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

Objective: High cholesterol levels in pancreatic β-cells cause oxidative stress and decrease insulin secretion. β-cells can internalize apolipoprotein (apo) A-I, which increases insulin secretion. This study asks whether internalization of apoA-I improves β-cell insulin secretion by reducing oxidative stress. Approach: Ins-1E cells were cholesterol-loaded by incubation with cholesterol-methyl-β-cyclodextrin. Insulin secretion in the presence of 2.8 or 25 mM glucose was quantified by radioimmunoassay. Internalization of fluorescently labelled apoA-I by β-cells was monitored by flow cytometry. The effects of apoA-I internalization on β-cell gene expression was evaluated by RNA sequencing. ApoA-I binding partners on the β-cell surface were identified by mass spectrometry. Mitochondrial oxidative stress was quantified in β-cells and isolated islets with MitoSOX and confocal microscopy. Results: An F1-ATPase β-subunit on the β-cell surface was identified as the main apoA-I binding partner. β-cell internalization of apoA-I was time-, concentration-, temperature-, cholesterol- and F1-ATPase β-subunit-dependent. β-cells with internalized apoA-I (apoA-I+ cells) had higher cholesterol and cell surface F1-ATPase β-subunit levels than β-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 β-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 β-cells are functionally heterogeneous and apoA-I restores insulin secretion in β-cells with elevated cholesterol levels by improving mitochondrial redox balance.

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