Fibroblast Growth Factor Inhibits Insulin-Like Growth Factor-ll (IGF-II) Gene Expression and Increases IGF-I Receptor Abundance in BC3H-1 Muscle Cells
Stephen M. Rosenthal, Eric J. Brown, Antonio Brunetti, and Ira D. Goldfine Department of Pediatrics (S.M.R., E.J.B.) Department of Medicine and Physiology (I.D.G.) University of California San Francisco, California 94143 Division of Diabetes and Endocrine Research Mount Zion Medical Center University of California (A.B., I.D.G.) San Francisco, California 94120
INTRODUCTION
Muscle is an important target tissue for insulin-like growth factor (IGF) action. We have previously reported that muscle cell differentiation is associated with down-regulation of the IGF-I receptor at the level of gene expression that is concomitant with an increase in the expression and secretion of IGF-II. Furthermore, treatment of myoblasts with IGF-II resulted in a similar decrease in IGF-I receptor mRNA abundance, suggesting an autocrine role of IGF-II in IGF-I receptor regulation. To explore further the role of IGF-II in IGF-I receptor regulation, BC3H-1 mouse muscle cells were exposed to differentiation medium in the presence of basic fibroblast growth factor (FGF), a known inhibitor of myogenic differentiation. FGF treatment of cells resulted in a 50% inhibition of IGF-II gene expression compared to that in control myoblasts and markedly inhibited IGF-II secretion. Concomitantly, FGF resulted in a 60-70% increase in IGF-I binding compared to that in control myoblasts. Scatchard analyses and studies of gene expression demonstrated that the increased IGF-I binding induced by FGF reflected parallel increases in IGF-I receptor content and mRNA abundance. These studies indicate that FGF may up-regulate IGF-I receptor expression in muscle cells through inhibition of IGF-II peptide expression and further support the concept of an autocrine role of IGF-II in IGF-I receptor regulation. In addition, these studies suggest that one mechanism by which FGF inhibits muscle cell differentiation is through inhibition of IGF-II expression. (Molecular Endocrinology 5: 678684,1991)
Insulin-like growth factor-l (IGF-I) and IGF-II are distinct members of a family of peptide hormones with a broad range of metabolic and mitogenic effects (1). Recent studies indicate that muscle is not only a site of IGF production (2-8), but is also an important target tissue for IGF action. Specific high affinity receptors for IGF-I and -II have been identified in various muscle cell lines as well as in primary monolayers of normal muscle (613), and both IGF peptides stimulate growth and differentiation of muscle cells (14-18). While IGF-II may signal through its own receptor in some tissues (19-21), studies in muscle have demonstrated that both IGF peptides can induce mitogenic effects through the IGF-I receptor (18, 22, 23). This receptor has significant structural and sequence homology with the insulin receptor and is a heterotetramer composed of two extracellular a-subunits (135 kDa) which bind the ligand and two transmembrane /3-subunits (90-97 kDa) which have intrinsic tyrosine kinase activity in their intracellular domains (24-26). We have recently reported that the differentiation of BC3H-1 cells, a nonfusing mouse cell line with characteristics of both smooth and skeletal muscle (27), is associated with down-regulation of the IGF-I receptor at the level of gene expression that is concomitant with an increase in the expression and secretion of IGF-II (8). Furthermore, treatment of BC3H-1 myoblasts with IGF-II resulted in a similar decrease in IGF-I receptor mRNA abundance, suggesting an autocrine role of IGFII in IGF-I receptor regulation (8). To explore further the role of IGF-II in IGF-I receptor regulation, BC3H-1 cells were exposed to differentiation medium in the presence
0888-8809/91 /0678-0684$03.00/0 Molecular Endocrinology Copyright © 1991 by The Endocrine Society
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The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 15 November 2015. at 13:53 For personal use only. No other uses without permission. . All rights reserved.
679
Effects of FGF on IGF-II Peptide and IGF-I Receptor
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of basic fibroblast growth factor (FGF), a known inhibitor of myogenic differentiation (28-30).
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Stephen M. Rosenthal, Eric J. Brown, Antonio Brunetti, and Ira D. Goldfine Department of Pediatrics (S.M.R., E.J.B.) Department of Medicine and Physiology (I.D.G.) University of California San Francisco, California 94143 Division of Diabetes and Endocrine Research Mount Zion Medical Center University of California (A.B., I.D.G.) San Francisco, California 94120
INTRODUCTION
Muscle is an important target tissue for insulin-like growth factor (IGF) action. We have previously reported that muscle cell differentiation is associated with down-regulation of the IGF-I receptor at the level of gene expression that is concomitant with an increase in the expression and secretion of IGF-II. Furthermore, treatment of myoblasts with IGF-II resulted in a similar decrease in IGF-I receptor mRNA abundance, suggesting an autocrine role of IGF-II in IGF-I receptor regulation. To explore further the role of IGF-II in IGF-I receptor regulation, BC3H-1 mouse muscle cells were exposed to differentiation medium in the presence of basic fibroblast growth factor (FGF), a known inhibitor of myogenic differentiation. FGF treatment of cells resulted in a 50% inhibition of IGF-II gene expression compared to that in control myoblasts and markedly inhibited IGF-II secretion. Concomitantly, FGF resulted in a 60-70% increase in IGF-I binding compared to that in control myoblasts. Scatchard analyses and studies of gene expression demonstrated that the increased IGF-I binding induced by FGF reflected parallel increases in IGF-I receptor content and mRNA abundance. These studies indicate that FGF may up-regulate IGF-I receptor expression in muscle cells through inhibition of IGF-II peptide expression and further support the concept of an autocrine role of IGF-II in IGF-I receptor regulation. In addition, these studies suggest that one mechanism by which FGF inhibits muscle cell differentiation is through inhibition of IGF-II expression. (Molecular Endocrinology 5: 678684,1991)
Insulin-like growth factor-l (IGF-I) and IGF-II are distinct members of a family of peptide hormones with a broad range of metabolic and mitogenic effects (1). Recent studies indicate that muscle is not only a site of IGF production (2-8), but is also an important target tissue for IGF action. Specific high affinity receptors for IGF-I and -II have been identified in various muscle cell lines as well as in primary monolayers of normal muscle (613), and both IGF peptides stimulate growth and differentiation of muscle cells (14-18). While IGF-II may signal through its own receptor in some tissues (19-21), studies in muscle have demonstrated that both IGF peptides can induce mitogenic effects through the IGF-I receptor (18, 22, 23). This receptor has significant structural and sequence homology with the insulin receptor and is a heterotetramer composed of two extracellular a-subunits (135 kDa) which bind the ligand and two transmembrane /3-subunits (90-97 kDa) which have intrinsic tyrosine kinase activity in their intracellular domains (24-26). We have recently reported that the differentiation of BC3H-1 cells, a nonfusing mouse cell line with characteristics of both smooth and skeletal muscle (27), is associated with down-regulation of the IGF-I receptor at the level of gene expression that is concomitant with an increase in the expression and secretion of IGF-II (8). Furthermore, treatment of BC3H-1 myoblasts with IGF-II resulted in a similar decrease in IGF-I receptor mRNA abundance, suggesting an autocrine role of IGFII in IGF-I receptor regulation (8). To explore further the role of IGF-II in IGF-I receptor regulation, BC3H-1 cells were exposed to differentiation medium in the presence
0888-8809/91 /0678-0684$03.00/0 Molecular Endocrinology Copyright © 1991 by The Endocrine Society
678
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 15 November 2015. at 13:53 For personal use only. No other uses without permission. . All rights reserved.
679
Effects of FGF on IGF-II Peptide and IGF-I Receptor
20
of basic fibroblast growth factor (FGF), a known inhibitor of myogenic differentiation (28-30).
n
y E o
