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 st imulated proliferation of K 562 leukemic cells even at as high a conce ntration as 0.1 mM. This effect was accompanied by a significant incre ase 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 ex pression of the ODC gene. The participation of ODC in early events evo ked by OA in leukemic cells was confirmed by the decrease of the stimu latory effect of OA on cell proliferation in the presence of alpha-dif luoromethylornithine (DFMO) - an irreversible inhibitor of ODC. The in volvement of protein kinase C (PK-C) and cyclic nucleotide-dependent k inases in OA action on K 562 leukemic cells was demonstrated by a sign ificant reduction of cell proliferation by addition of H-7 (1-(5-isoqu inolinesulphonyl)-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 eve n in the presence of OA and exogenous putrescine. The importance of ex tracellular sources of polyamines for leukemic cell growth was shown b y 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 decre ased specific lysis when targets were preincubated with it. On the oth er hand, pretreatment of PBMC effector cells with OA, regardless of th e applied concentration, did not affect the amount of Cr-51 released f rom lysed cells. OA as a promoter of carcinogenesis stimulates prolife ration of leukemic cells and impairs their responsiveness to NK activi ty. ODC/polyamine system and PK-C appear to be involved in OA action o n K 562 cells. The presented observations are important from a practic al point of view, since an elevated blood concentration of OA resultin g from the impaired kidney function in hematological proliferative dis eases may accelerate their progression.