ApoA-I Protects Pancreatic β-cells from Cholesterol-induced Mitochondrial Damage and Restores their Ability to Secrete Insulin

Abstract

Objective: High cholesterol levels in pancreatic {beta}-cells cause oxidative stress and decrease insulin secretion. {beta}-cells can internalize apolipoprotein (apo) A-I, which increases insulin secretion. This study asks whether internalization of apoA-I improves {beta}-cell insulin secretion by reducing oxidative stress. Approach: Ins-1E cells were cholesterol-loaded by incubation with cholesterol-methyl-{beta}-cyclodextrin. Insulin secretion in the presence of 2.8 or 25 mM glucose was quantified by radioimmunoassay. Internalization of fluorescently labelled apoA-I by {beta}-cells was monitored by flow cytometry. The effects of apoA-I internalization on {beta}-cell gene expression was evaluated by RNA sequencing. ApoA-I binding partners on the {beta}-cell surface were identified by mass spectrometry. Mitochondrial oxidative stress was quantified in {beta}-cells and isolated islets with MitoSOX and confocal microscopy. Results: An F1-ATPase {beta}-subunit on the {beta}-cell surface was identified as the main apoA-I binding partner. {beta}-cell internalization of apoA-I was time-, concentration-, temperature-, cholesterol- and F1-ATPase {beta}-subunit-dependent. {beta}-cells with internalized apoA-I (apoA-I+ cells) had higher cholesterol and cell surface F1-ATPase {beta}-subunit levels than {beta}-cells without internalized apoA-I (apoA-I- cells). The internalized apoA-I co-localized with mitochondria and was associated with reduced oxidative stress and increased insulin secretion. The ATPase inhibitory factor 1, IF1, attenuated apoA-I internalization and increased oxidative stress in Ins-1E {beta}-cells and isolated mouse islets. Differentially expressed genes in apoA-I+ and apoA-I- Ins-1E cells were related to protein synthesis, the unfolded protein response, insulin secretion and mitochondrial function. Conclusions: These results establish that {beta}-cells are functionally heterogeneous and apoA-I restores insulin secretion in {beta}-cells with elevated cholesterol levels by improving mitochondrial redox balance.