The 14 aminoacid N-terminal peptide of S100A8 (NT-S100A8) was isolated by us from pancreatic cancer tissue samples of patients with an associated diabetes mellitus and demonstrated to alter myoblasts glucose metabolism in vitro at the dosage of 50 nM. We hypothesized that NT-S100A8 might cause diabetes mellitus by interfering with insulin secretion and/or signalling. This study was performed to verify this hypothesis. The following cell lines were used: βTC6 (rat insulinoma), C2C12 (mice myoblasts), HepG2 (human hepatocellular carcinoma) and CAPAN1 (human pancreatic carcinoma). Hyperglycemia (20 mM) stimulated βTC6 cells alone or in co-culture with C2C12 (1:1 ratio) were treated without (control) or with CAPAN1 conditioned medium, and 50 nM NT-S100A8. In the culture media insulin, glucose and lactate concentrations were measured. HepG2 cells were cultured in the presence or absence of insulin (50 mU/mL) and treated with increasing amounts of NT-S100A8 (25, 50, 100, 200, 500 nM). After 24, 48 and 72 hrs, HepG2 cells were harvested and counted. Hyperglycemia induced a significant early (3-60 minutes) and late (24 hrs) insulin release from βTC6 cells in control, CAPAN1 conditioned and NT-S100A8 treated cells (Repeated measured analysis of variance, test of within subjects effects: F=2.71, p<0.05). In the presence of NT-S100A8 insulin exocytosis from secretory vescicles (3-60 min release), was significantly reduced (test of between subjects effects: F=4.02, p<0.05). In co-culture experiments the levels of glucose remained slightly higher, those of lactate slightly lower, in the presence of NT-S100A8 with respect to control, after 24 (mean±SD: glucose=17±1.6 and 15±2.2 mM, lactate=11±3.2 and 12±3.0 mM) and 48 hrs (glucose=9±3.2 and 8±2.7 mM, lactate=24±6.2 and 26±4.3 mM) of culture. Insulin stimulated HepG2 cell growth. In insulin non stimulated cells NT-S100A8 at 500 nM significantly enhanced cell growth (test of between subjects effects: F=2.71, p<0.05). This stimulatory effect was more pronounced in insulin treated cells (F=3.6, p<0.005). In
Conclusion: the NT-S100A8 inhibits glucose stimulated insulin exocytosis (early response); this finding supports the hypothesis that this pancreatic tumor derived peptide alters β-cell insulin secretion. We choose the hepatocellular carcinoma cell line HepG2 to test insulin signalling events, since in this line the activation of the insulin receptor evokes the mitogenic signalling through MAPK phosphorylation and activation. The NT-S100A8 at a 10 fold higher dosage (500 nM) than that able to inhibit insulin release, was shown to be a growth factor for HepG2 cells, being this effect synergic with that of insulin.