Evidence supporting a role for dihydroorotate dehydrogenase, bioenergetics, and p53 in selective teriflunomide-induced apoptosis in transformed versus normal human keratinocytes

We have demonstrated previously that the dihydroorotate dehydrogenase (DHODH) inhibitor teriflunomide (TFN) encourages apoptosis in transformed human keratinocytes. Here we sought to determine if this cytotoxic effect could be restricted to transformed keratinocytes relative to their normal human epidermal keratinocyte (NHEK) counterparts, and ascertain a potential mechanistic basis for the selectivity. The NHEK cells proliferated much slower than the premalignant HaCaT and malignant COLO 16 keratinocytes, and exogenous uridine added to the culture medium did not affect this growth. Similarly, DHODH expression and the bioenergetic characteristics of the normal cells were markedly dissimilar from those observed in the transformed cells indicating that de novo pyrimidine synthesis was involved with keratinocyte proliferation. Moreover, a short-term exposure to TFN caused a wild-type p53 response in the NHEK cells illustrating that pyrimidine metabolic stress could regulate this tumor suppressor protein in the normal cells. TFN-induced apoptosis occurred primarily in S phase HaCaT cells. This cell death was sensitive to uridine, an antioxidant, and a caspase inhibitor, and the suppression of Bcl-XL and the induction of Mn superoxide dismutase preceded it. These events suggested that mitochondrial/redox stress was involved with the cytotoxic effect of TFN. Conversely, a long-term exposure to TFN caused G0/G1 arrest in the NHEK cells, which supported a cytoprotective role for p53 against TFN-induced apoptosis. Together, these results propose that TFN could be useful in the prevention or therapy of non-melanoma skin cancers and possibly other hyperproliferative keratinocytic diseases.