Modo batch

Diabetes affects haemostasis in a variety of ways resulting ultimately in thrombosis and hypofibrinolysis. This occurs through increased levels of numerous key elements of coagulation and fibrinolysis along with post- translational modifications and alteration of the clot structure itself leading to a pro-thrombotic state. Many of the changes seen are also associated with increased risk of cardiovascular disease. The changes associated with diabetes

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outlined below contribute to the formation of dense clot with thinner fibres and fewer pores ex vivo, that prove more resistant to lysis by plasmin132;133. Improving glycaemic control can favourably alter the clot structure 134.

1.7.1 Quantitative changes in coagulation and fibrinolysis proteins

Both diabetes and CVD are independently associated with alterations in the level of coagulation and fibrinolysis proteins.

1.7.1.1 Thrombin

Hyperglycaemia is linked with increased thrombin production and elevated levels are seen in T1DM and T2DM. Reducing glucose levels has been shown to reduce thrombin production135-137. Thrombin levels are also involved in clot structure and enhanced concentrations play a role in the dense clots described above37.

1.7.1.2 Fibrinogen

Elevated levels of fibrinogen are associated with increased risk for CVD and have been used as a marker of clinical and subclinical disease 138-141. Both T1DM and T2DM are associated with high levels of fibrinogen142 which may be a result of inflammation causing hepatocyte stimulation9. Elevated insulin levels associated with insulin resistance is also likely to play a part in T2DM through increased hepatic production of fibrinogen 143;144. This is supported by the finding of higher levels of fibrinogen in healthy first degree relatives of T2DM patients145 and that plasma levels were able to predict development of T2DM in healthy individuals which implies that hyperinsulinaemia rather than hyperglycaemia alone is important 146. This finding may also explain why improving glucose levels through the use of insulin does not correlate with a

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reduction in fibrinogen levels147;148 whereas metformin use has been associated with lowering plasma levels149. Higher levels of fibrinogen also alter final clot structure and, in part at least, help explain the differences seen in clot structure in CVD and DM as above.

1.7.1.3 Tissue Factor

The levels of TF have been shown to be elevated in patients with CAD, with highest levels seen in those with unstable angina or MI compared with stable angina9. Diabetes is also associated with elevated levels of TF 137, possibly as a result from diminished inhibition of TF synthesis by insulin in diabetes compared with non-diabetes subjects 150.

1.7.1.4 Plasminogen activator inhibitor-1

Elevated levels are seen in CAD, and levels are strongly correlated with risk factors for the MetS, namely BMI, blood pressure, plasma triglycerides and insulin levels35. In diabetes subjects there are increased levels of PAI-1, which is positively correlated with glycaemic control151, and elevated levels are also thought to be an independent risk factor for the development of T2DM146, adding further weight to the suggestion that insulin resistance and hyperinsulinaemia, in the face of normoglycaemia, is sufficient to trigger hypofibrinolysis152. A combination of inflammatory cytokines, insulin, free fatty acids and very low density lipoproteins are likely to cause increased release of PAI-1 from the liver and also adipocytes11;153-157. It has not been clearly demonstrated that elevated PAI-1 levels are a risk factor for CVD158, however tPA/PAI-1complexes, which correlates with PAI-1 activity, has been shown to

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be a risk factor for MI, supporting the role of PAI-1 in reduced fibrinolysis and CVD159;160.

1.7.1.5 Factor VII

Factor VII forms a complex with TF and initiates the formation of thrombin. Studies investigating the effect of coagulant activity of FVII (FVII:c) on cardiovascular events have yielded conflicting results although there is a suggestion that elevated FVII:c levels are associated with fatal cardiovascular events35;161;162. Levels of FVII:c are raised in T2DM and also in healthy first degree relatives of T2DM and those with the MetS163-165, again suggesting that observed changes precede the development of T2DM and hyperglycaemia. Possible explanations for this observation is that triglycerides (TG), which are associated with elevated FVII:c163;164;166 along with poor glycaemic control and MetS, are involved in activation of FVII167. FVII is also thought to bind to TG rich particles and reduced post-prandial breakdown of these fatty proteins lead to increased plasma FVII levels35.

1.7.1.6 Factor VIII and vWf

The clotting factor VIII circulates in the plasma bound to vWf, which offers stability and increases half life of the protein 11. Elevated levels of FVIII and vWf have been associated with CVD but as with other clotting factors, this association is lost following adjustment for traditional CVS risk factors 168;169. Levels are elevated in diabetes and in these individuals the correlation with CVD remains after adjustment for cardiovascular risk factors 129;170;171. It is likely that raised levels of FVIII and vWf are an indication of EC damage resulting from inflammation and insulin resistance 130;131.

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Table 1-3 Summary of changes to haemostatic components in diabetes

1.7.1.7 Plasmin

As described in section 1.4.2.2.1, plasminogen conversion to plasmin is reduced in diabetes as a result of increased glycation, along with the efficacy of plasmin itself once generated. Lower levels of plasmin are also expected due to increased activity of PAI-1 and cross linking of PI in diabetes, all of which contributes to the hypofibrinolytic environment associated with diabetes.

Haemostatic component Function Changes in DM Effect in DM TF Initiates clotting cascade ↑levels ↑ thrombosis

FVII Forms complex with TF ↑levels ↑thrombosis

FVIII & vWf complex

Adherence of platelets to endothelial cell wall

↑levels ↑platelet activation

Thrombin Converts fibrinogen to

fibrin

↑ levels Altered clot structure

Fibrinogen Forms fibrin clot ↑levels

↑glycation

Altered clot structure ↓fibrinolysis

Plasmin Breaks down fibrin clot ↓levels ↓fibrinolysis

PAI-1 Inhibits production of

plasmin

↑levels ↓fibrinolysis

Platelets Activation of

coagulation factors and forms fibrin mesh

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1.7.2 Qualitative changes in coagulation and fibrinolysis proteins

1.7.2.1 Fibrinogen

In vivo hyperglycaemia in poorly controlled diabetes patients in known to cause glycation of fibrinogen172;173 which causes an alteration in the structure and function of the molecule and contributes to the development of the rigid clot architecture seen in diabetes132;174 which is associated with premature CVD175;176. Improving blood glucose control can modulate fibrinogen glycation 134;172

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1.7.2.2 Plasminogen

Increased glycation of plasminogen has been observed in diabetes compared with healthy controls, with lysine residues 107 and 557 preferentially glycated. These sites are involved in fibrin binding and cleavage of plasminogen to form plasmin which may explain the reduction in plasmin generation and activity56.

1.7.2.3 Plasmin inhibitor

The glycation of PI in diabetes has not been reported, but its anti-fibrinolytic properties may be enhanced in these patients by increased FXIII mediated cross linking to fibrinogen during clot formation133. The mechanism underpinning this has not been elucidated but it may be related to structural changes that occur within the fibrinogen molecule as a result of hyperglycaemia or through increased activation of FXIII by thrombin133.