Increased levels of soluble thrombomodulin (TM) in patients with diabetes mellitus are considered a marker of endothelial damage. It is unknown whether the loss of endothelial TM-function contributes to the progression of vascular complications in diabetes mellitus. To address whether the loss of TM-dependent protein C (PC) activation contributes to diabetic complications such as diabetic nephropathy two animal models were employed: (1) TMPro mice, which have been previously described and carry a point mutation in the TM-gene (E404P), resulting in a loss of TM-dependent PC-activation, and (2) hPC mice, which carry a transgene resulting in the expression of a mutant “hyperactivatable” PC, which can be activated by thrombin in the absence of TM. The mutant PC could be captured from plasma samples of hPC mice and activated ex vivo by thrombin in the absence of TM. hPC mice had a prolonged bleeding time. Following induction of diabetes by streptozotocin TAT levels were increased in diabetic control (wild type) mice and to a larger extent in TMPro mice, but not in diabetic hPC mice. In comparison to diabetic control mice the kidney weight was increased in diabetic TMPro mice, but not in diabetic hPC mice. Albuminuria was increased in diabetic TMPro mice and reduced in diabetic hPC mice in comparison to diabetic control mice, indicating increased glomerular damage in TMPro mice and partial protection from glomerular damage in hPC mice. Consistently, using a histological score glomerular damage was more severe in diabetic TMPro mice in comparison to diabetic control mice, while diabetic hPC mice were protected. Preliminary data suggest that the observed changes are associated with increased apoptosis in glomeruli of diabetic TMPro mice. Using HUVECs we were able to establish that high glucose concentrations (30 mM) reduce TM-dependent PC activation. The reduced TM-dependent PC activation is associated with increased apoptosis. Glucose induced apoptosis in HUVECs is associated with an increased Bax/Bcl-2 ratio, increased translocation of Bax into mitochondria, and increased caspase-3 activation. Activated PC normalizes the Bax/Bcl-2 ratio, prevents translocation of Bax, and reduces caspase-3 activity. Further studies using TRAPs and inhibitory antibodies established that the antiapoptotic effect of aPC in glucose stressed endothelial cells is mediated through a Par-1 and EPCR-dependent mechanism. The current data strongly suggest that the loss of the endothelial TM-PC system is not just a marker of endothelial damage in diabetic patients, but rather contributes to the progression of diabetic vascular complications.