Relative resistance to methotrexate by proliferating normal rabbit epidermal cells in vitro

Primary cell cultures of normal rabbit epidermal cells (keratinocytes) were established without the use of enzymatic techniques. Six experiments were carried out on cells from six different rabbits. When these cells were exposed to methotrexate (MTX) for 24 h at 1 micrograms/ml, proliferation, as measured by cells entering mitosis, was significantly inhibited (P less than 0.05) in only one experiment. When the dose of MTX was elevated to 100 micrograms/ml, only two experiments showed significant inhibition of mitosis. This minimal inhibition of mitosis by MTX was contrasted by the dramatic inhibitory effect of this antimetabolite on DNA synthesis. At 1 micrograms/ml MTX for 24 h, DNA synthesis, as measured by [3H]deoxyuridine uptake, was inhibited greater than 95%. We can conclude that under certain conditions, the rabbit keratinocyte may represent a normal cell type that is inherently resistant to the anti-proliferative effects of methotrexate.