The role of IGF-I in the development of cardiovascular disease in type 2 diabetes mellitus: is prevention possible?

The incidence of peripheral, cerebro- and cardiovascular disease (CVD) in patients with type 2 diabetes mellitus is approximately twice as high as in the non-diabetic population. Conventional cardiovascular risk factors such as plasma lipids, lipoproteins and hypertension only partially explain this excessive risk of developing atherosclerosis and CVD. Meta-analysis of studies performed in non-diabetic populations indicates that the risk of CVD increases continuously with glucose levels above 4.2 mmol/l. The glucose hypothesis suggests that treatment which normalizes glucose levels prevents or delays the long-term complications of diabetes mellitus. However, the outcome of the UK Prospective Diabetes Study demonstrates that glucose control does not completely prevent CVD.In healthy subjects, serum IGF-I levels peak in early adulthood, after which they gradually decrease with increasing age. Several observations suggest that there is a premature and progressive age-related decline in serum IGF-I bioactivity in type 2 diabetics, which eventually results in a (relative) IGF-I deficiency. In type 2 diabetics, close relationships have been demonstrated between glycaemic control and serum IGF-I levels, with worse control being associated with lower IGF-I levels. Several studies (in non-diabetics) suggest that lowered circulating IGF-I levels account for a poor outcome of CVD. We previously observed in a population-based study that a genetically determined lowered IGF-I expression increases the risk of myocardial infarction with type 2 diabetes. This genetic approach overcomes the problem that cross-sectional studies cannot distinguish whether changes in IGF-I levels are a cause or a consequence of a disease. IGF-I is an important metabolic regulatory hormone. In addition, IGF-I suppresses myocardial apoptosis and improves myocardial function in various models of experimental cardiomyopathy. Compared with other growth factors, the 'survival' effect of IGF-I on myocardium seems rather unique.Therefore, we hypothesize that the premature and progressive decline in serum IGF-I bioactivity in ageing patients with type 2 diabetics is an important pathophysiological abnormality. It contributes not only to elevated glucose and lipid levels, but also to the progression and the poor outcome of CVD. If this hypothesis is proven to be right, treatment with IGF-I as an adjunct to insulin offers great potential and might not only improve metabolic control but also reduce the incidence and prevalence of CVD in type 2 diabetes patients. However, there is as yet no experimental evidence that long-term (replacement) treatment with IGF-I prevents, delays or reduces CVD in type 2 diabetes patients. Clinical trials are necessary to prove that long-term IGF-I treatment, preferably in the form of a better-tolerated IGF-I/IGF-binding protein-3 complex, improves the overall cardiovascular risk in type 2 diabetes.