Aberrant activation of bone marrow Ly6C ^high monocytes in diabetic mice contributes to impaired glucose tolerance

Accumulating evidence indicates that diabetes and obesity are associated with chronic low-grade inflammation and multiple organ failure. Tissue-infiltrated inflammatory M1 macrophages are aberrantly activated in these conditions and contribute to hyperglycemia and insulin resistance. However, it is unclear at which stage these cells become aberrantly activated: as precursor monocytes in the bone marrow or as differentiated macrophages in tissues. We examined the abundance, activation state, and function of bone marrow-derived Ly6C^high monocytes in mice with diabetes and/or obesity. Ly6C^high monocytes were FACS-purified from six groups of male mice consisting of type 2 diabetes model db/db mice, streptozotocin (STZ) induced insulin depletion mice, high fat diet (HFD) induced obesity mice and each control mice. Ly6C^high monocytes were then analyzed for the expression of inflammation markers by qRT-PCR. In addition, bone marrow-derived Ly6C^high monocytes from db/+ and db/db mice were fluorescently labeled and injected into groups of db/db recipient mice. Cell trafficking to tissues and levels of markers were examined in the recipient mice. The expression of many inflammation-related genes was significantly increased in Ly6C^high monocytes from db/db mice, compared with the control. Bone marrow-derived Ly6C^high monocytes isolated from db/db mice, but not from db/+ mice, displayed prominent infiltration into peripheral tissues at 1 week after transfer into db/db mice. The recipients of db/db Ly6C^high monocytes also exhibited significantly increased serum glucose levels and worsening tolerance compared with mice receiving db/+ Ly6C^high monocytes. These novel observations suggest that activated Ly6C^high monocytes may contribute to the glucose intolerance observed in diabetes.