ABCA1 deficiency and cellular cholesterol accumulation increases islet amyloidogenesis in mice

Islet amyloid, a pathological feature of type 2 diabetes, forms from the aggregation of islet amyloid polypeptide (IAPP), a beta cell peptide that is produced and co-secreted with insulin. Cholesterol regulates amyloid-β processing, deposition and clearance, promoting amyloidogenesis in the brain. ATP-binding cassette transporter 1 (ABCA1) is a cholesterol efflux transporter that when absent increases and when overexpressed reduces brain amyloid-β deposition in mouse models of Alzheimer’s disease. We examined whether alterations in ABCA1 expression and islet cholesterol content could also modulate islet amyloidogenesis.

Thioflavin S staining for amyloid was performed in islets isolated from mice with beta cell expression of human IAPP (hIAPP ^Tg/o) and cultured for 8 days following cholesterol loading, microRNA-33 overexpression (to reduce ABCA1 expression) or palmitate treatment in the presence or absence of ABCA1 overexpression or mevastatin treatment (to reduce cholesterol synthesis). hIAPP ^Tg/o mice were crossed with beta cell-specific Abca1-knockout mice (hIAPP ^Tg/o Abca1 ^βKO) and glucose tolerance and amyloid formation were assessed.

Cholesterol loading and microRNA-33-induced reduction in islet ABCA1 expression increased Thioflavin S-positive amyloid in hIAPP ^Tg/o islets. Palmitate treatment also increased amyloid formation and this was reduced by both ABCA1 overexpression and mevastatin treatment. hIAPP ^Tg/o Abca1 ^βKO mice had increased islet cholesterol, accompanied by fasting hyperglycaemia, glucose intolerance, impaired in vivo insulin secretion and an increased islet proinsulin:insulin ratio. Amyloid area was increased in cultured hIAPP ^Tg/o Abca1 ^βKO islets compared with hIAPP ^Tg/o controls.

These data suggest that elevations in islet cholesterol may lead to increases in IAPP aggregation and islet amyloid formation, further worsening beta cell function and glucose homeostasis.