Synergistic Regulation of Schwann Cell Proliferation by Heregulin and Forskolin

Abstract

A peptide corresponding to the epidermal growth factor homology domain of ␤-heregulin stimulated autophosphorylation of the heregulin receptors erbB2 and erbB3 in Schwann cells and activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2. Heregulin-dependent activation of PAK65, a component of the stress-activated signaling pathway, ribosomal S6 kinase, and a cyclic AMP (cAMP) response element binding protein (CREB) kinase, identified as p95 RSK2, was also observed. Receptor phosphorylation and activation of these kinases in response to heregulin occurred in the absence of forskolin stimulation and were not augmented in cells treated with forskolin, a direct activator of adenylyl cyclase. Schwann cell proliferation in response to heregulin was observed only when the cells were also exposed to an agent that elevates cAMP levels. In the absence of heregulin, elevation of cAMP levels failed to stimulate Schwann cell proliferation. Forskolin significantly enhanced heregulin-stimulated expression of cyclin D and phosphorylation of the retinoblastoma gene product. In cells treated with both heregulin and forskolin there was a sustained accumulation of phospho-CREB, which was not observed in cells treated with either agent alone. Heregulin and forskolin synergistically activated transcription of a cyclin D promoter construct. These results demonstrate that heregulin-stimulated activation of MAP kinase is not sufficient to induce maximal Schwann cell proliferation. Expression of critical cell cycle regulatory proteins and cell division require activation of both heregulin and cAMP-dependent processes.Myelination of axons by Schwann cells is critical for the proper functioning of the peripheral nervous system. The correct ratio of Schwann cells to axons is achieved during development through a combination of Schwann cell proliferation (26) and programmed cell death (29). Studies with primary cultures of Schwann cells and embryonic sensory neurons have shown that molecular signals that stimulate Schwann cell proliferation are associated with axonal membranes (24,27,35).Several lines of evidence suggest that the axonal Schwann cell mitogen is a member of the heregulin family of growth factors (5,9,17,21). A common structural feature of heregulins is a cysteine-rich domain of approximately 50 amino acids that is homologous to the active domain of epidermal growth factor (EGF) (18). Heregulins stimulate cell proliferation by binding to and activating transmembrane receptor tyrosine kinases with homology to the EGF receptor, called erbB2, erbB3, and erbB4 (10, 25). A synthetic peptide corresponding to the heregulin EGF homology domain is sufficient to mediate binding to erbB receptors (2). Ligand-dependent activation of erbB receptors leads to activation of the mitogen-activated protein (MAP) kinase pathway, which is critical for cell division in many cell types (22).Schwann cell proliferation can also be stimulated by other polypeptide growth factors (6), including basic fibroblast growth factor and platelet-d...

Protein Kinase A-Induced Phosphorylation of the p65 Subunit of Nuclear Factor-κB Promotes Schwann Cell Differentiation into a Myelinating Phenotype

“…Increasing cAMP alone, in the absence of serum or exogenous growth factors, does not increase proliferation but induces markers of differentiation, e.g., P0 (Morgan et al, 1991). However, when cAMP is elevated in concert with exposure to serum or growth factors, such as neuregulin, then a synergistic effect is seen and there is robust cell proliferation (Lemke and Brockes, 1984;Stewart et al, 1991;Kim et al, 1997;Rahmatullah et al, 1998;Monje et al, 2006).…”

Trypanosoma cruzitrans-sialidase: A potent and specific survival factor for human Schwann cells by means of phosphatidylinositol 3-kinase/Akt signaling

“…monkey kidney) were grown at 37°C in DMEM͞2.5% (vol͞vol) FCS. T. cruzi trypomastigotes, Silvio strain, were maintained in Vero cell cultures (16). T. cruzi epimastigotes were grown in acellular medium at 26°C (18).…”

“…Like primary cultures of rat Schwann cells (11,16), immortalized Schwann cells, when grown in serum-free medium, underwent apoptosis in a time-dependent manner, such that about 50% of the cells exhibited apoptosis by 72 hr (Fig. 1A Left and Fig.…”

“…3B). Thus, TS is a potent trophic agent for Schwann cells, with an efficacy (Ն2.5 nM) equaling or surpassing that of Schwann-cell survival factors such as neuregulin (16) and lysophosphatidic acid, which was active at Ն10 nM for primary cultures of rat Schwann cells (11). cTS also protected primary cultures of rat Schwann cells in a dose-response curve similar to that of human Schwann cells (data not shown), indicating that the action of TS is not restricted to human Schwann cells.…”

Activation of the neuregulin-1/ErbB signaling pathway promotes the proliferation of neoplastic Schwann cells in human malignant peripheral nerve sheath tumors

“…We have hypothesized that inappropriate signaling by proteins in the neuregulin-1 (NRG-1) family of growth and differentiation factors contributes to the pathogenesis of peripheral nerve sheath tumors. NRG-1 proteins stimulate the proliferation, survival and migration of developing Schwann cells (Dong et al, 1995;Morrissey et al, 1995;Rahmatullah et al, 1998), and activating mutations of the NRG-1 receptor subunit erbB2 promote MPNST formation in rodents exposed to the mutagen N-ethyl-N-nitrosourea (EtNU) in utero (Bargmann et al, 1986;Hudziak et al, 1987;Bargmann and Weinberg, 1988;Weiner et al, 1989;Di Marco et al, 1990;Nikitin et al, 1991;Sherman et al, 1999). We have found that JS1 cells, a rat cell line derived from an EtNU-induced MPNST, lack this erbB2 mutation and express erbB2 in combination with the NRG-1 receptor subunit erbB3 and multiple NRG-1 isoforms.…”