Action of exogenous differentiating agents on gene expression in cancer cells

Identification of a molecular signature predictive of sensitivity to differentiation induction in acute myeloid leukemia

Acute myeloid leukemia (AML) blasts are immature committed myeloid cells unable to spontaneously undergo terminal maturation, and characterized by heterogeneous sensitivity to natural differentiation inducers. Here, we show a molecular signature predicting the resistance or sensitivity of six myeloid cell lines to differentiation induced in vitro with retinoic acid or vitamin D. The identified signature was further validated by TaqMan assay for the prediction of response to an in vitro differentiation assay performed on 28 freshly isolated AML blast populations. The TaqMan assay successfully predicts the in vitro resistance or responsiveness of AML blasts to differentiation inducers. Furthermore, performing a meta-analysis of publicly available microarray data sets, we also show the accuracy of our prediction on known phenotypes and suggest that our signature could become useful for the identification of patients eligible for new therapeutic strategies.

A functionally active RARalpha nuclear receptor is expressed in retinoic acid non responsive early myeloblastic cell lines

Although all-trans retinoic acid (ATRA) can restore the differentiation capacity of leukemic promyelocytes, early leukemic myeloblasts are conversely not responsive to ATRA induced granulocytic differentiation. To assess whether this resistance to ATRA is related to an impaired function of the Retinoic Acid Receptor alpha (RARalpha), we performed an analysis of RARalpha expression and transactivation activity, in several myeloid leukemic cell lines, representative of different types of spontaneous acute myeloid leukemias. Our results indicate that a functionally active RARalpha nuclear receptor is expressed in all the analyzed cell lines, regardless of their differentiation capacity following exposure to ATRA. The observation that ATRA treatment is able to induce the expression of retinoic acid target genes, in late- but not in early-myeloblastic leukemic cells, raises the possibility that the differentiation block of these cells is achieved through a chromatin mediated mechanism. Acetylation is apparently not involved in this process, since the histone deacetylase inhibitor trichostatin A, is not able to restore the differentiation capacity of early leukemic myeloblasts. Further investigation is needed to clarify whether myeloid transcription factors, distinct to RARalpha, play a role in the resistance of these cells to ATRA treatment.

Drug-induced reversion of progression phenotype is accompanied by reversion of AP-1 phenotype in JB6 cells

Transformed JB6 cells can be stably reverted to nontransformed phenotype by AP-1 inhibiting gluccorticoid fluocinolone (FA) and cAMP elevator forskolin (FN), yielding stable revertants of promotion resistant (P-) and promotion sensitive (P+) phenotypes. AP-1 activity of nontransformed P- and P+ revertant clones was decreased under a variety of experimental conditions compared with their transformed counterparts. Moreover, AP-1 activity in P+ cells under anchorage-independent conditions was induced by 12-0-tetradecanoyl-phorbol-13-acetate (TPA) while AP-1 activity in the reverted P- cells was not induced, just as observed for the original P+ and P- variants. Taken together these data suggest that changes in AP-1 activity may be one key mediator not only of forward progression but also of reversion of tumor cells to nontransformed phenotype. In addition, the higher transfection efficiency of the new reverted P- and P+ cells renders them useful for studying the role of transcription factors in tumor promotion.

Modulation of growth supportive and oncogenic properties of murine leukemia cells

A leukemoid reaction occurs after inoculation of L1210 leukemic cells into recipient mice and the degree of granulocytosis is correlated with tumor progression. It was found that the sera of leukemic mice contained elevated levels of colony stimulating activity (CSA) when compared with normal mouse sera. Media conditioned by L1210 cells in vitro (L1210-CM) contained CSA which stimulated normal bone marrow myeloid colony growth and an auto-stimulatory activity (ASA) which stimulated L1210 cell proliferation. We studied the effects of trans-retinoic acid (RA) and 1,25 dihydroxyvitamin D3 (VD3) on the production of growth substances by L1210 cells. When L1210-CM was prepared in the presence of RA and VD3, the CSA and ASA were markedly inhibited. A combination of the two agents was more effective than either agent. Mice inoculated with 1 x 10(5) L1210 suspension culture cells treated with either agent or both combined survived significantly longer than controls. Mice inoculated with L1210 cells treated with the two agents combined survived longest. By using antibodies, preliminary analysis of growth substances generated in L1210-CM showed that it contains primarily GM-CSF and M-CSF-like activities which were distinct from ASA. Combination antibody titer assays revealed that ASA was not significantly inhibited with anti-GM-CSF and anti-M-CSF antibodies, while CSA was inhibited by between 61 and 84%. We conclude that RA and VD3 synergistically inhibit the release of growth-enhancing substances by L1210 cells which may reduce the growth advantage of leukemic cells and the resulting leukocytosis in lymphocytic leukemia.