The biological activity of growth factors that act through receptor tyrosine kinases (RTKs) can differ dramatically, depending on both the properties of the RTKs and the cellular environment in which the RTKs are expressed. To determine the ability of different RTKs to elicit ras- independent responses central to neuronal differentiation, we analyzed voltage-dependent sodium (Na) channel expression in rat pheochromocytoma (PC12) cells after activation of a variety of endogenously and exogenously expressed RTKs. In PC12 cells expressing trkB (Ip et al., 1993), the increase in Na current density caused by brain-derived neurotrophic factor (BDNF) was similar to that observed upon activation of endogenous trkA by NGF. BDNF also increased type II Na channel mRNA expression, as did neurotrophin-3 in PC12 cells expressing trkC (Tsoulfas et al., 1993). In contrast, insulin did not increase type II Na channel mRNA expression or Na current density in PC12 cells, while epidermal growth factor (EGF) elicited small, yet reproducible increases in type II Na channel mRNA and Na current density when compared to NGF, even upon coexpression of an EGF receptor/p75 receptor chimera (Yan et al., 1991). Finally, in PC12 cells expressing beta-platelet-derived growth factor (PDGF) receptors (Heasley and Johnson, 1992), PDGF increased type II Na channel mRNA and Na current density to the same extent as NGF. The results show the capabilities of these RTKs in eliciting Na channel expression and the specificity arising due to differences in their intrinsic properties.

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Note I would like to dedicate this essay with gratitude to the faculty of the Department of Romance Languages and Literatures at Washington University. My appreciation to my associate J. Randall Phillips for his unstinting efforts in improving this essay.