INHIBITION OF T-CELL PROLIFERATION BY A MYB ANTISENSE OLIGOMER IS ACCOMPANIED BY SELECTIVE DOWN-REGULATION OF DNA-POLYMERASE ALPHA-EXPRESSION

We recently found that inhibition of MYB protein synthesis in human peripheral blood mononuclear cells (PBMC) exposed to human c-myb (designated MYB) antisense oligodeoxynucleotides prevents entry into S phase and cell proliferation. To determine the mechanism(s) by which down-regulation of human c-myb protein (MYB) synthesis interferes with DNA synthesis, we analyzed mRNA levels of DNA polymerase α and proliferating cell nuclear antigen (PCNA), transcripts of two genes required for DNA synthesis, in normal and leukemic T lymphocytes exposed to MYB antisense oligodeoxynucleotides. Expression of DNA polymerase α was inhibited both in normal T lymphocytes progressing from G0 to S phase and in exponentially growing CCRF-CEM leukemic cells, whereas expression of PCNA was inhibited only in mitogen-stimulated PBMC and remained essentially unaffected in the leukemia T-cell line. The functional link between expression of MYB and DNA polymerase α mRNAs was further demonstrated by analyzing DNA polymerase α mRNA levels in a temperature-sensitive (ts) fibroblast cell line (TK-ts13; TK is thymidine kinase) constitutively expressing human MYB mRNA driven by the simian virus 40 (SV40) promoter. In the MYB-expressing TK-ts13 cells, DNA polymerase α mRNA levels were unaffected following shift to the nonpermissive temperature of 39.6°C, whereas in the parental line, DNA polymerase α mRNA levels were readily down-regulated. These findings indicate that the expression of MYB is related to that of DNA polymerase α in cells expressing MYB at high levels and suggest that there is a functional link between c-myb and DNA polymerase α mRNA expression during cell cycle progression of normal T lymphocytes.