Insulin resistance in platelets and monocytes

Abstract

Patients with type 2 diabetes have an increased risk of cardiovascular disease and suffer from thrombotic complications. The aim of the studies presented in this thesis is to elucidate the cause of the prothrombotic situation in type 2 diabetes patients. The studies were focused on platelets and monocytes, two important cell types involved in thrombosis. Normally, insulin inhibits platelets by interfering with the suppression of cAMP, a potent intracellular inhibitor of aggregation and secretion. In type 2 diabetes, this interference is much reduced and platelets show faster aggregation, secretion of granule content, increased synthesis of thromboxane A2 and generation of a procoagulant surface than controls. Obesity is a major risk factor of insulin resistance. Blood from obese individuals contains elevated levels of adipokines, cytokines and hormones derived from adipocytes and macrophages. We identified four adipokines that induce insulin resistance in megakaryocytes, the cells that produce platelets. Short contact with resistin, leptin, PAI-1 and RBP4 but not with visfatin, IL-6 and TNF-alpha aborts insulin signaling through IRS-1 and is reversible; prolonged incubation causes IRS-1 degradation and is irreversible. Plasma samples from obese men mimick the effect of megakaryocyte-disturbing adipokines. Platelets from type 2 diabetic patients are insulin-resistant and have reduced expression of IRS-1. These results suggest that increased release of resistin, leptin, PAI-1 and RBP4 by adipocytes causes synthesis of insulin-resistant platelets by megakaryocytes. Coagulation is initiated by Tissue Factor (TF), which is produced in activated platelets and monocytes. We investigated whether platelets and monocytes are responsive to insulin and if so, whether insulin resistance affects TF production in platelets and monocytes. Recent findings suggest a new source for TF expression. The anucleate platelet has long been considered incapable of synthesizing proteins, but the presence of a splicing machinery together with pre-mRNAs for a number of proteins including TF make them a source for TF synthesis. In healthy individuals, TF synthesis is inhibited by insulin but in patients with type 2 diabetes inhibition is impaired. This leads to the novel finding that platelets from type 2 diabetes patients produce more TF than platelets from matched controls. TF is predominantly produced in activated monocytes. We investigated whether insulin interferes with TF expression in monocytes and monocytic THP-1 cells. We show that insulin concentrations in the physiological range interfere with cAMP suppression and TF formation in cells and monocytes-derived microparticles. The inhibition of TF synthesis in monocytes by insulin has disappeared in type 2 diabetes patients. The cause is impaired formation of an insulin receptor – Gi alpha 2 complex, which in normal monocytes interferes with the Gi mediated suppression of cAMP. Together with the disappearance of suppression of platelet functions, the upregulated TF synthesis might contribute to the atherothrombotic complications observed in type 2 diabetes patients. The results in this study show that platelets and monocytes from type 2 diabetes patients are insulin-resistant, which might explain the prothrombotic situation in these patients. Type 2 diabetes patients have hyperactive platelets and increased levels of TF expressed by platelets and monocytes compared to healthy controls.