The aim of this study was to deeper investigate the mechanisms through which
ENPP1, a negative modulator of insulin receptor (IR) activation, plays a role on
insulin signaling, insulin secretion and eventually glucose metabolism. ENPP1
cDNA (carrying either K121 or Q121 variant) was transfected in HepG2 liver-, L6
skeletal muscle- and INS1E beta-cells. Insulin-induced IR-autophosphorylation
(HepG2, L6, INS1E), Akt-Ser473,
ERK1/2-Thr202/Tyr204 and GSK3-beta Ser9
phosphorylation (HepG2, L6), PEPCK mRNA levels (HepG2) and
2-deoxy-D-glucose uptake (L6) was studied. GLUT 4 mRNA
(L6), insulin secretion and caspase-3 activation (INS1E) were also investigated.
Insulin-induced IR-autophosphorylation was decreased in HepG2-K, L6-K, INS1E-K
(20%, 52% and 11% reduction vs. untransfected cells) and
twice as much in HepG2-Q, L6-Q, INS1E-Q (44%, 92% and 30%).
Similar data were obtained with Akt-Ser473,
ERK1/2-Thr202/Tyr204 and GSK3-beta Ser9 in
HepG2 and L6. Insulin-induced reduction of PEPCK mRNA was progressively lower in
untransfected, HepG2-K and HepG2-Q cells (65%, 54%, 23%).
Insulin-induced glucose uptake in untransfected L6 (60% increase over
basal), was totally abolished in L6-K and L6-Q cells. GLUT 4 mRNA was slightly
reduced in L6-K and twice as much in L6-Q (13% and 25% reduction
vs. untransfected cells). Glucose-induced insulin secretion was 60%
reduced in INS1E-K and almost abolished in INS1E-Q. Serum deficiency activated
caspase-3 by two, three and four folds in untransfected INS1E, INS1E-K and
INS1E-Q. Glyburide-induced insulin secretion was reduced by 50% in
isolated human islets from homozygous QQ donors as compared to those from KK and
KQ individuals. Our data clearly indicate that ENPP1, especially when the Q121
variant is operating, affects insulin signaling and glucose metabolism in
skeletal muscle- and liver-cells and both function and survival of insulin
secreting beta-cells, thus representing a strong pathogenic factor predisposing
to insulin resistance, defective insulin secretion and glucose metabolism
abnormalities.
