RB PHOSPHORYLATION IN SODIUM BUTYRATE-RESISTANT HL-60 CELLS - CROSS-RESISTANCE TO RETINOIC ACID BUT NOT VITAMIN-D3

To examine the potential coupling between inducible cellular changes i n RE (retinoblastoma) tumor suppressor protein phosphorylation and abi lity to CO growth arrest and differentiate, HL-60 promyelocytic leukem ia cells were cultured in incremental sodium butyrate (NaB) concentrat ions and thereby made resistant to the growth inhibitory effects of so dium butyrate, which normally induces GO arrest and monocytic differen tiation in wild type HL-6O cells. The resistant cells were also unable to differentiate in response to NaB, indicating that a regulatory fun ction controlling both GO growth arrest and differentiation had been a ffected. The induced resistance was not genetic in origin since the ce lls regained the ability to GO arrest and differentiate after being re cultured in medium free of sodium butyrate for only three days. The re sistant cells had similar cell cycle phase durations as the original w ild type cells. The resistant cells retained the ability to both GO ar rest and differentiate in response to 1,25-dihydroxy vitamin D3 (VD3), normally an inducer of GO arrest and monocytic differentiation in wil d type cells. However, they were cross-resistant to retinoic acid (RA) , another ligand for the same steroid thyroid hormone receptor family, which induces GO arrest and myeloid differentiation in wild type cell s. The ability to CO arrest and phenotypically differentiate in respon se to RA were both grossly impaired. Unlike wild type cells which unde rgo early down-regulation and then hypophosphorylation of the RE prote in when induced to differentiate, in resistant cells, hypophosphorylat ion of RE in response to NaB was grossly retarded. These changes in RE protein occurred faster when the cells were treated with VD3. In cont rast, the changes in RB phosphorylation occurred significantly slower when the cells were treated with RA. The results suggest a coupling be tween the ability to GO growth arrest and phenotypically convert and t he ability to hypophosphorylate RE. (C) 1995 Wiley-Liss, Inc.