Abstract #3279: Omacetaxine induces the rapid loss of anti-apoptotic short-lived proteins in BCR-ABL positive cells

Omacetaxine mepesuccinate (formerly homoharringtonine), a natural product alkaloid, is currently undergoing evaluation in clinical trials as a therapy for patients with tyrosine kinase inhibitor resistant chronic myeloid leukemia (CML). Omacetaxine has been shown to inhibit protein synthesis (via inhibition the elongation step of polypeptide synthesis) and induce apoptosis in leukemic cells. Long term exposure (>16hr) of leukemic cells to omacetaxine has been found to alter the expression of bcl-2 family proteins and activate proapoptotic caspases. However, the key initial proapoptotic events induced by omacetaxine are not completely characterized. As omacetaxine inhibits protein synthesis, anti-apoptosis proteins with a short half life may be preferentially lost and result in apoptosis induction. Therefore, the expression of a number of short-lived proteins including c-Myc, Mcl-1 and NOXA was investigated in K562 cells during an 8 hour treatment with omacetaxine. For comparison, the expression of these proteins was also assessed in cells treated with imatinib mesylate (imatinib). Western blot analysis of K562 cells following exposure to 30nM omacetaxine or 10uM imatinib for 2, 4 and 6 hr showed that omacetaxine rapidly induced the loss of c-myc and Mcl-1 protein expression after 2 hr. The decrease in these proteins was most pronounced 4 and 6hr after the addition of omacetaxine. In contrast, treatment with imatinib did not markedly alter c-myc or Mcl-1 expression in the first 6 hr after treatment. Notably, protein expression of the c-Myc responsive gene, NOXA, was also decreased in omacetaxine but not imatinib treated cells. Expression of proteins with long half lives such as the anti-apoptotic protein Bcl-XL or the pro-apoptotic proteins PUMA, Bid, Bik, Bmf or Bok were not affected by omacetaxine or imatinib treatment. These observations indicate that omacetaxine but not imatinib induces a rapid loss of specific short lived anti-apoptotic proteins such as c-Myc and Mcl-1 in CML cells. These findings raise the possibility that these early events are critical to omacetaxine induced apoptosis in BCR-ABL positive leukemic cells.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3279.