Autocrine production of prostaglandin F_2α enhances phenotypic transformation of normal rat kidney fibroblasts

We have used normal rat kidney (NRK) fibroblasts as an in vitro model system to study cell transformation. These cells obtain a transformed phenotype upon stimulation with growth-modulating factors such as retinoic acid (RA) or transforming growth factor-β (TGF-β). Patch-clamp experiments showed that transformation is paralleled by a profound membrane depolarization from around -70 to -20 mV. This depolarization is caused by a compound in the medium conditioned by transformed NRK cells, which enhances intracellular Ca ²⁺ levels and thereby activates Ca²⁺-dependent Cl ^- channels. This compound was identified as prostaglandin F _2α (PGF_2α) using electrospray ionization mass spectrometry. The active concentration in the medium conditioned by transformed NRK cells as determined using an enzyme immunoassay was 19.7 ± 2.5 nM (n = 6), compared with 1.5 ± 0.1 nM (n = 3) conditioned by nontransformed NRK cells. Externally added PGF_2α was able to trigger NRK cells that had grown to density arrest to restart their proliferation. This proliferation was inhibited when the FP receptor (i.e., natural receptor for PGF₂α) was blocked by AL-8810. RA-induced phenotypic transformation of NRK cells was partially (∼25%) suppressed by AL-8810. Our results demonstrate that PGF_2α acts as an autocrine enhancer and paracrine inducer of cell transformation and suggest that it may play a crucial role in carcinogenesis in general.