Theophylline incorporation into the nucleic acids of theophylline-stimulated melanoma cells.

Theophyllin, an inhibitor of cAMP-degrading phosphodiesterase, stimulates melanin biosynthesis in cultures of RPMI 3460 hamster melanoma cells. Although theophylline does produce an initial transient elevation of intracellular cAMP levels, long-term treatment with theophylline produces a significant decrease in cAMP content. There is an inhibition of the theophylline stimulation by dibutyryl-cAMP; this is apparently caused by interference of dibutyryl-cAMP with the uptake and incorporation of theophylline, as shown by experiments with 3H-theophylline. An alternative theory is that theophylline, being a methylxanthine compound, is metabolized by the cell and possibly causes melanotic stimulation by becoming incorporated into cellular nucleic acids or by altering the normal nucleic acid metabolism. The following observations are consistent with this theory: (u) 3H-theophylline was incorporated into both trichloroacetic acid (TCA)-soluble and TCA-insoluble cell fractions; most of the insoluble label became soluble after digestion with ribonuclease and deoxyribonuclease. (2) These nuclease digests of the 3H-theophylline-labeled TCA-insoluble cell fractions contained 3H-labeled material that chromatographed differently from normal nucleotides on ion exchange thin layer sheets. (3) The acid-soluble pool of 3H label disappeared rapidly while both the insoluble label and the induction of melanogenesis remained stable for 50 hr after the removal of exogenous 3H-theophylline.