DECREASED EXPRESSION OF PROTEIN PHOSPHATASE TYPE 2A IN HL-60 VARIANT (HL-6ORA) CELLS RESISTANT TO INDUCTION OF CELL-DIFFERENTIATION BY ALL-TRANS-RETINOIC ACID

To evaluate the molecular basis for susceptibility of the cell differe ntiation induced by all-trans retinoic acid (ATRA), we examined bioche mical activities and expression of protein phosphatases type 1 (PP1) a nd type 2A (PP2A) from HL-60 cells that are susceptible to differentia tion induced by ATRA and HL-60RA(r) cells, HL-60 variant cells that ar e resistant to such induction. One nM of calyculin-A (CAL-A) achieved the enhancement of granulocytic differentiation in ATRA-treated HL-60 (1 mu M) cells. ATRA exerted no differential action in HL-60RA(r) cell s, but when used in combination with CAL-A, the differential activity was partly resumed at functional and phenotypic levels without change in morphology. The phosphatase activity in the cytosol from HL-60RA(r) cells was 50% of that from parental HL-60 cells, but the enzyme activ ities in either membrane or nuclear fractions showed similar values. T he decreased phosphatase activity in the cytosol of HL-60RA(r) cells w as mainly due to the decreased expression of the PP2A catalytic subuni t. This low level of PP2A protein was reflected at a relative deficien cy in expression of the PP2A beta gene in HL-60RA(r) cells. The exposu re to 1 mu M ATRA resulted in downregulation of PP2A catalytic subunit protein in HL-60 cells, but ATRA did not affect PP2A expression in HL 60RA(r) cells. Both cell Lines expressed the proteins of each PP1 cata lytic subunit isozyme (i.e., PP1 alpha, PP1 gamma, and PP1 delta) at c omparable levels. ATRA treatment had no effect on the levels of PP1 is ozymes. Our results show a correlation between the extent of PP2A expr ession and the response of HL-60 and HL-60RA(r) cells to the different iative effects of ATRA.