Apolipoprotein A-I increases association of cytosolic cholesterol and caveolin-1 with microtubule cytoskeletons in rat astrocytes

Apolipoprotein A-I priming via SR-BI and ABCA1 receptor binding upregulates mitochondrial metabolism to promote insulin secretion in INS-1E cells

Apolipoprotein A-I (ApoA-I), the primary component of high-density lipoprotein (HDL) cholesterol primes β-cells to increase insulin secretion, however, the mechanisms involved are not fully defined. Here, we aimed to confirm ApoA-I receptors in β-cells and delineate ApoA-I-receptor pathways in β-cell insulin output. An LRC-TriCEPS experiment was performed using the INS-1E rat β-cell model and ApoA-I for unbiased identification of ApoA-I receptors. Identified targets, alongside ATP binding cassette transporter A1 (ABCA1) (included control) were silenced in the same cells, and insulin secretion (ELISA) and mitochondrial metabolism (seahorse) were assessed with/without ApoA-I priming. Human β-cell expression data was used to investigate ApoA-I receptor pathways in type 2 diabetes (T2D). Scavenger receptor B1 (SR-BI) and regulator of microtubule dynamics 1 were identified as ApoA-I targets. SR-BI or ABCA1 silencing abolished ApoA-I induced increases in insulin secretion. ApoA-I priming increased mitochondrial OXPHOS, however this was greatly attenuated with SR-BI or ABCA1 silencing. Supporting this, human β-cell expression data investigations found SR-BI and ABCA1 to be correlated with genes associated with mitochondrial pathways. In all, SR-BI and ABCA1 correlated with 73 and 3 genes differentially expressed in T2D, respectively. We confirm that SR-BI and ABCA1 are the primary β-cell ApoA-I receptors and demonstrate that ApoA-I priming enhances β-cell insulin secretion via the upregulation of mitochondrial metabolism through ApoA-I-SR-BI and ApoA-I-ABCA1 pathways. We propose that SR-BI relies on mitochondrial and exocytotic pathways, while ABCA1 depends solely on mitochondrial pathways. Our findings uncover new targets in ApoA-I β-cell mechanism for T2D therapies.

24-Hydroxycholesterol Moderates the Effects of Amyloid-β on Expression of HMG-CoA Reductase and ABCA1 Proteins in Mouse Astrocytes

Background

Elevated brain cholesterol increases the risk of Alzheimer's disease. Production of 24-hydroxycholesterol (24s-OHC) by neurons prevents cholesterol accumulation in the brain. In this study, we investigated the effect of 24s-OHC on the HMG-COA reductase and ABCA1 which are involved in the brain cholesterol homeostasis with or without β-amyloid in astrocytes.

Methods and Materials

Astrocytes were treated with 24s-OHC with or without Aβ. Western blot and real-time polymerase chain reaction were done to detect protein and gene expression of β-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) and ABCA1, respectively. Cholesterol release was determined using a quantitation kit.

Results

Protein levels of HMGCR and ABCA1 were significantly increased by Aβ; however, the 24s-OHC was able to restore their levels and diminish the effect of amyloid-β. Aβ did not have a significant effect on HMGCR expression, while 24s-OHC reduced it by 68%. Aβ-induced ABCA1 expression did not increase cholesterol efflux as the lower levels of cholesterol in conditioned medium of Aβ-treated cells were found.

Conclusion

Our novel findings show that Aβ affects two key elements in the brain cholesterol homeostasis, HMGCR and ABCA1, which are crucial in cholesterol synthesis and efflux. Since 24s-OHC could suppress the Aβ effects on enhancement of HMGCR and ABCA1, therefore the cytochrome P450 46A1 (Cyp46A1), which is exclusively expressed in the central nervous system and responsible for producing of 24s-OHC, could consider as a therapeutic target in the cholesterol-related neurodegenerative diseases such as Alzheimer's disease.

Caveolin-1 facilitates internalization and degradation of ABCA1 and probucol oxidative products interfere with this reaction to increase HDL biogenesis

BACKGROUND AND AIMS

Expression of ATP binding cassette transporter (ABC) A1, a key membrane protein for biogenesis of high-density lipoprotein (HDL), is regulated not only by its gene transcription but also by its intracellular degradation to modulate plasma HDL concentration. We previously showed that inhibition of ABCA1 degradation by probucol oxidative products, spiroquinone (SQ) and diphenoquinone (DQ), increased HDL biogenesis and reverse cholesterol transport, and achieved reduction of atherosclerosis in animal models. The background mechanism has thus been investigated.

METHODS

Involvement of caveolin-1, a protein of multiple functions in cell biology, particularly in cholesterol trafficking, has been examined for its roles in ABCA1 degradation as well as the effects of SQ and DQ on the reaction.

RESULTS

ABCA1 protein was increased in caveolin-1-deficient mouse embryonic fibroblasts, not by increase of transcription but by decrease in its internalization and degradation. Transfection and expression of caveolin-1 normalized the protein level and the rate of degradation of ABCA1. Immunoprecipitation experiments demonstrated association between ABCA1 and caveolin-1 and SQ and DQ disrupted this interaction. The effects of SQ and DQ to increase ABCA1 and cell cholesterol release induced by apolipoprotein A-I were dependent on expression of caveolin-1. Fluorescence imaging of ABCA1 and caveolin-1 in cultured cells demonstrated their co-localization as well as its disruption by SQ and DQ, being consistent with the biochemical findings.

CONCLUSIONS

Caveolin-1 enhances internalization and degradation of ABCA1 by its association with ABCA1. Interference of this interaction by probucol oxidative products suppresses ABCA1 degradation and increase HDL biogenesis.

ApoA-I/HDL Generation and Intracellular Cholesterol Transport through Cytosolic Lipid-Protein Particles in Astrocytes

Exogenous apolipoprotein A-I (apoA-I) associates with ATP-binding cassette transporter A1 (ABCA1) on the cell surface of astrocytes like various peripheral cells and enhances the translocation of newly synthesized cholesterol from the endoplasmic reticulum/Golgi apparatus (ER/Golgi) to the cytosol. The cholesterol translocated to the cytosol is incorporated to cytosolic lipid-protein particles (CLPP) together with phospholipids and proteins such as sphingomyelin, phosphatidylcholine, caveolin-1, protein kinase Cα (PK-Cα), and cyclophilin A. The CLPP are high density lipoproteins- (HDL-)like cytosolic lipid-protein complex with densities of 1.09–1.16 g/mL and diameters of 17-18 nm. The association of exogenous apoA-I with cellular ABCA1 induces tyrosine phosphorylation, activation, and translocation to the CLPP of ABCA1-associated phospholipase Cγ (PL-Cγ) in rat astrocytes. Furthermore, PK-Cα is translocated and activated to/in the CLPP through theproduction of diacylglyceride in the CLPP. ApoA-I enhances both the association of CLPP with microtubules and the phosphorylation of α-tubulin as a component of microtubules. The CLPP are dissociated from microtubules after α-tubulin in microtubules is phosphorylated by the CLPP-associated PK-Cα. The association and dissociation between CLPP and microtubules may participate in the intracellular transport of cholesterol to the plasma membrane.

Caveolin-1--a novel interacting partner of organic cation/carnitine transporter (Octn2): effect of protein kinase C on this interaction in rat astrocytes

OCTN2 - the Organic Cation Transporter Novel family member 2 (SLC22A5) is known to be a xenobiotic/drug transporter. It transports as well carnitine - a compound necessary for oxidation of fatty acids and mutations of its gene cause primary carnitine deficiency. Octn2 regulation by protein kinase C (PKC) was studied in rat astrocytes - cells in which β-oxidation takes place in the brain. Activation of PKC with phorbol ester stimulated L-carnitine transport and increased cell surface presence of the transporter, although no PKC-specific phosphorylation of Octn2 could be detected. PKC activation resulted in an augmented Octn2 presence in cholesterol/sphingolipid-rich microdomains of plasma membrane (rafts) and increased co-precipitation of Octn2 with raft-proteins, caveolin-1 and flotillin-1. Deletion of potential caveolin-1 binding motifs pointed to amino acids 14–22 and 447–454 as the caveolin-1 binding sites within Octn2 sequence. A direct interaction of Octn2 with caveolin-1 in astrocytes upon PKC activation was detected by proximity ligation assay, while such an interaction was excluded in case of flotillin-1. Functioning of a multi-protein complex regulated by PKC has been postulated in rOctn2 trafficking to the cell surface, a process which could be important both under physiological conditions, when carnitine facilitates fatty acids catabolism and controls free Coenzyme A pool as well as in pathology, when transport of several drugs can induce secondary carnitine deficiency.

ApoA-I enhances generation of HDL-like lipoproteins through interaction between ABCA1 and phospholipase Cγ in rat astrocytes

In the previous paper, we reported that apolipoprotein (apo) A-I enhances generation of HDL-like lipoproteins in rat astrocytes to be accompanied with both increase in tyrosine phosphorylation of phospholipase Cγ (PL-Cγ) and PL-Cγ translocation to cytosolic lipid-protein particles (CLPP) fraction. In this paper, we studied the interaction between apoA-I and ATP-binding cassette transporter A1 (ABCA1) to relate with PL-Cγ function for generation of HDL-like lipoproteins in the apoA-I-stimulated astrocytes. ABCA1 co-migrated with exogenous apoA-I with apparent molecular weight over 260kDa on SDS-PAGE when rat astrocytes were treated with apoA-I and then with a cross-linker, BS3. The solubilized ABCA1 of rat astrocytes was associated with the apoA-I-immobilized Affi-Gel 15. An LXR agonist, To901317, increased the cellular level of ABCA1, association of apoA-I with ABCA1 and apoA-I-mediated lipid release in rat astrocytoma GA-1/Mock cells where ABCA1 expression at baseline is very low. PL-Cγ was co-isolated by apoA-I-immobilized Affi-Gel 15 and co-immunoprecipitated by anti-ABCA1 antibody along with ABCA1 from the solubilized membrane fraction of rat astrocytes. The SiRNA of ABCA1 suppressed not only the PL-Cγ binding to ABCA1 but also the tyrosine phosphorylation of PL-Cγ. A PL-C inhibitor, U73122, prevented generation of apoA-I-mediated HDL-like lipoproteins in rat astrocytes. To901317 increased the association of PL-Cγ with ABCA1 in GA-1/Mock cells dependently on the increase of cellular level of ABCA1 without changing that of PL-Cγ. These findings suggest that the exogenous apoA-I augments the interaction between PL-Cγ and ABCA1 to stimulate tyrosine phosphorylation and activation of PL-Cγ for generation of HDL-like lipoproteins in astrocytes.

Caveolin-1: a critical regulator of lung injury

Caveolin-1 (cav-1), a 22-kDa transmembrane scaffolding protein, is the principal structural component of caveolae. Cav-1 regulates critical cell functions including proliferation, apoptosis, cell differentiation, and transcytosis via diverse signaling pathways. Abundant in almost every cell type in the lung, including type I epithelial cells, endothelial cells, smooth muscle cells, fibroblasts, macrophages, and neutrophils, cav-1 plays a crucial role in the pathogenesis of acute lung injury (ALI). ALI and its severe form, acute respiratory distress syndrome (ARDS), are responsible for significant morbidity and mortality in intensive care units, despite improvement in ventilation strategies. The pathogenesis of ARDS is still poorly understood, and therapeutic options remain limited. In this article, we summarize recent data regarding the regulation and function of cav-1 in lung biology and pathology, in particular as it relates to ALI. We further discuss the potential molecular and cellular mechanisms by which cav-1 expression contributes to ALI. Investigating the cellular functions of cav-1 may provide new insights for understanding the pathogenesis of ALI and provide novel targets for therapeutic interventions in the future.

Regulation of amino acid/carnitine transporter B 0,+ (ATB 0,+) in astrocytes by protein kinase C: independent effects on raft and non-raft transporter subpopulations

Neutral and basic amino acid transporter B(0,+) belongs to a Na,Cl-dependent superfamily of proteins transporting neurotransmitters, amino acids and osmolytes, known to be regulated by protein kinase C (PKC). The present study demonstrates an increased phosphorylation of B(0,+) on serine moiety after treatment of rat astrocytes with phorbol 12-myristate 13-acetate, a process correlated with an augmented activity of l-leucine transport and an enhanced presence of the transporter at the cell surface. After solubilization with Triton X-100 and sucrose gradient centrifugation, B(0,+) was detected in non-raft as well as in detergent-resistant raft fractions under control conditions, while phorbol 12-myristate 13-acetate treatment resulted in a complete disappearance of the transporter from the raft fraction. B(0,+) was observed to interact with caveolin-1 and flotillin-1 (reggie-2) proteins, the markers of detergent-resistant microdomains of plasma membrane. As verified in immunocytochemistry and immunoprecipitation experiments, modification of PKC activity did not affect these interactions. It is proposed that PKC reveals different effects on raft and non-raft subpopulations of B(0,+). Phorbol ester treatment results in trafficking of the transporter from the intracellular pool to non-raft microdomains and increased activity, while B(0,+) present in raft microdomains undergoes either internalization or is transferred laterally to non-raft domains.

Platelet Gs hypofunction and abnormal morphology resulting from a heterozygous RGS2 mutation

SUMMARY BACKGROUND

Regulator of G-protein signaling (RGS) 2 negatively regulates Gs signaling by inhibiting the activation of adenylyl cyclase (AC). RGS2 mRNA contains four translation initiation sites, leading to four isoforms with different abilities to inhibit AC activity; the largest isoform is the most pronounced inhibitor. A role for RGS2 in platelets is not known.

OBJECTIVE

To describe a heterozygous RGS2 mutation (G23D) in three related patients, leading to Gs hypofunction in their platelets, and to study the mechanism behind the effect of the RGS2 mutation on platelet function and morphology.

METHODS

Gs signaling was studied ex vivo in platelets and in vitro in transfected cells. Translation initiation was evaluated in vitro, and the interaction of wild-type and G23D RGS2 with AC was unraveled via immunoprecipitation. Platelet granule content was analyzed with proteomics.

RESULTS

The mutation leads to reduced cAMP production after stimulation of Gs-coupled receptors. The largest RGS2 isoforms, with strong AC inhibitor activity, are enriched when the mutation is present, as compared with wild-type RGS2. Moreover, the mutation results in a stronger interaction of RGS2 with AC. G23D RGS2 carriers have enlarged, round platelets with abnormal alpha-granules. Proteomics of the platelet releasate revealed altered expression of some proteins involved in actin assembly, and carriers seemed to have a reduced platelet shape change.

CONCLUSIONS

We present the first platelet Gs signaling defect caused by a heterozygous RGS2 variant that results in a unique mutational mechanism, such as the differential use of translation initiation sites resulting in different functional RGS2 isoforms.

Host cell factors in HIV replication: meta-analysis of genome-wide studies

We have analyzed host cell genes linked to HIV replication that were identified in nine genome-wide studies, including three independent siRNA screens. Overlaps among the siRNA screens were very modest (<7% for any pairwise combination), and similarly, only modest overlaps were seen in pairwise comparisons with other types of genome-wide studies. Combining all genes from the genome-wide studies together with genes reported in the literature to affect HIV yields 2,410 protein-coding genes, or fully 9.5% of all human genes (though of course some of these are false positive calls). Here we report an “encyclopedia” of all overlaps between studies (available at http://www.hostpathogen.org), which yielded a more extensively corroborated set of host factors assisting HIV replication. We used these genes to calculate refined networks that specify cellular subsystems recruited by HIV to assist in replication, and present additional analysis specifying host cell genes that are attractive as potential therapeutic targets.

Do caveolins regulate cells by actions outside of caveolae?

Caveolae (caveolin-containing lipid rafts) are plasma membrane domains that scaffold and organize a variety of important proteins in eukaryotic cells. Recent work shows that caveolins can act independently of caveolae, both in cells that lack caveolae (e.g. neurons and leukocytes) and in non-caveolar regions of cells that have caveolae (e.g. cardiac myocytes and fibroblasts). Phosphorylation of caveolins can influence the scaffolding of protein partners, and caveolins appear to participate in the protection and trafficking of proteins to and from the plasma membrane. Together, these results suggest that, despite their name, caveolins should now be thought of as proteins that scaffold signaling and other proteins in both caveolar and non-caveolar regions.