Cyclic AMP‐dependent and ‐independent protein kinases and protein phosphorylation in human promyelocytic leukemia (HL60) cells induced to differentiate by retinoic acid

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Abstract
The human leukemia cell line HL60 which resembles promyelocytes can be induced to differentiate to cells displaying features of the mature myeloid phenotype by a variety of agents including retinoic acid (RA) and agents that elevate intracellular adenosine 3:5 cyclic monophosphate (cyclic AMP) levels, e.g., 8‐bromo‐cyclic adenosine 3:5 monophosphate (8‐Br‐cyclic AMP), cholera toxin. Since most, if not all the effects of cyclic AMP, are mediated by adenosine 3:5 cyclic monophosphate‐dependent protein kinase (cyclic AMP‐dPK), we investigated the role of cyclic AMP‐dPK and adenosine 3:5 cyclic monophosphate‐independent protein kinase (cyclic AMP‐iPK) in the induced differentiation of HL60 cells. Marked stimulation of cyclic AMP‐dPK and cyclic AMP‐iPK appears to be intimately involved with and specific for HL60 myeloid differentiation as evidenced by: (1) Stimulation of cyclic AMP‐dPK and cyclic AMP‐iPK early during HL60 myeloid differentiation and prior to phenotypic changes. (2) RA and dimethylformamide (DMF), agents that induce differentiation along the myeloid pathway, cause a marked increase in the type I cytosolic cyclic AMP‐dPK and cyclic AMP‐iPK (protamine kinase) while no such increases are noted in cells treated with 12‐0‐tetradecanoyl‐phorbol‐13‐acetate (TPA) which induces differentiation along the monocyte/macrophage pathway. (3) Both native polyacrylamide gel electrophoresis as well as photoaffinity labeling with 8‐azido‐cyclic AMP demonstrate marked increases in type I cyclic AMP‐dPK in the cytosols of cells exposed to agents that induce myeloid differentiation but no increase in TPA‐differentiated cells. (4) The appearance and disappearance of specific cyclic AMP‐dependent and ‐independent protein phosphorylations are associated with the induced myeloid differentiated state.

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