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...