| Abstrakt: |
Cell cultures derived from human neonatal foreskins (HF cells) are susceptible to phorbol-12,13-didecanoate- (PDD) induced inhibition of queuine uptake, but this inhibition is pronounced only in early passage HF cells. The present analysis of five different primary cultures demonstrated that, between 10 and 30 population doublings beyond the primary cultures, HF cells gradually became refractile to PDD-induced inhibition of queuine uptake, after which PDD begins to stimulate queuine uptake. Treating late passage HF cells with conditioned medium from early passage HF cells partially restored the PDD-induced inhibition of queuine uptake. This indicates the existence of a factor produced by early passage HF cells that permits PDD to inhibit queuine uptake. The tumor promoter, teleocidin, mimics the effects of PDD on queuine uptake. Both PDD and teleocidin are known to activate protein kinase C; therefore, this kinase may be an intermediary in tumor promoter-induced effects on queuine uptake. Epidermal growth factor, platelet-derived growth factor, and transforming growth factor beta stimulated queuine uptake in both early and late passage HF cells. Growth factor stimulation of uptake was enhanced by PDD in late passage cells but inhibited by PDD in early passage cells. Polyinosinic polycytidylic acid treatment of late passage HF cells partially restored PDD-induced inhibition of queuine uptake. Human recombinant beta-interferon, plus or minus PDD, had no effect on queuine uptake. PDD did not inhibit queuine uptake in the immortal human and non-human cell lines examined. |
