Macrophage alterations in islets of obese mice linked to beta cell disruption in diabetes

Mild islet inflammation has been suggested as a contributing factor to beta cell failure in type 2 diabetes. Macrophage levels are elevated in the islets of humans and mice with type 2 diabetes, but their effects on beta cells are not understood. Our goal was to examine the gene expression changes in islet-associated macrophages in obesity models with opposing disposition to diabetes development and to assess their potential contribution to beta cell (mal)adaptation.

Islets were isolated from lean control mice, obese diabetes-prone db/db mice and obese diabetes-resistant ob/ob mice. Macrophages were sorted using flow cytometry. Islets were treated ex vivo with clodronate-containing liposomes to deplete macrophages. Gene expression was assessed by real-time RT-PCR.

Macrophage levels were increased in islets from db/db mice but not in islets from ob/ob mice compared with lean control mice. Macrophages from db/db and ob/ob islets displayed distinct changes in gene expression compared with control islet macrophages, suggesting differential shifts in functional state. Macrophages from db/db islets displayed increased expression of interferon regulatory factor 5 (Irf5), IL-1 receptor antagonist (Il1rn) and mannose receptor C-type 1 (Mrc1), whereas macrophages from ob/ob islets showed elevated levels of transforming growth factor beta 1 (Tgfb1) and reduced IL-1β (Il1b). Clodronate-liposome treatment of islets depleted macrophages, as evidenced by reduced mRNA expression of Cd11b (also known as Itgam) and F4/80 (also known as Adgre1) compared with PBS-liposome-treated islets. The depletion of macrophages in db/db islets increased the expression of genes related to beta cell identity. The mRNA levels of islet-associated transcription factors (Mafa and Pdx1), glucose transporter (Glut2 [also known as Slc2a2]), ATP-sensitive K⁺ channel (Kcnj11), incretin receptor (Gipr) and adaptive unfolded protein response (UPR) genes (Xbp1, Hspa5, Pdia4 and Fkbp11) were increased in db/db islets after macrophage depletion, whereas the mRNA levels of the deleterious UPR effector, Ddit3, were reduced. In contrast, depletion of macrophages in islets of ob/ob mice did not affect beta cell identity gene expression.

The findings of this study suggest that distinct alterations in islet macrophages of obese mice are critically important for the disruption of beta cell gene expression in diabetes.