title: Serum and forskolin cooperate to promote G1 progression in Schwann cells by differentially regulating cyclin D1, cyclin E1, and p27Kip expression.
abstract: Proliferation of Schwann cells in vitro, unlike most mammalian cells, is not induced by serum alone but additionally requires cAMP elevation and mitogenic stimulation. How these agents cooperate to promote progression through the G1 phase of the cell cycle is unclear. We studied the integrative effects of these compounds on receptor-mediated signaling pathways and regulators of G1 progression. We show that serum alone induces strong cyclical expression of cyclin D1 and E1, 6 and 12 h after addition, respectively. Serum also promotes strong but transient erbB2, ERK, and Akt phosphorylation, but Schwann cells remain arrested in G1 due to high levels of the inhibitor, p27(Kip). Forskolin with serum promotes G1 progression in 22% of Schwann cells between 18 and 24 h by inducing a steady decline in p27(Kip) levels that reaches a nadir at 12 h coinciding with peak cyclin E1 expression. Forskolin also delays neuregulin-induced loss of erbB2 receptors allowing strong acute activation of PI3K, sustained erbB2 phosphorylation and G1 progression in 31% of Schwann cells. We find that the ability of forskolin to decrease p27(Kip) is associated with its ability to decrease Krox-20 expression that is induced by serum and further increased by neuregulin. Our results explain why serum is required but insufficient to stimulate proliferation and identify two routes by which forskolin promotes proliferation in the presence of serum and neuregulin. These findings provide insights into how G1 progression and, cell cycle arrest leading to myelination are regulated in Schwann cells.
authors.affiliation: Department of Molecular Biology and Microbiology, University of Central Florida, Orlando, Florida, USA.
authors.forename: Jared
authors.initials: J
authors.lastname: Iacovelli
authors.forename: Jorge
authors.initials: J
authors.lastname: Lopera
authors.forename: Marga
authors.initials: M
authors.lastname: Bott
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Baldwin
authors.forename: Annette
authors.initials: A
authors.lastname: Khaled
authors.forename: Nizam
authors.initials: N
authors.lastname: Uddin
authors.forename: Cristina
authors.initials: C
authors.lastname: Fernandez-Valle
mesh_terms: D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D001769:Blood; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology*; D019938:Cyclin D1 / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D004357:Drug Synergism; D051767:Early Growth Response Protein 2 / Q000037:antagonists & inhibitors; D016193:G1 Phase / Q000187:drug effects; D006023:Glycoproteins / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D020890:Neuregulin-1; D010766:Phosphorylation / Q000187:drug effects; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018719:Erb-b2 Receptor Tyrosine Kinases; D012583:Schwann Cells / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D000077191:Wortmannin
chemical_list: D000730:Androstadienes; D005576:Colforsin; D019938:Cyclin D1; D019927:Cyclin E; D050760:Cyclin-Dependent Kinase Inhibitor p27; D016213:Cyclins; D051767:Early Growth Response Protein 2; D006023:Glycoproteins; D009419:Nerve Tissue Proteins; D020890:Neuregulin-1; D047428:Protein Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt; D018719:Erb-b2 Receptor Tyrosine Kinases; D000077191:Wortmannin; C497328:Egr2 protein, rat; C490580:Nrg1 protein, rat; C583044:cyclin E1, rat; C483804:Erbb2 protein, rat