Bryostatin 1 potentiates 1-[β-D-arabinofuranosyl] cytosine-mediated antiproliferative effects in c-jun dominant-negative human myeloid leukemia cells (U937/TAM67) through a nonapoptotic mechanism

Academic Article

Abstract

  • Recent studies suggest that exposure of leukemic cells to a differentiating stimulus following a DNA-damaging agent leads to potentiation of apoptosis or programmed cell death. The present studies were undertaken to evaluate the contribution of the transcription factor c-Jun to apoptosis and growth inhibition induced by the sequential administration of 1-β-D- arabinofuranosylcytosine (ara-C) and the protein kinase C activator bryostatin 1 in human monocytic leukemia cells (U937). To address this issue, a U937 cell line stably transfected with a dominant-negative, c-Jun transactivation domain-deficient mutant (TAM67), was employed. The mutant TAM67 protein interferes with normal c-Jun function and AP-1 activation through a 'quenching' mechanism. TAM67 expressing cells and cells containing empty vector (pMM) were equally susceptible to apoptosis induced by exposure to ara-C (1 μM; 6 h); moreover, this effect was not altered by subsequent exposure of cells to bryostatin 1 (10 nM; 24 h). However, clonogenic TAM67- expressing cells were less susceptible to the antiproliferative effects of ara-C and more susceptible to growth inhibition by bryostatin 1 than their empty vector counterparts. In addition, subsequent exposure to bryostatin 1 substantially increased growth inhibition by ara-C in TAM67-expressing cells despite failing to potentiate apoptosis. Whereas 10 nM bryostatin 1 was ineffective in triggering maturation of pMM cells, it partially induced differentiation in their TAM67-expressing counterparts, manifested by increased expression of the maturation marker CD11b, modest up-regulation of native c-Jun, and limited dephosphorylation of the retinoblastoma protein pRb. Sequential administration of ara-C followed by bryostatin 1 led to further up-regulation of native c-Jun, particularly in TAM67-expressing cells, but failed to induce pRb hypophosphorylation in either cell line. Collectively, these findings indicate that bryostatin 1 reverses, at least in part, the reduced susceptibility of clonogenic U937 cells to ara-C conferred by c-Jun dysregulation, and further suggest that this phenomenon proceeds via nonapoptotic mechanisms.
  • Author List

  • Freemerman AJ; Maloney NJ; Birrer MJ; Szabo E; Grant S
  • Start Page

  • 247
  • End Page

  • 262
  • Volume

  • 4
  • Issue

  • 3