Altered gravity conditions affect early EGF-induced signal transduction in human epidermal A431 cells

Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in human A431 epidermal carcinoma cells. Among the early responses evoked by EGF are receptor clustering, cell rounding, and early gene expression. These processes have been studied under various gravity conditions. In addition, we have investigated signalling pathways as induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), forskolin, and A23187 that bypass the EGF receptor, but mimic the partial activation of signal transduction pathways. Hypergravity, simulated microgravity, and real microgravity have been obtained by means of centrifuge, fast-rotating clinostat, and sounding rocket, respectively. EGF-induced c-fos gene expression is suppressed in simulated microgravity (clinostatting) and even more so in real microgravity, while hypergravity increases early gene expression. This indicates that gravity inhibits early EGF-induced signal transduction. However, neither microgravity nor clinostatting affect EGF-induced EGF receptor clustering, suggesting that inhibition of EGF-induced signal transduction by microgravity and clinostatting is independent of EGF receptor clustering. EGF-induced cell rounding is enhanced under clinostatting, while hypergravity does not significantly influence this process. Furthermore, both under clinostatting and real microgravity, EGF- and TPA-induced c-fos expression is decreased, while forskolin and A23187-induced c-fos expression remains unaltered. These observations demonstrate that gravity affects specific components in the EGF-induced signal transduction circuitry, in particular the protein kinase C pathway which is common to EGF and TPA activated intracellular signalling.