Analysis by Polymerase Chain Reaction of c- myc Expression in Human Leukemia Cells Induced to Differentiate by Heptachlor and 12- O-Tetradecanoylphorbol-13-acetate

Abstract

Heptachlor, a chlorinated hydrocarbon insecticide, was previously demonstrated to be able to induce differentiation of human myeloblastic leukemia ML-1 cells into monocyte- or macrophage-like cells, a phenomenon similar to that caused by the known tumor promoter 12- O-tetradecanoylphorbol-13-acetate (TPA). Studies have also shown that chemically induced myeloid differentiation of human promyclocytic leukemia HL-60 cells is associated with a decline in the steady-state c- myc proto-oncogene expression. In this study, the polymerase chain reaction (PCR) was combined with primer-extension reverse transcription for the detection of subtle changes in the level of myc mRNA transcription in ML-1 cells exposed to either heptachlor or TPA. Three sets of 5′ primers (S1, S2, and S3) within the region bordered by two promoter sequences (P1 and P2) and a fourth 5′ sequence (S4) downstream of P2 were individually paired with a 3′ primer sequence (S5) for the synthesis of portions of the first exon of the c- myc oncogene. The results indicated that when S2, S3, and S4 5′ primers were used, an initial increase of myc gene expression was detected and was followed by a rapid decline of myc transcripts in cells which had been exposed to TPA. However, when the S1 5′ primer was used in the PCR assay, RNA transcription was shown to be less efficient. Heptachlor, under the same assay conditions, decreased the expression of the myc oncogene, but did not cause an initial stimulation. Marked depression in c- myc expression was further detected in ML-1 cells exposed to chlordane, a heptachlor-related, chlorinated hydrocarbon insecticide, but not to toxaphene, another chlorinated hydrocarbon insecticide. In addition, heptachlor was shown to stimulate the in vitro protein kinase C (PKC) activity and the binding of [ 3H]PDBu to ML-1, indicating a dissociation of PKC activation and myc expression in ML-1 induced to differentiate by heptachlor.