Insulin-Dependent Regulation of mTORC2-Akt-FoxO Suppresses TLR4 Signaling in Human Leukocytes: Relevance to Type 2 Diabetes.

Leukocyte signaling in patients with systemic insulin resistance is largely unexplored. We recently discovered the presence of multiple Toll-like receptor 4 (TLR4) signaling intermediates in leukocytes from patients with type 2 diabetes or acute insulin resistance associated with cardiopulmonary bypass surgery. Here, we extend this work to show that in addition to MMP9, HIF-1α, and cleaved AMPKα, patient leukocytes also express IRS-1 phosphorylated on Ser 312 , Akt phosphorylated on Thr 308 , and elevated TLR4 expression. Similar signaling intermediates were detected in leukocytes and neutrophils treated with lipopolysaccharide (LPS), a ligand of TLR4, in vitro. In contrast, insulin, but not LPS, induced mTORC2-dependent phosphorylation of Akt on Ser 473 and FoxO1/O3a on Thr 24/32 in leukocytes and neutrophils. Insulin suppressed LPS-induced responses in a dose- and time-dependent manner. AS1842856, a FoxO1 inhibitor also suppressed TLR4 signaling. We propose that insulin is a homeostatic regulator of leukocyte responses to LPS/TLR4, and that the signaling intermediates expressed in type 2 diabetic patient leukocytes are indicative of TLR4 signaling dominance and deficient insulin signaling. Our data suggest that insulin suppresses LPS/TLR4 signals in leukocytes via the mTORC2-Akt-FoxO signaling axis. Better understanding of type 2 diabetic patient leukocyte signaling may shed new light on disease causation and progression.