Identification and characterization of insulin-like growth factor receptors on adult rat cardiac myocytes: linkage to inositol 1,4,5-trisphosphate formation.

Cultured cardiac myocytes from adult Sprague-Dawley rats express
both insulin-like growth factor-I (IGF-I) receptors and
insulin-like growth factor-II/mannose 6-phosphate (IGF-II/Man6P)
receptors and respond to IGF-I with a dose-dependent accumulation
of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol
1,4-bisphosphate [Ins(1,4)P2]. Specific binding of [125I]IGF-I to
isolated membranes from cultured cardiac myocytes amounted to
1-1.2%. Binding of [125I]IGF-I was inhibited by unlabeled IGF-I at
nanomolar concentrations and insulin at much higher concentrations.
These data suggest that IGF-I binds to its own receptor on rat
cardiac myocytes. Competitive binding studies using isolated
membranes from cardiac myocytes and [125I]IGF-II showed 2-4%
specific binding. Binding of [125I]IGF-II was inhibited by IGF-II
and much less potently by IGF-I and insulin. Immunoglobulin G (IgG)
3637 (an IgG directed against the IGF-II/Man6P receptor) partially
inhibited binding of [125I]IGF-II whereas nonimmune IgG did not.
Affinity cross-linking studies with [125I]IGF-II and cardiac
myocyte membranes and subsequent analysis of the ligand-receptor
complex using SDS-PAGE and autoradiography showed a radiolabeled
band of approximately 250 kilodalton (kDa). The formation of the
[125I]IGF-II-receptor complex was inhibited by incubation with
IGF-II and IgG 3637 but not by insulin or nonimmune IgG. Western
blotting of protein extracts from cultured cardiac myocytes was
performed using IgG 3637 and an immunoperoxidase technique for the
visualization of the IGF-II/Man6P receptor protein. A specific band
at 220 kDa under nonreducing conditions was detected on the blots,
providing further evidence for the expression of the IGF-II/Man6P
receptor by cardiac myocytes. The effect of IGFs on the
accumulation of inositol phosphates was measured by HPLC analysis
of perchloric acid extracts from myo-[3H]inositol-labeled cultured
cardiac myocytes. IGF-I (50 ng/ml) stimulated the accumulation both
of Ins(1,4,5)P3 and Ins(1,4)P2 after 30 sec by 43% and 63%. IGF-II
(up to 500 ng/ml) had no significant effect on inositol phosphate
accumulation under the same conditions. However, in the presence of
millimolar concentrations of Man6P, IGF-II (500 ng/ml) also
increased Ins(1,4,5)P3 accumulation by 59%. We conclude that
cardiac myocytes from adult rats express IGF receptors and respond
to IGFs with the accumulation of Ins(1,4,5)P3 and Ins(1,4)P2. This
effect seems to be mediated by an IGF-I receptor-specific pathway.