation associated with human AV development and pediatric end-stage CAVS (pCAVS). Histological studies identified collagen fiber realignment and unique regions of high-density collagen in pCAVS. Proteomic analysis reported specific collagen peptides are modified by hydroxylated prolines (HYP), a post-translational modification critical to stabilizing the collagen triple helix. Quantitative data analysis reported significant regulation of collagen HYP sites across patient categories. Non-collagen type ECM proteins identified (26 of the 44 total proteins) have direct interactions in collagen synthesis, regulation, or modification. Network analysis identified BAMBI (BMP and Activin Membrane Bound Inhibitor) as a potential upstream regulator of the collagen interactome. This is the first study to detail the collagen types and HYP modifications associated with human AV development and pCAVS. We anticipate that this study will inform new therapeutic avenues that inhibit valvular degradation in pCAVS and engineered options for valve replacement.

Deregulation of TLR4 signaling pathway characterizes Bicuspid Aortic valve syndrome

Bicuspid aortic valve (BAV) disease is recognized to be a syndrome with a complex and multifaceted pathophysiology. Its progression is modulated by diverse evolutionary conserved pathways, such as Notch-1 pathway. Emerging evidence is also highlighting the key role of TLR4 signaling pathway in the aortic valve pathologies and their related complications, such as sporadic ascending aorta aneurysms (AAA). Consistent with these observations, we aimed to evaluate the role of TLR4 pathway in both BAV disease and its common complication, such as AAA. To this aim, 70 subjects with BAV (M/F 50/20; mean age: 58.8 ± 14.8 years) and 70 subjects with tricuspid aortic valve (TAV) (M/F 35/35; mean age: 69.1 ± 12.8 years), with and without AAA were enrolled. Plasma assessment, tissue and gene expression evaluations were performed. Consistent with data obtained in the previous study on immune clonotypic T and B altered responses, we found reduced levels of systemic TNF-α, IL-1, IL-6, IL-17 cytokines in BAV cases, either in the presence or absence of AAA, than TAV cases (p < 0.0001 by ANOVA test). Interestingly, we also detected reduced levels of s-TLR4 in BAV cases with or without AAA in comparison to the two groups of TAV subjects (p < 0.0001 by ANOVA test). These results may suggest a deregulation in the activity or in the expression of TLR4 signaling pathway in all BAV cases. Portrait of these data is, indeed, the significantly decreased gene expression of inflammatory cytokines and TLR4, in both normal and aneurysmatic tissue samples, from BAV with AAA than TAV with AAA. In conclusion, our study demonstrates that subjects with BAV display a significant deregulation of TLR4 signaling pathway paralleled by a deregulation of Notch-1 pathway, as previously showed. This data suggests that the crosstalk between the Notch-1 and TLR4 signaling pathways may play a crucial role in both physiological embryological development, and homeostasis and functionality of aortic valve in adult life.

Toll-Like Receptors, Inflammation, and Calcific Aortic Valve Disease

Inflammation, the primary response of innate immunity, is essential to initiate the calcification process underlying calcific aortic valve disease (CAVD), the most prevalent valvulopathy in Western countries. The pathogenesis of CAVD is multifactorial and includes inflammation, hemodynamic factors, fibrosis, and active calcification. In the development of CAVD, both innate and adaptive immune responses are activated, and accumulating evidences show the central role of inflammation in the initiation and propagation phases of the disease, being the function of Toll-like receptors (TLR) particularly relevant. These receptors act as sentinels of the innate immune system by recognizing pattern molecules from both pathogens and host-derived molecules released after tissue damage. TLR mediate inflammation via NF-κB routes within and beyond the immune system, and play a crucial role in the control of infection and the maintenance of tissue homeostasis. This review outlines the current notions about the association between TLR signaling and the ensuing development of inflammation and fibrocalcific remodeling in the pathogenesis of CAVD. Recent data provide new insights into the inflammatory and osteogenic responses underlying the disease and further support the hypothesis that inflammation plays a mechanistic role in the initiation and progression of CAVD. These findings make TLR signaling a potential target for therapeutic intervention in CAVD.

High mobility group box 1 induces calcification of aortic valve interstitial cells via toll-like receptor 4

Chronic inflammation and the calcification of aortic valve interstitial cells (AVICs) are the primary etiologies of calcific aortic valve disease (CAVD). However, the underlying mechanism remains to be elucidated. The present study investigated the importance of high mobility group box 1 (HMGB1) via toll-like receptor 4 (TLR4) for the regulation of inflammation and calcification in AVICs. It was determined that the expression levels of HMGB1 and TLR4 were increased in the calcific region of aortic valves with CAVD. In cultured primary AVICs from wild-type mice, HMGB1 treatment demonstrated a dose-dependent increase in mineralization levels and osteogenic gene expression. These effects were significantly reduced in AVICs obtained from TLR4 knockout mice (TLR4^−/−). In addition, calcification was inhibited by TLR4-specific antibodies in primary AVICs. HMGB1 induced the activation of p38 and nuclear factor-κB (NF-κB) in TLR4^−/− primary AVICs, and inhibited p38 and NF-κB in wild-type AVICs treated with TLR4-specific antibodies. The present study demonstrated that TLR4 may function as an essential mediator of HMGB1-induced calcification and in the activation of p38 and NF-κB.

Rapamycin Decreases the Osteogenic Response in Aortic Valve Interstitial Cells Through the Stat3 Pathway

BACKGROUND

Calcific aortic valve disease (CAVD) is an age-related and slowly progressive valvular disorder. We have previously found that the increased inflammatory and osteogenic responses to Toll-like receptor 4 (TLR4) stimulation is correlated with lower signal transducer and activator of transcription 3 (Stat3) activity in aortic valve interstitial cells (AVICs). Rapamycin, a drug used clinically, induces feedback activation of Akt. Akt in turn may upregulate Stat3. Therefore we hypothesized that rapamycin will decrease TLR4-induced osteogenic response in human AVICs through modulation of Stat3 activity.

METHODS

AVICs were isolated from normal valves taken from the explanted hearts of patients undergoing transplantation. Cells were treated with TLR4 ligand lipopolysaccharide (LPS) or rapamycin, or both. The osteogenic markers runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), and bone morphogenetic protein 2 (BMP-2), as well as activation of Stat3 and its associated signaling molecules, were analyzed.

RESULTS

LPS induces the expression of RUNX2, ALP, and BMP-2. Rapamycin decreased both the baseline and LPS-induced expression of RUNX2, ALP, and BMP-2. Rapamycin also decreased calcium deposit formation. Rapamycin activated both Stat3 and Akt in AVICs. Suppression of Akt resulted in abolishment of Stat3 activation. Inhibition of Stat3 enhanced expression of RUNX2, ALP, and BMP-2 at baseline and in response to LPS.

CONCLUSIONS

Rapamycin inhibits TLR4-induced osteogenic responses in AVICs by activation of Stat3 through Akt. Rapamycin may alleviate inflammation-induced initiation and progression of CAVD.