Activation of caspase-8 and Erk-1/2 in domes regulates cell death induced by confluence in MDCK cells

Under normal culture conditions, cells adhere to culture dish, spread out, proliferate, and finally cover all areas and reach confluence. During the confluent stage, cell proliferation ceases and differentiation is enhanced. Meanwhile, cell death also appears as the monolayer confluence proceeds. To delineate the mechanism of cell death induced by the confluent process, we employed Madin-Darby canine kidney (MDCK) cells. When approaching confluence, MDCK cells exhibited increase the levels of caspase-2 and enhanced the activity of caspase-8. Using various caspase inhibitors to block apoptosis, we found that only z-VAD-fmk and z-IETD-fmk can inhibit confluent cell death, indicating that confluent cell death is mediated by activation of caspase-8. Overexpression of Bcl-2 inhibited confluent cell death, suggesting the involvement of mitochondria-dependent pathway in confluent cell death. Interestingly, the activity of phospho-Erk (p-Erk) was initially decreased before confluence, but markedly increased after confluence. Immunofluorescence staining studies showed that p-Erk was expressed exclusively on dome-forming cells that underwent apoptosis. Treatment of confluent MDCK cells with PD98059 and UO126, the inhibitors of MEK, enhanced apoptosis as well as activity of caspase-8. These data indicate that elevation of p-Erk activity during confluence may serve to suppress confluent cell death. Taken together, activation of caspase-8 contributes to and results in confluent cell death, whereas elevated p-Erk activity serves to prevent confluent cell death by regulating activation of caspase-8.