EGF induces ornithine decarboxylase transcription in Balb/MK keratinocytes: a proposed mechanism for the TGFα autocrine loop
Epidermal hyperproliferation (psoriasis, wound repair) is the result of quiescent (G0) keratinocytes being recruited into the cell cycle. A detailed characterization of the cell cycle kinetic parameters of the mouse keratinocyte line (Balb/MK) has been carried out with the aid of bivariate iododeoxyuridine (IdUrd) and DNA analysis using flow cytometry, in order to establish whether it might provide a useful model for the study of the biochemical events controlling recruitment into the cell cycle. Balb/MK keratinocytes were cultured using low Ca2+ Dulbecco's modified Eagle's medium/F 12 in the presence of 10% dialysed fetal bovine serum and 4 ng/ml epidermal growth factor (EGF). IdUrd labelling followed by flow cytometric analysis of trypsinized cells showed that about 95% of the population were actively cycling, with a cell cycle time of around 14h and no significant contact inhibition. Omission of serum and EGF followed by IdUrd pulse-labelling indicated that the cells progressively withdrew from the cycle and, after 16h, less than 10% incorporated IdUrd. Subsequent restimulation with serum resulted in a synchronized cohort of cells being recruited. Entry into the S phase of the cell cycle (IdUrd incorporation) started at 8 h and was maximal between 12 h and 16h after stimulation. Restimulation with EGF alone reached a growth fraction of 87% after 24 h continuous labelling compared with 97% using serum together with EGF. Epidermal growth factor already showed a significant stimulation at 10 pg/ml compared with the controls. There is a broad plateau centred on 5 ng/ml, followed by a slight decline above this level. We conclude that the use of a cell line with defined cell cycle kinetic parameters which can be switched between the quiescent and cycling states in a fully defined medium will provide an ideal model for biochemical studies of the relevant signal transduction pathways in keratinocytes.