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Dual control of muscle cell survival by distinct growth factor-regulated signaling pathways
M A Lawlor1, X Feng, D R Everding
1Molecular Medicine Division, Oregon Health Sciences University, Portland, Oregon 97201-3098, USA.
Abstract:
In addition to their ability to stimulate cell proliferation, polypeptide growth factors are able to maintain cell survival under conditions that otherwise lead to apoptotic death. Growth factors control cell viability through regulation of critical intracellular signal transduction pathways. We previously characterized C2 muscle cell lines that lacked endogenous expression of insulin-like growth factor II (IGF-II). These cells did not differentiate but underwent apoptotic death in low-serum differentiation medium. Death could be prevented by IGF analogues that activated the IGF-I receptor or by unrelated growth factors such as platelet-derived growth factor BB (PDGF-BB). Here we analyze the signaling pathways involved in growth factor-mediated myoblast survival. PDGF treatment caused sustained activation of extracellular-regulated kinases 1 and 2 (ERK1 and -2), while IGF-I only transiently induced these enzymes. Transient transfection of a constitutively active Mek1, a specific upstream activator of ERKs, maintained myoblast viability in the absence of growth factors, while inhibition of Mek1 by the drug UO126 blocked PDGF-mediated but not IGF-stimulated survival. Although both growth factors activated phosphatidylinositol 3-kinase (PI3-kinase) to similar extents, only IGF-I treatment led to sustained stimulation of its downstream kinase, Akt. Transient transfection of a constitutively active PI3-kinase or an inducible Akt promoted myoblast viability in the absence of growth factors, while inhibition of PI3-kinase activity by the drug LY294002 selectively blocked IGF- but not PDGF-mediated muscle cell survival. In aggregate, these observations demonstrate that distinct growth factor-regulated signaling pathways independently control myoblast survival. Since IGF action also stimulates muscle differentiation, these results suggest a means to regulate myogenesis through selective manipulation of different signal transduction pathways.
Insights
Growth factors like insulin-like growth factor (IGF) and platelet-derived growth factor (PDGF) promote muscle cell survival through distinct signaling pathways. Understanding these pathways, including ERK and PI3-kinase/Akt, offers insights into regulating myogenesis.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Polypeptide growth factors are crucial for cell proliferation and survival, preventing apoptotic death.
- Growth factors regulate cell viability via intracellular signal transduction pathways.
- Previous studies identified C2 muscle cells lacking insulin-like growth factor II (IGF-II) that undergo apoptosis in low-serum medium, preventable by IGF analogues or platelet-derived growth factor BB (PDGF-BB).
Purpose of the Study:
- To investigate the specific signaling pathways mediating growth factor-induced myoblast survival.
- To differentiate the roles of extracellular-regulated kinases (ERKs) and phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathways in survival signaling.
- To explore potential therapeutic strategies for regulating myogenesis through targeted pathway manipulation.
Main Methods:
- Analysis of signaling pathways activated by PDGF-BB and IGF-I in C2 muscle cells.
- Utilizing transient transfection with constitutively active Mek1 (ERK activator) and PI3-kinase/Akt.
- Employing specific inhibitors (UO126 for Mek1, LY294002 for PI3-kinase) to block pathway activation.
- Assessing myoblast viability under various growth factor treatments and pathway inhibitions.
Main Results:
- PDGF-BB induced sustained activation of extracellular-regulated kinases 1 and 2 (ERK1/2), while IGF-I caused only transient activation.
- IGF-I led to sustained stimulation of phosphatidylinositol 3-kinase (PI3-kinase) and its downstream kinase Akt, whereas PDGF-BB did not.
- Inhibition of Mek1 blocked PDGF-mediated survival, while PI3-kinase inhibition selectively blocked IGF-I-mediated survival.
- Constitutively active Mek1 or PI3-kinase/Akt promoted myoblast viability independently of growth factors.
Conclusions:
- Distinct signaling pathways, ERK and PI3-kinase/Akt, independently regulate myoblast survival in response to different growth factors.
- PDGF-BB primarily utilizes the ERK pathway for survival, while IGF-I employs both ERK (transiently) and PI3-kinase/Akt (sustained) pathways.
- Selective manipulation of these signal transduction pathways offers a potential method to regulate myogenesis, considering IGF's role in differentiation.