AN X-RAY MICROSCOPIC STUDY OF THE BLOOD SUPPLY TO THE VALVES OF THE HUMAN HEART

Hypoxia-inducible factor activation promotes osteogenic transition of valve interstitial cells and accelerates aortic valve calcification in a mice model of chronic kidney disease

Introduction

Valve calcification (VC) is a widespread complication in chronic kidney disease (CKD) patients. VC is an active process with the involvement of in situ osteogenic transition of valve interstitial cells (VICs). VC is accompanied by the activation of hypoxia inducible factor (HIF) pathway, but the role of HIF activation in the calcification process remains undiscovered.

Methods and result

Using in vitro and in vivo approaches we addressed the role of HIF activation in osteogenic transition of VICs and CKD-associated VC. Elevation of osteogenic (Runx2, Sox9) and HIF activation markers (HIF-1α and HIF-2α) and VC occurred in adenine-induced CKD mice. High phosphate (Pi) induced upregulation of osteogenic (Runx2, alkaline-phosphatase, Sox9, osteocalcin) and hypoxia markers (HIF-1α, HIF-2α, Glut-1), and calcification in VICs. Down-regulation of HIF-1α and HIF-2α inhibited, whereas further activation of HIF pathway by hypoxic exposure (1% O₂) or hypoxia mimetics [desferrioxamine, CoCl₂, Daprodustat (DPD)] promoted Pi-induced calcification of VICs. Pi augmented the formation of reactive oxygen species (ROS) and decreased viability of VICs, whose effects were further exacerbated by hypoxia. N-acetyl cysteine inhibited Pi-induced ROS production, cell death and calcification under both normoxic and hypoxic conditions. DPD treatment corrected anemia but promoted aortic VC in the CKD mice model.

Discussion

HIF activation plays a fundamental role in Pi-induced osteogenic transition of VICs and CKD-induced VC. The cellular mechanism involves stabilization of HIF-1α and HIF-2α, increased ROS production and cell death. Targeting the HIF pathways may thus be investigated as a therapeutic approach to attenuate aortic VC.

Aortic valve: anatomy and structure and the role of vasculature in the degenerative process

Aortic valve stenosis is a degenerative disease affecting increasing number of individuals and characterised by thickening, calcification and fibrosis of the valve resulting in restricted valve motion. Degeneration of the aortic valve is no longer considered a passive deposition of calcium, but an active process that involves certain mechanisms, that is endothelial dysfunction, inflammation, increased oxidative stress, calcification, bone formation, lipid deposition, extracellular matrix (ECM) remodelling and neoangiogenesis. Accumulating evidence indicates an important role for neoangiogenesis (i.e. formation of new vessels) in the pathogenesis of aortic valve stenosis. The normal aortic valve is generally an avascular tissue supplied with oxygen and nutrients via diffusion from the circulating blood. In contrast, presence of intrinsic micro-vasculature has been demonstrated in stenotic and calcified valves. Importantly, presence and density of neovessels have been associated with inflammation, calcification and bone formation. It remains unclear whether neoangiogenesis is a compensatory mechanism aiming to counteract hypoxia and increased metabolic demands of the thickened tissue or represents an active contributor to disease progression. Data extracted mainly from animal studies are supportive of a direct detrimental effect of neoangiogenesis, however, robust evidence from human studies is lacking. Thus, there is inadequate knowledge to assess whether neoangiogenesis could serve as a future therapeutic target for a disease that no effective medical therapy exists. In this review, we present basic aspects of anatomy and structure of the normal and stenotic aortic valve and we focus on the role of valve vasculature in the natural course of valve calcification and stenosis.

Vascularity of human atrioventricular valves: a myth or fact?

BACKGROUND

Knowledge of heart valve vascularity is an important factor for understanding the valvular pathology and to develop tissue-engineered valves for repair procedures. Some investigators believe that blood vessels may exist in normal human heart valves whereas some recent publications have proposed that the presence of blood vessels in the valves is secondary to inflammation.

METHODS

Tissues from 60 normal formalin-fixed human hearts were examined microscopically for type, location, and number of vessels in atrioventricular valves. The age of the patient ranged from 10 to 70 years, and an attempt was made to study the age-related morphologic changes in atrioventricular valves.

RESULTS

Of the 60 tricuspid and 60 mitral valves examined, 12 tricuspid (20%) and 14 mitral (23.33%) valves were found to have vessels without the presence of an inflammatory process. In tricuspid valves the vessels were observed mainly in the fibrosa layer with a range of 1 to 4 vessels, whereas in mitral valves the vessels were situated mainly in the spongiosa layer with a range of 1 to 2 vessels. The maximum vascularity was seen in the fourth decade of life, in which the vessels were found in 40% of both tricuspid and mitral valves. The mean transverse diameter of these vessels was 0.23 ± 0.18 mm, with a range of 0.06 to 0.79 mm in tricuspid valves, whereas it was 0.15 ± 0.08 mm, with a range of 0.04 to 0.4 mm in mitral valves. The capillaries (3-11 capillaries) were found scattered in the fibrosa and spongiosa with an average lumen area of 0.39 ± 0.18 mm(2).

CONCLUSIONS

The blood vessels in atrioventricular valves also can be seen in the absence of inflammation and are likely to be a necessary component of valve leaflets. Thus, when performing procedures involving in situ tissue engineering and valve repair the physician needs to be aware of the presence of these vessels in human heart valves.

Pathogenesis of "senile" nodular sclerosis of atrio-ventricular valves

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Pathology of the human aortic valve homograft

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