Small applied electric fields, growth factors and corneal epithelial cell behaviour
Wounding of the cornea generates lateral electric fields (EFs) and initiates the expression of hepatocyte growth factor (HGF) and keratinocyte growth factor (KGF; Chiang et al., 1992; Wilson et al., 1999a). Therefore, these biologically generated EFs and endogenous growth factors may be of particular significance during wound healing. In the presence of an EF (150 mV/mm) cultured corneal epithelial cells (CECs) oriented perpendicular, directed cathodally and migrated at an enhanced rate. The induction times, induction thresholds and response patterns for these behaviours in increasing field strengths, indicated that they may operate through separate and parallel pathways. The application of either HGF or KGF enhanced the rate of CEC migration but neither affected the extent of CEC orientation or directionality. The distribution of HGF receptors (HGFR) was found to be exclusive to the cell body in the presence of an EF, the receptors accumulated cathodally. Moreover, the asymmetrical accumulation of HGFR in the presence of an EF correlated with the direction of CEC migration. The application of both HGF and an EF activated extracellular-signal regulated kinase (ERK) a mitogen-activated protein kinase. Furthermore, in the presence of an EF the observed ERK activation was greater in the cathodal facing half of the CECs. Inhibition of ERK reduced the extent of HGF and EF-enhanced CEC migration rate but did not alter EF-induced CEC cathodal directionality or perpendicular orientation. The HGF- and EF-enhancement of CEC migration rate may involve the activation of ERK and with the downstream liberation of leukotrienes and phosphorylation of MLCK, would culminate in actin contraction and polymerisation respectively. The potential clinical relevance for this work would concern the topical application of HGF and exogenous application of EFs to corneal epithelial wounds in order to augment healing in patients where this process is slow or deficient.