ABCC6 deficiency promotes dyslipidemia and atherosclerosis

ABCC6 deficiency promotes ectopic calcification; however, circumstantial evidence suggested that ABCC6 may also influence atherosclerosis. The present study addressed the role of ABCC6 in atherosclerosis using Ldlr-/- mice and pseudoxanthoma elasticum (PXE) patients. Mice lacking the Abcc6 and Ldlr genes were fed an atherogenic diet for 16 weeks before intimal calcification, aortic plaque formation and lipoprotein profile were evaluated. Cholesterol efflux and the expression of several inflammation, atherosclerosis and cholesterol homeostasis-related genes were also determined in murine liver and bone marrow-derived macrophages. Furthermore, we examined plasma lipoproteins, vascular calcification, carotid intima-media thickness and atherosclerosis in a cohort of PXE patients with ABCC6 mutations and compared results to dysmetabolic subjects with increased cardiovascular risk. We found that ABCC6 deficiency causes changes in lipoproteins, with decreased HDL cholesterol in both mice and humans, and induces atherosclerosis. However, we found that the absence of ABCC6 does not influence overall vascular mineralization induced with atherosclerosis. Decreased cholesterol efflux from macrophage cells and other molecular changes such as increased pro-inflammation seen in both humans and mice are likely contributors for the phenotype. However, it is likely that other cellular and/or molecular mechanisms are involved. Our study showed a novel physiological role for ABCC6, influencing plasma lipoproteins and atherosclerosis in a haploinsufficient manner, with significant penetrance.

Should a homeless person become a deceased organ donor?

Efforts to increase deceased donation have included the use of US Public Health Service (PHS) high-risk donors. The homeless have high rates of medical and substance abuse issues that are often unrecognized. This study investigates whether the homeless should become suitable organ donors. We retrospectively reviewed 193 brain-dead prospective donors from Hawaii's organ procurement organization (OPO; 2013-2018) and compared two groups: homeless (n = 13) and non-homeless (n = 180) prospective donors. The homeless prospective donors were older (48.0 vs 40.7 years, P = .009) and had more substance abuse (30.8% vs 10%, P = .046), methamphetamine use (53.8% vs 12.2%, P = .001), cocaine use (23.1% vs 3.9%, P = .022), and urine with amphetamines (54.5% vs 17.9%, P = .049). The homeless prospective donors trended toward more PHS high-risk designation (50% vs 19%, P = .062). There was no difference in medical history, gender/race, hepatitis serologies, authorization for donation, and organs procured/transplanted between prospective donors. We have provided evidence that the homeless should become prospective organ donors; however, they have more high-risk behaviors and often have limited information. Larger studies from OPOs are needed to better characterize organ donation and track disease transmission in this population.

Analysis of pseudoxanthoma elasticum-causing missense mutants of ABCC6 in vivo; pharmacological correction of the mislocalized proteins

Mutations in the ABCC6 gene cause soft-tissue calcification in pseudoxanthoma elasticum (PXE) and, in some patients, generalized arterial calcification of infancy (GACI). PXE is characterized by late onset and progressive mineralization of elastic fibers in dermal, ocular, and cardiovascular tissues. GACI patients present a more severe, often prenatal arterial calcification. We have tested 10 frequent disease-causing ABCC6 missense mutants for the transport activity by using Sf9 (Spodoptera frugiperda) cells, characterized the subcellular localization in MDCKII (Madin-Darby canine kidney (cell line)) cells and in mouse liver, and tested the phenotypic rescue in zebrafish. We aimed at identifying mutants with preserved transport activity but with improper plasma membrane localization for rescue by the chemical chaperone 4-phenylbutyrate (4-PBA). Seven of the mutants were transport-competent but mislocalized in mouse liver. The observed divergence in cellular localization of mutants in MDCKII cells versus mouse liver underlined the limitations of this 2D in vitro cell system. The functionality of ABCC6 mutants was tested in zebrafish, and minimal rescue of the morpholino-induced phenotype was found. However, 4-PBA, a drug approved for clinical use, restored the plasma membrane localization of four ABCC6 mutants (R1114P, S1121W, Q1347H, and R1314W), suggesting that allele-specific therapy may be useful for selected patients with PXE and GACI.

ABCC6 is a basolateral plasma membrane protein

RATIONALE

ABCC6 plays a crucial role in ectopic calcification; mutations of the gene cause pseudoxanthoma elasticum and general arterial calcification of infancy. To elucidate the role of ABCC6 in cellular physiology and disease, it is crucial to establish the exact subcellular localization of the native ABCC6 protein.

OBJECTIVE

In a recent article in Circulation Research, ABCC6 was reported to localize to the mitochondria-associated membrane and not the plasma membrane. As the suggested mitochondrial localization is inconsistent with published data and the presumed role of ABCC6, we performed experiments to determine the cellular localization of ABCC6 in its physiological environment.

METHODS AND RESULTS

We performed immunofluorescent labeling of frozen mouse and human liver sections, as well as primary hepatocytes. We used several different antibodies recognizing human and mouse ABCC6. Our results unequivocally show that ABCC6 is in the basolateral membrane of hepatocytes and is not associated with the mitochondria, mitochondria-associated membrane, or the endoplasmic reticulum.

CONCLUSIONS

Our findings support the model that ABCC6 is in the basolateral membrane, mediating the sinusoidal efflux of a metabolite from the hepatocytes to systemic circulation.