Physiological Epidermal Growth Factor Concentrations Activate High Affinity Receptors to Elicit Calcium Oscillations

Signaling mediated by the epidermal growth factor (EGF) is crucial in tissue development, homeostasis and tumorigenesis. EGF is mitogenic at picomolar concentrations and is known to bind its receptor on high affinity binding sites depending of the oligomerization state of the receptor (monomer or dimer). In spite of these observations, the cellular response induced by EGF has been mainly characterized for nanomolar concentrations of the growth factor, and a clear definition of the cellular response to circulating (picomolar) concentrations is still lacking. We investigated Ca(2+) signaling, an early event in EGF responses, in response to picomolar doses in COS-7 cells where the monomer/dimer equilibrium is unaltered by the synthesis of exogenous EGFR. Using the fluo5F Ca(2+) indicator, we found that picomolar concentrations of EGF induced in 50% of the cells a robust oscillatory Ca(2+) signal quantitatively similar to the Ca(2+) signal induced by nanomolar concentrations. However, responses to nanomolar and picomolar concentrations differed in their underlying mechanisms as the picomolar EGF response involved essentially plasma membrane Ca(2+) channels that are not activated by internal Ca(2+) store depletion, while the nanomolar EGF response involved internal Ca(2+) release. Moreover, while the picomolar EGF response was modulated by charybdotoxin-sensitive K(+) channels, the nanomolar response was insensitive to the blockade of these ion channels.