Aims/hypothesis. To explain the mechanisms whereby mutations in the HNF-1 a
lpha gene cause insulin secretory defects.
Methods. A truncated mutant HNF-1 alpha (HNF-1 alpha 288t) was overexpresse
d in hepatoma cells (HepG2) and murine insulinoma cells (MING) using a reco
mbinant adenovirus system and expression of the HNF-1 alpha target genes an
d insulin secretion were examined.
Results. Expression of phenylalanine hydroxylase and alpha 1-antitrypsin ge
nes, the target genes of HNF-1 alpha, was suppressed in HepG2 cells by over
expression of HNF-1 alpha 288t. In MIN6 cells, overexpression of HNF-1 alph
a 288t did not change insulin secretion stimulated by glucose (5 mmol/l and
25 mmol/l) or leucine (20 mmol/l). Potentiation of insulin secretion by ar
ginine (20 mmol/l, in the presence of 5 mmol/l or 25 mmol/l glucose) was, h
owever, reduced (p < 0.0001 and p = 0.027, respectively). Similarly reduced
responses were observed when stimulated with homoarginine. Expression of t
he cationic amino acid transporter-2 was not reduced and insulin secretory
response to membrane depolarization by 50 mmol/l KCl was intact. Conclusion
/interpretation. The HNF-1 alpha 288t, which is structurally similar to the
mutant HNF-1 alpha expressed from the common MODY3 allele, P291fsinsC, exe
rts a dominant negative effect. Suppression of HNF-1 alpha in MIN6 cells se
verely impaired potentiation of insulin secretion by arginine, whereas gluc
ose-stimulated and leucine-stimulated insulin secretion was intact. Our fin
dings delineate the complex nature of beta-cell failure in patients with MO
DY3. This cell model will be useful for further investigation of the mechan
ism of insulin secretory defects in these patients.