The atypical alpha2beta2 IGF receptor expressed in inducible c2.7 myoblasts is derived from post-translational modifications of the mouse IGF-I receptor.
OBJECTIVE
Unlike parental permissive C2.7 myoblasts, inducible C2.7 myoblasts require IGF-I or IGF-II to differentiate and expression of MyoD is not constitutive. Our previous studies indicated that inducible myoblasts express an atypical alpha2beta2 IGF receptor that differs from the classical IGF-I receptor by its higher affinity for IGF-II compared with IGF-I and the higher molecular weight of its alpha and beta subunits. Expression of this atypical IGF-I receptor is developmentally regulated; hence this receptor is lost upon terminal differentiation. Muscle cell differentiation is a system in which IGF-II plays an essential role and developmentally regulated atypical IGF-I receptor may represent a candidate for mediating differentiation signals provided by IGF-II. To further understand the structure and the role of the atypical IGF-I receptor, (i) we investigated for a putative IGF-I receptor transcript polymorphism by extensive sequencing of RT-PCR products; (ii) we overexpressed cloned mouse IGF-I receptor in permissive and inducible C2.7 myoblasts and characterized the binding and structural properties of overexpressed IGF-I receptor and (iii) we analysed the effects of this overexpression on myoblasts differentiation.


METHODS
Cultured mouse myoblasts C2.7 and subclone variant inducible C2.7 cell lines were used. Mouse IGF-I receptor cDNA was cloned by cDNA library screening. Gene expression was measured by semi-quantitative RT-PCR analysis and receptor affinity by ligand binding. Receptor protein autophosphorylation of IGF-IR was analysed by immunoprecipitation and Western blot. Myoblastic differentiation was accessed by myogenic factors expression and immunofluorescence study.


RESULTS
Atypical IGF-I receptor may correspond to a new receptor belonging to the insulin/IGF-I receptor family, or it may also derive from alternate splicing of the gene of the insulin/IGF-I receptors and/or post-translational modifications of the insulin/IGF-I receptors. Our results exclude the existence of a polymorphism of the IGF-I receptor transcripts in inducible and permissive myoblasts. In embryo and cancer cells IGF-II binds to insulin receptor (IR) isoform A, RT-PCR experiments show that IR is expressed in permissive but not in inducible myoblasts. We demonstrated here that post-translational processing of the mouse IGF-I receptor is responsible for the existence of the mouse atypical IGF-I receptor in inducible myoblasts. Overexpressed mouse IGF-I receptor in permissive myoblasts has the same biochemical and binding characteristics as the classical IGF-I receptor whereas in inducible myoblasts, overexpressed mouse IGF-I receptor has the biochemical, binding and functional characteristics of the atypical IGF-I receptor.


CONCLUSIONS
Our results provide experimental evidence that the atypical IGF-I receptor variant expressed in subclone inducible C2.7 is issued from a post-translational processing of mouse IGF-I receptor. We show that this post-translational modification is closely associated with the cell lines indeed permissive C2.7 myoblasts process mouse cDNA IGF-I receptor as a classical IGF-I receptor whereas inducible C2.7 myoblasts process mouse cDNA IGF-I receptor as an atypical IGF-I receptor. On other hand, we show that overexpression of mouse IGF-I receptor in inducible myoblasts does not abrogate IGF-I or IGF-II requirement to differentiate.
Navarro
Magali
M
UMR866 INRA, Laboratoire de Différenciation Cellulaire et Croissance, 2 place Viala, 34060 Montpellier Cedex 1, France.
Joulia
Dominique
D
CeDReS, CHU de Treichville, 01 BP V03 Abidjan, Cote d'Ivoire.
Fedon
Yann
Y
UMR866 INRA, Laboratoire de Différenciation Cellulaire et Croissance, 2 place Viala, 34060 Montpellier Cedex 1, France.
Levin
Jonathan
J
UMR866 INRA, Laboratoire de Différenciation Cellulaire et Croissance, 2 place Viala, 34060 Montpellier Cedex 1, France.
Barenton
Bruno
B
Unité 418 INSERM, Hopital Desbrousse, 29 rue Sœur Bouvier, 69322 Lyon Cedex 5, France.
Bernardi
Henri
H
UMR866 INRA, Laboratoire de Différenciation Cellulaire et Croissance, 2 place Viala, 34060 Montpellier Cedex 1, France.
D000818:Animals; D002454:Cell Differentiation; D002460:Cell Line; D018076:DNA, Complementary / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000378:metabolism; D051379:Mice; D032446:Myoblasts / Q000378:metabolism*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D011972:Receptor, Insulin / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction
D018076:DNA, Complementary; D007335:Insulin-Like Growth Factor II; D017526:Receptor, IGF Type 1; D011972:Receptor, Insulin