Oxysterols have long been known for their important role in cholesterol homeostasis, where they are involved in both transcriptional and posttranscriptional mechanisms for controlling cholesterol levels. However, they are increasingly associated with a wide variety of other, sometimes surprising cell functions. They are activators of the Hedgehog pathway (important in embryogenesis), and they act as ligands for a growing list of receptors, including some that are of importance to the immune system. Oxysterols have also been implicated in several diseases such as neurodegenerative diseases and atherosclerosis. Here, we explore the latest research into the roles oxy-sterols play in different areas, and we evaluate the current evidence for these roles. In addition, we outline critical concepts to consider when investigating the roles of oxysterols in various situations, which includes ensuring that the concentration and form of the oxysterol are relevant in that context—a caveat with which many studies have ...

Respiratory diseases including asthma and chronic obstructive pulmonary disease (COPD) are characterised by excessive and persistent inflammation. Current treatments are often inadequate for symptom and disease control, and hence new therapies are warranted. Recent emerging research has implicated dyslipidaemia in pulmonary inflammation. Three ATP-binding cassette (ABC) transporters are found in the mammalian lung – ABCA1, ABCG1 and ABCA3 – that are involved in movement of cholesterol and phospholipids from lung cells. The aim of this review is to corroborate the current evidence for the role of ABC lipid transporters in pulmonary lipid homeostasis and inflammation. Here, we summarise results from murine knockout studies, human diseases associated with ABC transporter mutations, and in vitro studies. Disruption to ABC transporter activity results in lipid accumulation and elevated levels of inflammatory cytokines in lung tissue. Furthermore, these ABC-knockout mice exhibit signs of respiratory distress. ABC lipid transporters appear to have a crucial and protective role in the lung. However, our knowledge of the underlying molecular mechanisms for these benefits requires further attention. Understanding the relationship between cholesterol and inflammation in the lung, and the role that ABC transporters play in this may illuminate new pathways to target for the treatment of inflammatory lung diseases.

Maintenance of the tightly regulated homeostatic environment of the brain is facilitated by the blood–brain barrier (BBB). P-glycoprotein (P-gp), an ATP-binding cassette transporter, is expressed on the luminal surface of the endothelial cells in the BBB, and actively exports a wide variety of substrates to limit exposure of the vulnerable brain environment to waste buildup and neurotoxic compounds. Downregulation of P-gp expression and activity at the BBB have been reported with ageing and in neurodegenerative diseases. Upregulation of P-gp at the BBB contributes to poor therapeutic outcomes due to altered pharmacokinetics of CNS-acting drugs. The regulation of P-gp is highly complex, but unravelling the mechanisms involved may help the development of novel and nuanced strategies to modulate P-gp expression for therapeutic benefit. This review summarises the current understanding of P-gp regulation in the brain, encompassing the transcriptional, post-transcriptional and post-translational mechanisms that have been identified to affect P-gp expression and transport activity.

The accumulation of amyloid‐β (Aβ) peptides is a key histopathological feature of the Alzheimer's brain. Defective clearance mechanisms result in toxic levels of soluble Aβ₄₀ and Aβ₄₂ oligomers, leading to impaired synaptic function, neurodegeneration and cognitive decline. Growing evidence points to the involvement of P‐glycoprotein (P‐gp or ABCB1), an ATP‐binding cassette transporter highly expressed on the luminal side of the blood–brain barrier, in facilitating the clearance of Aβ from the brain. In this review, we summarise evidence from human, animal and in vitro studies examining the contribution of P‐gp to Aβ clearance, and discuss the potential for P‐gp as a novel pharmacological target in Alzheimer's disease (AD). P‐gp expression and activity in the brain are inversely correlated with ageing, Aβ deposition and AD. Moreover, Aβ itself has been found to compromise the expression of P‐gp, thereby exacerbating Aβ deposition and disease. Despite decades of research, the pathophysiology of AD remains elusive. Understanding the normal versus impaired processing and clearance mechanisms affecting Aβ peptides will assist the development of more effective therapeutic agents to combat this progressive neurodegenerative condition that continues to devastate millions of patients globally.

The ATP-binding cassette transporter ABCG1 has an essential role in cellular cholesterol homeostasis, and dysregulation has been associated with a number of high burden diseases. Previous studies reported that ABCG1 is ubiquitinated and degraded via the ubiquitin proteasome system. However, so far the molecular mechanism, including the identity of any of the rate limiting ubiquitination enzymes, or E3 ligases, is unknown. Using liquid chromatography mass spectrometry, we identified two HECT domain E3 ligases associated with ABCG1, named HUWE1 (HECT, UBA and WWE domain containing 1, E3 ubiquitin protein ligase) and NEDD4-1 (Neural precursor cell-expressed developmentally down regulated gene 4), of which the latter is the founding member of the NEDD4 family of ubiquitin ligases. Silencing both HUWE1 and NEDD4-1 in cells overexpressing human ABCG1 significantly increased levels of the ABCG1 monomeric and dimeric protein forms, however ABCA1 protein expression was unaffected. In addition, ligase silencing increased ABCG1-mediated cholesterol export to HDL in cells overexpressing the transporter as well as in THP-1 macrophages. Reciprocally, overexpression of both ligases resulted in a significant reduction in protein levels of both the ABCG1 monomeric and dimeric forms. Like ABCG1, ABCG4 protein levels and cholesterol export activity were significantly increased after silencing both HUWE1 and NEDD4-1 in cells overexpressing this closely related ABC half-transporter. In summary, we have identified for the first time two E3 ligases that are fundamental enzymes in the post-translational regulation of ABCG1 and ABCG4 protein levels and cellular cholesterol export activity.

Defective clearance mechanisms lead to the accumulation of amyloid-beta (Aβ) peptides in the Alzheimer’s brain. Though predominantly generated in neurons, little is known about how these hydrophobic, aggregation-prone, and tightly membrane-associated peptides exit into the extracellular space where they deposit and propagate neurotoxicity. The ability for P-glycoprotein (P-gp), an ATP-binding cassette (ABC) transporter, to export Aβ across the blood-brain barrier (BBB) has previously been reported. However, controversies surrounding the P-gp–Aβ interaction persist. Here, molecular data affirm that both Aβ₄₀ and Aβ₄₂ peptide isoforms directly interact with and are substrates of P-gp. This was reinforced ex vivo by the inhibition of Aβ₄₂ transport in brain capillaries from P-gp-knockout mice. Moreover, we explored whether P-gp could exert the same role in neurons. Comparison between non-neuronal CHO-APP and human neuroblastoma SK-N-SH cells revealed that P-gp is expressed and active in both cell types. Inhibiting P-gp activity using verapamil and nicardipine impaired Aβ₄₀ and Aβ₄₂ secretion from both cell types, as determined by ELISA. Collectively, these findings implicate P-gp in Aβ export from neurons, as well as across the BBB endothelium, and suggest that restoring or enhancing P-gp function could be a viable therapeutic approach for removing excess Aβ out of the brain in Alzheimer’s disease.