Role for PI-3-Kinase in C-MYC expression by P210 BCR-ABL

Transcription of c-myc is a requirement for hematopoietic transformation by p210 BCR-ABL, product of the Philadelphia chromosome. Additionally, phosphatidyIinositol-3-kinase (PI-3-kinase), has been closely linked to cellular proliferation in hematopoietic systems. Whether the mechanism by which p210 BCR-ABL affects c-myc expression may involve PI-3-kinasc was investigated. By immunoprecipitation, the PI-3-kinase regulatory subunit, p85, was found in a complex with adaptor protein She, previously linked to p2lrus activation in BCRABL transformed cells. Use of specific PI-3-kinase inhibitors, wortmannin (WM) and LY294002, revealed dose-dependent, selective inhibition of c-myc mRNA expression at concentrations that did not affect expression of c-jun mRNA, an endpoint of p21ras activity. A transcriptional basis for downmodulation of c-myc mRNA expression by WM was proved by actinomycin D decay. Neither the cjun regulatory enzyme JNK, nor the ras/raf-dependcnt enzyme, ERK2/MAP kinase, were inhibited by WM, supporting a distinct enzymatic effector pathway for expression of c-myc mRNA compared to c-jun. On the other hand, the dosedependent inhibition of c-myc mRNA expression conformed to the dosedependence of PI-3-kinase inhibition in Bcr-Abl.A54 cells at 100-300 nM. The identity of downstream effector(s) responsible for c-myc transcription, influenced by PI-3-kinase, was investigated. Cyclin-dependent kinase 4 (cdk4) activity for phosphorylating retinoblastoma protein, pRb (ultimately responsible for E2F1dependent c-myc transcription) was highly sensitive to inhibition of PI-3-kinase. The cytosolic enzyme, akt or protein kinase B, was also enzymatically inactivated downstream of PI-3-kinase by WM treatment, suggesting it may represent an intermediate effector of cdk4 regulation sensitive to the intracellular concentration of PIP3, the PI-3-kinase product. Thus, PI-3-kinase regulation of substrate enzyme, akt, and of cdk4, defines a unique pathway for c-myc transcription from p210 BCR-ABL, which diverges from the pathway downstream of p21ras activation by BCR-ABL.