Hyperglycaemia-induced degeneration of bioprosthetic valve tissue and the pivotal role of the extracellular matrix molecule biglycan

Abstract

Abstract Background/Introduction Bioprosthetic valve degeneration is a growing clinical challenge in our aging society. Especially type 2 diabetes mellitus represents a risk factor for bioprosthetic valve deterioration with likewise worldwide rising prevalence. The proteoglycan biglycan has been shown to be associated with degenerative changes of the valve in diabetic patients even though underlying mechanisms are yet unknown. Purpose The impact of hyperglycaemia on bioprosthetic valve tissue was analysed in an established mouse model of ectopic calcification. Thereby, the role of biglycan in bioprosthetic valve degeneration was assessed in biglycan-deficient mice (Bgn-/0) in comparison to wild type littermates (WT). Methods Hyperglycaemia was induced in six weeks old mice by intraperitoneal injection of streptozotocin (STZ; 55 mg/kg body weight) on five consecutive days. Blood glucose concentration was verified two weeks after treatment and bioprosthetic valve material was implanted subcutaneously. After eight weeks, phenotype of invading cells and extracellular matrix remodelling of implants were analysed with histochemical and immunohistological staining. Furthermore, gene expression of invading cells, circulating cytokines in blood plasma and calcium accumulation in the tissue were quantified. Results STZ treatment significantly increased blood glucose levels in both genotype groups (WT-STZ: 156 mg/dl; WT+STZ: 353 mg/dl; Bgn-/0-STZ: 142 mg/dl; Bgn-/0+STZ: 396 mg/dl). Immunohistological staining identified most of cells invading the bioprosthetic tissue and the surrounded capsule as Mac2-positive and partly positive for vimentin, whereas alpha smooth muscle actin and von Willebrand factor were only detectable sporadically. Movat pentachrome staining exhibited an altered extracellular matrix composition of collagen and proteoglycans due to hyperglycaemia, while the elastin amount remained stable. Gene expression analysis showed an increased expression of the proteoglycan decorin in hyperglycaemic mice (WT-STZ: 0.55; WT+STZ: 1.13; Bgn-/0-STZ: 0.51; Bgn-/0+STZ: 1.70). Circulating cytokines (IL-1β, IL-2, IL-5, IL-10, INF-γ, TNF-α, GM-CSF) were influenced neither by hyperglycaemia nor by the genotype of the mice. Alizarin and von Kossa staining exhibited small to moderate calcium deposits in the bioprosthetic valve tissue. A colorimetric assay showed a significantly increased calcium accumulation in WT with hyperglycaemia versus normoglycemic WT (WT-STZ: 8.96 μg/mg; WT+STZ: 18.54 μg/mg), an observation that was lost in Bgn-/0 (Bgn-/0-STZ: 9.84 μg/mg; Bgn-/0+STZ: 12.97 μg/mg). Conclusion Hyperglycaemia significantly promotes destructive remodelling in bioprosthetic valve material. Biglycan-deficiency limits degenerative processes associated with diabetes, suggesting that biglycan as a component of the extracellular matrix has an adverse effect in diabetes-associated bioprosthetic valve deterioration. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): Dr. Rusche grant of the German Heart Foundation and the German Society for Thoracic and Cardiovascular Surgery