Page 111 - 76_01

VOL. 76 (1), 23-44, 2010 BETA-CELL HYPERPLASIA INDUCED BY HEPATIC INSULIN...

3.4. Progressive hepatic expression and plasma
3.4. concentration of IGF-1 in iLIRKO mice

Previous studies have suggested the presence of a circulating
islet growth factor in insulin resistant states, independent of glucose
and obesity (21). To address this important issue, we conducted
western blot analysis with anti-IGF-1 antibody in the liver. Thus,
iLIRKO induced expression depending on the level of IR deletion
by the liver (Figure 6, panel A). In parallel, iLIRKO mice induced
IGFBP1 and IGFBP3 also depending on the level of IR deletion
by the liver. These data were confirmed in 1 year-old mice (Figure
6, panel C). Noteworthy, iLIRKO mice bearing no liver IR expression
significantly induced plasma IGF-1 in 6 month- and 1 year-old mice
as compared with controls (Figure 6, panels B and D respectively).

3.5. Increase of IR-A in pancreatic islets: beta cell lines
3.5. expressing IR-A, but not IR-B, show increased
3.5. proliferation in response to insulin or IGF-1

The IR expression in pancreatic islets was significantly increased
in iLIRKO mice (Figure 7, panel A). More importantly, the percentage
of IR-A from total IR dramatically increased in these mice (Figure 7,
panel A). In order to investigate a possible role of this change in the
pattern of expression of the IR isoforms in the beta-cell hyperplasia,
we performed experiments of BrdU incorporation in pancreatic islets
of 6-month-old control and iLIRKO mice in presence of 10 nM insulin
or 10 nM IGF-1. The results showed that the islets of iLIRKO mice
were significantly more sensitive to the IGF-1-induced proliferation
than those control islets. Previous data indicated that IR-A was 2-fold
more sensitive than IR-B in response to insulin regarding glycogen
synthesis and also mitogenesis (22). To assess those results in mouse
beta cells, we have generated beta cell lines bearing IR (IRLoxP),
lacking IR (IRKO), expressing exclusively IR-A (Rec A), or alterna-
tively expressing IR-B (Rec B) (Figure 7, panel C). Moreover, the
generated cell lines were completely functional as assesed by IR and
IGF-1R phosphorylation experiments (Figure 7, panel D). The lack
of IR significantly decreased basal glucose uptake in beta cells.
Reconstitution with IR-A, but not with IR-B, restored basal glucose

35