A Twenty-First Century Cancer Epidemic Caused by Obesity: The Involvement of Insulin, Diabetes, and Insulin-Like Growth Factors

Figure 3

Adipose tissue as an endocrine organ. Adipocytes express enzymes involved in steroid metabolism, notably 17β-hydroxysteroid dehydrogenase and aromatase which converts androgens into oestrogens. There is a resultant increase in paracrine concentrations of oestrogens and a significant increase in the concentration of circulating oestrogens in obese men and in obese postmenopausal women compared to nonobese individuals. In the blood, oestrogen is complexed with sex hormone-binding globulin (SHBG). Adipocytes secrete several protein hormones including IGF-1 and IGF-2. IGF-1 and IGF-2 enter the blood stream where they can be bound by insulin-like binding proteins (IGFBPs). Local concentrations of IGF-1 and IGF-2 may be particularly high and are unlikely to be reduced by feedback mechanisms.