RAPID DECLINE IN FOLYLPOLYGLUTAMATE SYNTHETASE-ACTIVITY AND GENE-EXPRESSION DURING MATURATION OF HL-60 CELLS - NATURE OF THE EFFECT, IMPACT ON FOLATE COMPOUND POLYGLUTAMATE POOLS, AND EVIDENCE FOR PROGRAMMED DOWN-REGULATION DURING MATURATION

These studies in HL-60 cells examined the regulation of folylpolyglutamate synthetase (FPGS) activity at the level of gene expression during terminal maturation. Following addition of 210 mM Me(2)SO to cultures of HL-60 cells at a concentration that induces maturation of 85-90% of the cells, FPGS activity, but not folylpolyglutamate hydrolase (FPGH) activity, was reduced 2-7-fold within 1-5 days. The initial decline in FPGS activity preceded any effect of Me(2)SO on rate of growth and the increase in appearance of nitro blue tetrazolium-positive cells, a marker of cellular maturation, and the decrease after 5 days of exposure to Me(2)SO was solely accounted for by a 7-fold decrease in value for V-max. The same time and concentration dependence for Me(2)SO was shown for the decline in FPGS activity, increase in nitro blue tetrazolium-positive cells, and decline in the level of a 2.1-kilobase FPGS mRNA during exposure to this inducer. This decline in FPGS mRNA was reversible when Me(2)SO was removed from the culture medium but only until that time when an appreciable number of cells were committed to terminal maturation. Following growth of HL-60 cells with [H-3]MTX, used as a model folate compound, a large reduction in its intracellular polyglutamate pools was shown during maturation which quantitatively reflected the decline in FPGS activity as well as folate transport inward (Sirotnak, F. M., Jacobson, D. M., and Yang, C-H. (1986) J. Biol. Chem. 261, 11150-11156). Other data showed that folate status or obviation of the folate requirement during growth of these cells strongly influenced the rapidity of the onset of maturation following exposure to inducer. Overall, these results show that FPGS activity in HL 60 cells is a marker for proliferative capacity and that the underlying basis for the decline in FPGS activity during maturation is altered cognate gene expression which is manifested as early reversible and late irreversible phases. They also suggest that the coordinate reduction observed in folate transport, FPGS activity, dihydrofolate reductase, and probably other folate related enzymes by limiting macromolecular biosynthesis may be early programmed events in the maturation process that influence the switch from proliferation to senescence in these cells.