SYNERGISTIC INDUCTION OF NEURITE OUTGROWTH BY NERVE GROWTH-FACTOR OR EPIDERMAL GROWTH-FACTOR AND INTERLEUKIN-6 IN PC12 CELLS

Native PC12 cells respond differentially to nerve growth factor (NGF) but not interkeukin-6 (IL-6); PC12-E2 cells, a stable variant, respond to both stimuli (and more rapidly to NGF). Neither responds to epider mal growth factor (EGF), NGF primarily induces the RAS/extracellular s ignal-regulated kinase (ERK) pathway and IL-6 activates a JAK (Janus t yrosine kinase)/STAT (signal transducers and activators of transcripti on) response. EGF also stimulates RAS/ERK but in a transient manner. W hen either cell type is treated with combinations of NGF, EGF, and IL- 6, at concentrations that produce modest or no response, a substantial augmentation of neurite outgrowth is observed. With PC12-E2 cells, a subthreshold concentration of IL-6 increases NGF response by similar t o-3-fold after 1-2 days; the increase with EGF is more pronounced. Nat ive PC12 cells show even greater synergistic effects with NGF and IL-6 . The most dramatic effect was observed with low levels of EGF, where IL-6 increased the percentage of responsive cells from zero to similar to 60% after 3 days. In addition, two neural specific transcripts, GA P-43 and SCG-10, are synergistically increased by the combinations of growth factors. Importantly, IL-6 does not enhance ERK phosphorylation in the presence of either NGF or EGF. In contrast, NGF and EGF, in th e presence or absence of IL-6, cause mobility shifts of Stat3 that are consistent with serine phosphorylations. Although these modifications do not lead to activation and translocation by themselves, in the pre sence of the tyrosine phosphorylation induced by IL-6, they may play a role in the synergistic responses. These observations suggest a diffe rentially regulated two-stage mechanism for the differentiative respon se of PC12 cells to NGF.