Saturated phosphatidic acids mediate saturated fatty acid–induced vascular calcification and lipotoxicity

Abstract

R e s e a R c h a R t i c l e 4 5 4 4jci.org Volume 125 Number 12 December 2015 IntroductionVascular calcification is a major complication in patients who are aging, have diabetes, or have chronic kidney disease (CKD) and is an active process that differentiates vascular smooth muscle cells (VSMCs) into osteoblast-like cells (1, 2). This process is highly regulated by transcription factors involved in osteogenic differentiation, such as RUNX2, MSX2, and ATF4 (3-5). Several in vitro and in vivo studies have shown lipids to play a causative role in the pathogenesis of vascular calcification in addition to inorganic phosphate, inflammatory cytokines, and oxidative stress (6-13). Treatment with unsaturated fatty acids (UFAs) inhibits vascular mineralization and osteogenic differentiation (14, 15), whereas oxidized lipids, such as oxysterols and oxidized phospholipids, elicit procalcific effects in vascular cells (9,16,17). In addition to this evidence, we also previously reported that saturated fatty acids (SFAs) and calcium simultaneously accumulate during osteogenic differentiation of vascular cells (18,19). Ectopic accumulation of excess lipids, called lipotoxicity, plays a central role in the pathogenesis of cardio-metabolic diseases, including diabetes, obesity, atherosclerosis, and vascular calcification (20-24). However, evidence from a number of experimental systems is emerging, stating that SFAs and UFAs have distinct contributions to lipotoxicity (25,26). SFAs such as palmitic acid (16:0) and stearic acid (18:0) induce apoptosis, oxidative stress, and ER stress in a variety of mammalian cell lines (including hepatocytes, macrophages, and VSMCs), whereas UFAs such as oleic acid have no or minimal lipotoxic properties (5,(25)(26)(27)(28)(29)(30). In addition, cotreatment with UFAs blocks SFA-mediated lipotoxic effects (25,29). However, the specific metabolite of SFAs that induces lipotoxicity, the mechanism underlying SFA-mediated lipotoxicity, and how UFAs block SFA-mediated lipotoxicity are largely unknown.Proper intracellular SFA and UFA balance is controlled by a lipogenic enzyme called stearoyl-CoA desaturase (SCD) (31,32). SCD catalyzes the conversion of SFAs to monounsaturated FAs, mainly 16:0 into palmitoleate (16:1n-7), and 18:0 into oleate (18:1n-9) (33,34). This introduction of a double bond markedly impacts several chemical properties, including a decrease in melting point and an increase in solubility. The activation of SCD therefore neutralizes SFA-mediated lipotoxicity (5, 25). The expression of SCD is highly regulated by a number of hormonal and dietary factors (33,(35)(36)(37). Recently, we found that the accumulation of SFAs by either supplementation with exogenous SFAs or inhibition of SCD induces mineralization of VSMCs (5,19). In addition, SFA-mediated lipotoxicity and vascular calcification were completely blocked by an acyl-CoA synthetase inhibitor and were attenuated by the shRNA-mediated inhibition of fatty acid elongase-6 (Elovl6) (5), suggesting that stearoyl-CoA (18:0-CoA) or its ...

Activating transcription factor-4 promotes mineralization in vascular smooth muscle cells

“…To investigate whether global ATF4 deficiency affects medial calcification, global ATF4-deficient mice were generated by backcrossing to DBA/2J mice, which are susceptible to aortic medial calcification (10,30). Since ATF4 biallelic KO mice on DBA/2J background are embryonic lethal, we used ATF4-haplodeficient (Atf4…”

“…In addition, we recently reported that the saturated fatty acid stearic acid strongly induces vascular calcification through ER stress (9,10). We then determined whether saturated fatty acids induce the expression of PiT1 and C/EBPβ.…”

“…Recent evidence is emerging that ER stress contributes to the pathogenesis of vascular calcification (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). The ER is a major site for the regulation of calcium and lipid homeostasis (11).…”

“…The three ER resident sensors, PKR-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6) implement the ER stress response (12). Among the three pathways of the ER stress response, our previous studies strongly implicate the PERK-initiated pathway in the development of vascular calcification (7)(8)(9)(10). Activation of PERK leads to the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2α), which inhibits assembly of the 80S ribosome and protein synthesis.…”

“…Many key transcriptional regulators involved in skeletal osteogenesis, such as Msx-2, osterix, and Runx-2, are expressed in both calcified medial arterial layers and atherosclerotic plaques (22,23). In addition, many causative hormonal factors (TNF-α, FGF23/Klotho, and phosphorus) and lipid-derived factors (saturated fats and oxidized lipids) have been identified in vascular calcification (7)(8)(9)(10)(24)(25)(26)(27)(28)(29). We recently reported that these factors induce ATF4 activation through the ER stress response, resulting in osteogenic differentiation and mineralization of vascular smooth muscle cells (VSMCs) in vitro (8,9).…”