The Role of Insulin-Like Growth Factors and Their Binding Proteins in Tumor Hypoglycemia

Tumors of nonislet cell origin may overexpress insulin-like growth factor (IGF)-II, leading to hypoglycemia with suppressed serum insulin levels (NICTH). Most of the serum IGF-II in NICTH patients is in precursor forms of 10-15 kD, and may be abnormally glycosylated. In NICTH, IGFs and IGF-binding protein-3 (IGFBP-3) are mainly found in binary complexes of 50-60 kD, instead of the normal ternary complex of about 140 kD with the acid-labile subunit (ALS). Factors contributing to the defect are: (1) low ALS levels, secondary to suppressed growth hormone (GH); (2) defective IGFBP-3 binding to ALS; (3) reduced ability of pro-IGF-II forms to complex normally, and (4) very high levels of other IGFBPs, including IGFBP-2 and IGFBP-6, which might limit the formation of complexes with IGFBP-3. While both GH and glucocorticoids can restore normoglycemia and increase high-molecular-weight IGFBP-3 complexes, corticosteroid treatment suppresses tumor IGF-II, whereas GH can restore normoglycemia despite continuing high IGF-II levels. Both treatments increase serum ALS, IGFBP-3, and IGF-I levels, and decrease IGFBP-2, whereas IGFBP-6 is unaffected. The reversal of hypoglycemia, by surgery, GH, or glucocorticoid treatment, is always accompanied by improved ternary complex formation, emphasizing the importance of the components of this complex, in particular ALS, in normal blood sugar regulation.