Phosphate-induced autophagy counteracts vascular calcification by reducing matrix vesicle release. Kidney Int. 2013;83:1042-1051. [PMID: 23364520 DOI: 10.1038/ki.2012.482]

Autophagy protects end plate chondrocytes from intermittent cyclic mechanical tension induced calcification

Calcification of end plate chondrocytes is a major cause of intervertebral disc (IVD) degeneration. However, the underlying molecular mechanism of end plate chondrocyte calcification is still unclear. The aim of this study was to clarify whether autophagy in end plate chondrocytes could protect the calcification of end plate chondrocytes. Previous studies showed that intermittent cyclic mechanical tension (ICMT) contributes to the calcification of end plate chondrocytes in vitro. While autophagy serves as a cell survival mechanism, the relationship of autophagy and induced end plate chondrocyte calcification by mechanical tension in vitro is unknown. Thus, we investigated autophagy, the expression of the autophagy genes, Beclin-1 and LC3, and rat end plate chondrocyte calcification by ICMT. The viability of end plate chondrocytes was examined using the LIVE/DEAD viability/cytotoxicity kit. The reverse transcription-polymerase chain reaction and western blotting were used to detect the expression of Beclin-1; LC3; type I, II and X collagen; aggrecan; and Sox-9 genes. Immunofluorescent and fluorescent microscopy showed decreased autophagy in the 10- and 20-day groups loaded with ICMT. Additionally, Alizarin red and alkaline phosphatase staining detected the palpable calcification of end plate chondrocytes after ICMT treatment. We found that increased autophagy induced by short-term ICMT treatment was accompanied by an insignificant calcification of end plate chondrocytes. To the contrary, the suppressive autophagy inhibited by long-term ICMT was accompanied by a more significant calcification. The process of calcification induced by ICMT was partially resisted by increased autophagy activity induced by rapamycin, implicating that autophagy may prevent end plate chondrocyte calcification.

A current understanding of vascular calcification in CKD

Patients with chronic kidney disease (CKD) and end-stage renal disease (ESRD) have significant cardiovascular morbidity and mortality that is in part due to the development of vascular calcification. Vascular calcification is an active, highly regulated process that shares many similarities with normal bone formation. New discoveries related to extracellular vesicles, microRNAs, and calciprotein particles continue to reveal the mechanisms that are involved in the initiation and progression of vascular calcification in CKD. Further innovations in these fields are critical for the development of biomarkers and therapeutic options for patients with CKD and ESRD.

Loss of PiT-1 results in abnormal endocytosis in the yolk sac visceral endoderm

PiT-1 protein is a transmembrane sodium-dependent phosphate (Pi) transporter. PiT-1 knock out (KO) embryos die from largely unknown causes by embryonic day (E) 12.5. We tested the hypothesis that PiT-1 is required for endocytosis in the embryonic yolk sac (YS) visceral endoderm (VE). Here we present data supporting that PiT-1 KO results in a YS remodeling defect and decreased endocytosis in the YS VE. The remodeling defect is not due to an upstream cardiomyocyte requirement for PiT-1, as SM22αCre-specific KO of PiT-1 in the developing heart and the YS mesodermal layer (ME) does not recapitulate the PiT-1 global KO phenotype. Furthermore, we find that high levels of PiT-1 protein localize to the YS VE apical membrane. Together these data support that PiT-1 is likely required in YS VE. During normal development maternal immunoglobulin (IgG) is endocytosed into YS VE and accumulates in the apical side of the VE in a specialized lysosome termed the apical vacuole (AV). We have identified a reduction in PiT-1 KO VE cell height and a striking loss of IgG accumulation in the PiT-1 KO VE. The endocytosis genes Tfeb, Lamtor2 and Snx2 are increased at the RNA level. Lysotracker Red staining reveals a loss of distinct AVs, and yolk sacs incubated ex vivo with phRODO Green Dextran for Endocytosis demonstrate a functional loss of endocytosis. As yolk sac endocytosis is controlled in part by microautophagy, but expression of LC3 had not been examined, we investigated LC3 expression during yolk sac development and found stage-specific LC3 RNA expression that is predominantly from the YS VE layer at E9.5. Normalized LC3-II protein levels are decreased in the PiT-1 KO YS, supporting a requirement for PiT-1 in autophagy in the YS. Therefore, we propose the novel idea that PiT-1 is central to the regulation of endocytosis and autophagy in the YS VE.

Medial vascular calcification revisited: review and perspectives

Vascular calcifications (VCs) are actively regulated biological processes associated with crystallization of hydroxyapatite in the extracellular matrix and in cells of the media (VCm) or intima (VCi) of the arterial wall. Both patterns of VC often coincide and occur in patients with type II diabetes, chronic kidney disease, and other less frequent disorders; VCs are also typical in senile degeneration. In this article, we review the current state of knowledge about the pathology, molecular biology, and nosology of VCm, expand on potential mechanisms responsible for poor prognosis, and expose some of the directions for future research in this area.

c-Ski inhibits autophagy of vascular smooth muscle cells induced by oxLDL and PDGF

Autophagy is increasingly being recognized as a critical determinant of vascular smooth muscle cell (VSMC) biology. Previously, we have demonstrated that c-Ski inhibits VSMC proliferation stimulated by transforming growth factor β (TGF-β), but it is not clear whether c-Ski has the similar protective role against other vascular injury factors and whether regulation of autophagy is involved in its protective effects on VSMC. Accordingly, in this study, rat aortic A10 VSMCs were treated with 40 µg/ml oxidized low-density lipoprotein (oxLDL) or 20 ng/ml platelet-derived growth factor (PDGF), both of which were autophagy inducers and closely related to the abnormal proliferation of VSMCs. Overexpression of c-Ski in A10 cells significantly suppressed the oxLDL- and PDGF- induced autophagy. This action of c-Ski resulted in inhibiting the cell proliferation, the decrease of contractile phenotype marker α-SMA expression while the increase of synthetic phenotype marker osteopontin expression stimulated by oxLDL or PDGF. Inversely, knockdown of c-Ski by RNAi enhanced the stimulatory effects of oxLDL or PDGF on A10 cell growth and phenotype transition. And further investigation found that inhibition of AKT phosphorylation to downregulate proliferating cell nuclear antigen (PCNA) expression, was involved in the regulation of autophagy and associated functions by c-Ski in the oxLDL- and PDGF-stimulated VSMCs. Collectively, c-Ski may play an important role in inhibiting autophagy to protect VSMCs against some harsh stress including oxLDL and PDGF.

Inflammatory, metabolic, and genetic mechanisms of vascular calcification

This review centers on updating the active research area of vascular calcification. This pathology underlies substantial cardiovascular morbidity and mortality, through adverse mechanical effects on vascular compliance, vasomotion, and, most likely, plaque stability. Biomineralization is a complex, regulated process occurring widely throughout nature. Decades ago, its presence in the vasculature was considered a mere curiosity and an unregulated, dystrophic process that does not involve biological mechanisms. Although it remains controversial whether the process has any adaptive value or past evolutionary advantage, substantial advances have been made in understanding the biological mechanisms driving the process. Different types of calcific vasculopathy, such as inflammatory versus metabolic, have parallel mechanisms in skeletal bone calcification, such as intramembranous and endochondral ossification. Recent work has identified important regulatory roles for inflammation, oxidized lipids, elastin, alkaline phosphatase, osteoprogenitor cells, matrix γ-carboxyglutamic acid protein, transglutaminase, osteoclastic regulatory factors, phosphate regulatory hormones and receptors, apoptosis, prelamin A, autophagy, and microvesicles or microparticles similar to the matrix vesicles of skeletal bone. Recent work has uncovered fascinating interactions between matrix γ-carboxyglutamic acid protein, vitamin K, warfarin, and transport proteins. And, lastly, recent breakthroughs in inherited forms of calcific vasculopathy have identified the genes responsible as well as an unexpected overlap of phenotypes. Until recently, vascular calcification was considered a purely degenerative, unregulated process. Since then, investigative groups around the world have identified a wide range of causative mechanisms and regulatory pathways, and some of the recent developments are highlighted in this review.

Mono-allelic and bi-allelic ENPP1 deficiency promote post-injury neointimal hyperplasia associated with increased C/EBP homologous protein expression

OBJECTIVE

Bi-allelic function-inactivating ENPP1 mutations cause artery media calcification (AMC) with associated severe myointimal hyperplasia in generalized arterial calcification of infancy (GACI), whereas mono-allelic ENPP1 deficiency is phenotypically normal. Here, we tested if ENPP1 deficiency promotes abnormal vascular smooth muscle cell (VSMC)-driven responses to injury, with or without calcification. The ER stress mediator C/EBP homologous protein (CHOP) affects neointimal hyperplasia and atherosclerosis, and has paradoxical effects on bone formation. Hence, we assessed relationships between ENPP1 and CHOP in VSMCs.

METHODS

We studied ENPP1-deficient mice and control littermates subjected to left carotid artery ligation, and isolated and studied VSMCs from these and Chop-/- mice, or with CHOP siRNA treatment.

RESULTS

Normal Enpp1-/+ mice, in addition to Enpp1-/- mice prior to AMC development, had accelerated neointimal hyperplasia in response to carotid artery ligation at 7-8 weeks age. Neointimal hyperplasia was linked with robust artery media CHOP expression in situ, but with marked AMC only in injured Enpp1-/- arteries. Cultured, ENPP1-deficient and CHOP-deficient VSMCs had increased migration and proliferation to PDGF. Cultured Chop-/- VSMCs demonstrated increased Pi donor-induced calcification. CHOP was significantly increased in Pi donor treated Enpp1-/- and Enpp1-/+ cultured VSMCs. CHOP siRNA treatment of Enpp1-/- VSMCs increased calcification, associated with elevated expression of tissue nonspecific alkaline phosphatase and the master osteoblastic transcription factor RUNX2.

CONCLUSIONS

Both mono-allelic and bi-allelic ENPP1 deficiency promote dysregulated VSMC function, with robust lesion CHOP expression and enhanced neointimal hyperplasia after injury in vivo, but marked post-injury calcification limited to Enpp1-/- mice. Intimal hyperplasia in GACI appears regulated by biologic effects of ENPP1 deficiency other than calcification, including ER stress. VSMC CHOP excess in ENPP1 deficiency may primarily function to limit VSMC calcification.

Implications of autophagy for vascular smooth muscle cell function and plasticity

Vascular smooth muscle cells (VSMCs) are fundamental in regulating blood pressure and distributing oxygen and nutrients to peripheral tissues. They also possess remarkable plasticity, with the capacity to switch to synthetic, macrophage-like, or osteochondrogenic phenotypes when cued by external stimuli. In arterial diseases such as atherosclerosis and restenosis, this plasticity seems to be critical and, depending on the disease context, can be deleterious or beneficial. Therefore, understanding the mechanisms regulating VSMC phenotype and survival is essential for developing new therapies for vascular disease as well as understanding how secondary complications due to surgical interventions develop. In this regard, the cellular process of autophagy is increasingly being recognized as a major player in vascular biology and a critical determinant of VSMC phenotype and survival. Although autophagy was identified in lesional VSMCs in the 1960s, our understanding of the implications of autophagy in arterial diseases and the stimuli promoting its activation in VSMCs is only now being elucidated. In this review, we highlight the evidence for autophagy occurring in VSMCs in vivo, elaborate on the stimuli and processes regulating autophagy, and discuss the current understanding of the role of autophagy in vascular disease.

Autophagy and matrix vesicles: new partners in vascular calcification

Vascular calcification is an actively regulated process driven by vascular smooth muscle cell (VSMC) adaptation and ultimately dysfunction, leading to the induction of active osteogenic processes within the vessel wall. Dai et al., for the first time, identify autophagy as a novel adaptive mechanism that protects against phosphate-induced VSMC calcification, by acting to regulate apoptosis and the release of mineralizing matrix vesicles from VSMCs.