The effect of OA on proliferation and polyamine metabolism of K 562 leukemic cells and their responsiveness to natural killer cell activity.

Orotic acid (OA), a known promoter of carcinogenesis, significantly stimulated proliferation of K 562 leukemic cells even at as high a concentration as 0.1 mM. This effect was accompanied by a significant increase of the activity of two key enzymes of the polyamine pathway, i.e. ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC). The induction of ODC activity was associated with increased expression of the ODC gene. The participation of ODC in early events evoked by OA in leukemic cells was confirmed by the decrease of the stimulatory effect of OA on cell proliferation in the presence of alpha-difluoromethylornithine (DFMO)--an irreversible inhibitor of ODC. The involvement of protein kinase C (PK-C) and cyclic nucleotide-dependent kinases in OA action on K 562 leukemic cells was demonstrated by a significant reduction of cell proliferation by addition of H-7 (1-(5-isoquinolinesulphonyl)-2-methylpiperazine). Since PK-C is involved both in induction of ODC activity and in membrane transport of polyamines, H-7 significantly inhibited the proliferation of K 562 leukemic cells even in the presence of OA and exogenous putrescine. The importance of extracellular sources of polyamines for leukemic cell growth was shown by supplementation of the incubation medium with putrescine. Exogenous putrescine significantly enhanced the concentration of spermidine and spermine within the cell and increased the number of cells. The effect of OA on natural killer (NK) cell cytotoxicity was also examined. Rat peripheral blood mononuclear cells were used as effector cells and K 562 cells as targets. OA, progressively with dose, significantly decreased specific lysis when targets were preincubated with it. On the other hand, pretreatment of PBMC effector cells with OA, regardless of the applied concentration, did not affect the amount of 51Cr released from lysed cells. OA as a promoter of carcinogenesis stimulates proliferation of leukemic cells and impairs their responsiveness to NK activity. ODC/polyamine system and PK-C appear to be involved in OA action on K 562 cells. The presented observations are important from a practical point of view, since an elevated blood concentration of OA resulting from the impaired kidney function in hematological proliferative diseases may accelerate their progression.