INDUCTION AND TURNOVER OF FRUCTOSE 1,6-BISPHOSPHATASE IN HL-60 LEUKEMIA-CELLS BY CALCITRIOL

Fructose 1,6-bisphosphatase mRNA and enzyme activity in HL-60 cells we re rapidly and markedly induced by calcitriol (formerly known as 1 alp ha,25-dihydroxyvitamin D-3). The activity reached 70-80 times the basa l level after 96 h. The enzyme activity in the cells incubated for 96 h with calcitriol decreased immediately after its withdrawal but after a 24-h incubation the activity in the cells continued to increase sli ghtly and then decreased slowly. Calcitriol increased the enzyme activ ity dose-dependently with maximal stimulation at 10 nM and half-maxima l at 2.1 nM. The rate of synthesis of fructose 1,6-bisphosphatase almo st paralleled the increase in mRNA level during treatment with calcitr iol. When calcitriol was removed from media after incubation for eithe r 24 h or 96 h, fructose-1;6-bisphosphatase mRNA and fructose-1,6-bisp hosphatase synthesis decreased rapidly to the basal level. The enzyme was only slightly degraded in the cells incubated with calcitriol for 24 h followed by the subsequent culture without calcitriol but it was degraded with a half-life estimated to be approximately 64 h in the sa me cells followed by culturing with calcitriol. In the cells incubated for 96 h, the same degradation rate (i.e. half-life approximate to 64 h) was observed irrespective of the following culture with or without calcitriol. Calcitriol did not affect the degradation rate of total s oluble proteins.