Bioactive insulin-like growth factors as a possible molecular target for non-islet cell tumor hypoglycemia

Abstract

Non-islet cell tumor hypoglycemia (NICTH) is a paraneoplastic syndrome characterized by persistent, severe hypoglycemia with a wide variety of solid tumors. It is considered to cause hypoglycemia by increasing the insulin-like bioactivity of the circulating insulin-like growth factor (IGF) system, however, the precise mechanism of hypoglycemia remains unclear. In this manuscript, we report on a patient suffering from NICTH caused by a small cell carcinoma of the colon. This is the first report focusing on the role of bioactive IGFs for this pathological condition. First, we demonstrated that the IGF signal pathway has been activated in this tumor in an autocrine and/or paracrine manner using immunohistochemical analysis. Second, we confirmed that bioactive IGFs in the patient's serum were increased using a modified kinase receptor activation assay, thus bioactive IGFs (mainly IGF-2) could be considered to play a major pathogenic role in enhanced hypoglycemic insulin-like activity. Third, increased IGF bioactivity in the patient's serum was completely inhibited by an anti-IGF neutralizing antibody in vitro. These results suggest that neutralization of bioactive IGFs might become a novel therapeutic strategy for NICTH to relieve the hypoglycemic symptoms together with the tumor suppressive effect.

Keywords: Anti-IGF neutralizing antibody; HMW IGF-2, high molecular weight IGF-2; IGF, insulin-like growth factor; IGF-1R, IGF type 1 receptor; IGFBP, IGF binding protein; IR; Insulin-like growth factor (IGF-1); Insulin-like growth factor-2 (IGF-2); KIRA, the kinase receptor activation assay; Kinase receptor activation assay (KIRA); NICTH, non-islet cell tumor hypoglycemia; Non-islet cell tumor hypoglycemia (NICTH); bioactive IGF; free IGF; insulin receptor.

Figure 1.

Radiological findings of this case. (A) Abdominal computed tomography revealed multiple metastatic tumors in the liver and (B) thickening of the ascending colonic wall. (B, inset) Total colonoscopy demonstrated a large ulcerated and circumferential tumor at the same region.

Figure 2.

Immunohistochemical analysis of tumor cells. (A) The tumor cells were highly atypical small cells with hyperchromatic nuclei and scanty cytoplasm, and strongly positive for synaptophysin and Ki-67 (labeling index; 70%) (×400). (B) The tumor cells were positive for IGF-2, IGF-1R, and phosphorylated IGF-1R (×400).

Figure 3.

Western blot analysis and the kinase receptor activation assay of the patient's serum. (A) High molecular weight (HMW) and mature IGF-2 were detected in serum by western blot analysis using anti-IGF-2 antibody (ab9574). Each serum (40 μg of protein per lane) was separated by 15% SDS-PAGE. (B) IGF bioactivity in the patient's serum was greater than in a normal subject, and increased IGF bioactivity was inhibited to normal bioactivity level by the anti-IGF-2 specific antibody, ab9574, and increased IGF bioactivity was completely inhibited by the anti-IGF neutralizing antibody, KM1468. KRB; Krebs–Ringer bicarbonate buffer as a negative control, Ab; ab9574 (anti-IGF-2 specific antibody, final concentration 10 μg/ml), KM; KM1468 (anti-IGF neutralizing antibody, final concentration 10 μg/ml), hIGF-2; recombinant human IGF-2 (final concentration 10 ng/ml) as a positive control.