Lawlor M A, Feng X, Everding D R, Sieger K, Stewart C E, Rotwein P
Molecular Medicine Division, Oregon Health Sciences University, Portland, Oregon 97201-3098, USA.
Mol Cell Biol. 2000 May;20(9):3256-65. doi: 10.1128/MCB.20.9.3256-3265.2000.
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.
除了具有刺激细胞增殖的能力外，多肽生长因子还能够在其他情况下会导致凋亡性死亡的条件下维持细胞存活。生长因子通过调节关键的细胞内信号转导途径来控制细胞活力。我们之前鉴定了缺乏胰岛素样生长因子II（IGF-II）内源性表达的C2肌肉细胞系。这些细胞在低血清分化培养基中不会分化，而是会发生凋亡性死亡。可以通过激活IGF-I受体的IGF类似物或无关的生长因子（如血小板衍生生长因子BB，PDGF-BB）来防止细胞死亡。在此，我们分析了生长因子介导的成肌细胞存活所涉及的信号通路。PDGF处理导致细胞外调节激酶1和2（ERK1和-2）持续激活，而IGF-I仅短暂诱导这些酶。组成型活性Mek1（ERK的特异性上游激活剂）的瞬时转染在无生长因子的情况下维持了成肌细胞的活力，而药物UO126对Mek1的抑制则阻断了PDGF介导的但不是IGF刺激的细胞存活。尽管两种生长因子都能以相似的程度激活磷脂酰肌醇3激酶（PI3激酶），但只有IGF-I处理能导致其下游激酶Akt的持续刺激。组成型活性PI3激酶或可诱导的Akt的瞬时转染在无生长因子的情况下促进了成肌细胞的活力，而药物LY294002对PI3激酶活性的抑制则选择性地阻断了IGF介导的而非PDGF介导的肌肉细胞存活。总的来说，这些观察结果表明，不同的生长因子调节信号通路独立地控制成肌细胞的存活。由于IGF的作用还能刺激肌肉分化，这些结果提示了一种通过选择性操纵不同信号转导途径来调节肌发生的方法。
