C-peptide-induced cFos mRNA expression in the human retinal pigment epithelial cell line ARPE-19. ARPE-19 cells were incubated in either low (5.5 mM) or high (25 mM) glucose for 24 h prior to exposure to vehicle, 1 nM C-peptide, or 1 nM C-peptide with 1 nM insulin for 1 h. Cells were lysed, RNA isolated, and changes in cFos mRNA expression evaluated using quantitative PCR. **P < 0.01, ***P < 0.001 vs. low glucose, vehicle-treated cells; #P < 0.01 vs. low glucose, C-peptide-treated cells.