I found this paper quite intriguing, as it links together a number of themes in vascular aging and the similar forms of vascular dysfunction seen in metabolic syndrome and diabetes. The molecular damage of aging in blood vessel walls causes stiffness of blood vessels, which in turn causes hypertension. This is one of the more important means by which low level biochemical damage is translated to high level structural damage to tissues, as raised blood pressure causes all sorts of harm. The damage that leads to vascular stiffness includes (a) cross-linking, in which sugary metabolic byproducts form links between molecules of the extracellular matrix, impeding its elasticity, (b) calcification, in which cells begin to inappropriately deposit calcium into the extracellular matrix, also degrading elasticity, and (c) failure of the vascular smooth muscle cells to perform appropriately when constricting or dilating blood vessels.
This last item has a number of poorly mapped underlying causes, but chronic inflammation appears to be a contributing issue. Chronic inflammation is also implicated in calcification. Chronic inflammation is one of the downstream consequences of cellular senescence, and there is evidence for the presence of senescent cells to be involved in calcification in blood vessel walls. So these items are already quite well connected together. The paper here closes the loop further by finding a form of intracellular signaling that is likely present in hyperglycemic individuals, who also exhibit raised levels of cross-linking, that spurs the formation of more senescent cells in blood vessel walls. Hyperglycemia is just the excessive case: everyone who consumes the usual modern amount of dietary sugar is probably in an incrementally worse position over the long term than people who consume less sugar, due to this and related mechanisms.
A major determinant of vascular aging is vascular calcification, characterized by vascular smooth muscle cells (VSMCs) calcification. Transdifferentiation of VSMCs into osteoblasts is considered to be the most critical pathophysiological of VSMCs calcification. There is accumulating evidence suggesting that VSMCs calcification/senescence have central roles in the development and progression of diabetes-related cardiovascular disorders.
The vascular response to hyperglycemia is a multifactorial process involving endothelial cells (ECs) and VSMCs, although the mechanism by which the information in circulating blood are transferred from ECs to VSMCs is yet to be understood. Signaling between ECs and VSMCs is crucial for the pathogenesis of diabetic vascular calcification/aging. However, how does circulating high glucose affect the calcification/senescence of VSMCs that are not directly contact with the blood? Exosomes, small vesicles with a diameter of 40-100 nm released from various cell types, have gained much attention for their role in intercellular communication. Exosomes can transfer active proteins, lipids, small molecules, and RNAs from their cell of origin to the target cell. ECs have been demonstrated to secrete exosomes, and the transfer of signaling molecules by exosomes may thus provide a way for communicating between ECs and VSMCs. Similarly, prior study has demonstrated that exosomes from senescent ECs promotes VSMCs calcification.
Exosomes from human umbilical vein endothelial cells (HUVEC-Exos) were isolated from normal glucose (NG) and high glucose (HG) stimulated HUVECs (NG/HG-HUVEC-Exos). Exosomes isolated from HG-HUVEC-Exos induced calcification/senescence in VSMCs. HG-HUVEC-Exos significantly increased lactate dehydrogenase (LDH) activity, as well as the product of lipid peroxidation, and decreased oxidative stress marker activity, as compared with NG-HUVEC-Exos. Moreover, mechanism studies showed that mitochondrial membrane potential and the expression levels of mitochondrial function related protein HADHA and Cox-4 were significantly decreased in HG-HUVEC-Exos compared to controls. Proteomic analysis showed that HG-HUVEC-Exos consisted of higher level of versican (VCAN), as compared with NG-HUVEC-Exos.
VCAN is mainly localized to the mitochondria of VSMCs. Knockdown of VCAN with siRNA in HUVECs, inhibited HG-HUVEC-Exos-induced mitochondrial dysfunction and calcification/senescence of VSMCs. Our data suggest a functional role for VCAN inside VSMCs. VCAN carried by HG-HUVEC-Exos promotes VSMCs calcification/senescence, probably by inducing mitochondrial dysfunction. Since VSMCs calcification/senescence could induce vascular dysfunction, blockage of the exosome-mediated transfer of VCAN between these two cells may serve as a potential therapeutic target against diabetic vascular complications. More work will be needed to explore this possibility and to better understand the intracellular roles of VCAN.