Endothelin-mediated cell signaling and proliferation in cultured rabbit corneal epithelial cells

To determine if there is endothelin-mediated regulation of cell signaling and proliferation in rabbit corneal epithelium.

Endothelin-1 (ET-1) gene and protein expression by the rabbit corneal epithelial (RCE) cells were analyzed by polymerase chain reaction, sequence analysis, and enzyme immunoassay. DNA synthesis was characterized by [3H]-thymidine uptake. Endothelin receptor linkage to cell signaling pathways was determined based on measurements of the dose dependent effects of ET-1, ET-2, and ET-3 on intracellular Ca2+ concentration ([Ca2+]i) transients in fura-2-loaded cells, and of ET-1 on phosphoinositide turnover and cAMP accumulation in the isolated rabbit corneal epithelium.

The authors detected the mRNA for prepro ET-1 in RCE cells, and ET-like immunoreactivity was identified in conditioned culture medium. ET-1 (1 nM) maximally stimulated [3H]-thymidine uptake by twofold (EC50 = 0.3 nM). Endothelins elicited transient increases in [Ca2+]i with a rank order of potency of ET-1 > or = ET-2 >> ET-3. These increases consisted of both intracellular Ca2+ mobilization and influx of Ca2+ from the bathing solution. Intracellular mobilization was linked to increases in IP3 turnover because 1 microM ET-1 increased IP3 content by 48% from its control value (EC50 = 23 nM), whereas Ca2+ influx occurred through a non-L-type Ca2+ channel because preexposure to 1 microM nicardipine did not affect either the height or the duration of a [Ca2+]i transient. One micromolar of ET-1 was required to elicit a significant increase in cAMP accumulation of 69% from its control value. This increase was dependent on the presence of Ca2+ in the bathing solution and was comparable to and nonadditive with that of the Ca2+ ionophore, A23187 (1 microM).

These data suggest that endothelin production by primary cultures of RCE cells can mediate an increase in cell proliferation through an ETA receptor subtype. This receptor subtype appears to be involved based on the rank order of potency of ETs to elicit [Ca2+]i transients, increases in phosphoinositide turnover, and cAMP accumulation.