Rapid phosphorylation of cellular proteins during differentiation of neuroblastoma cells induced by recombinant human interferon-gamma

Interferon-gamma and retinoic acid down-regulate N-myc in neuroblastoma through complementary mechanisms of action

The N-myc oncogene plays a key role in the biology of neuroblastoma and the differentiation process. N-myc expression is associated with metastatic disease, as well as the undifferentiated state of normal neuroblasts migrating from the neural crest during embryogenesis. Its down-regulation is a pivotal event in the differentiation of neuroblastoma cells by retinoic acid (RA). Our previous work has shown that RA works synergistically with other agents, such as interferon-gamma (IFN-gamma), to down-regulate N-myc expression and induce differentiation. The present study demonstrates that IFN-gamma, like RA, decreases N-myc transcription. However, functional analysis of N-myc upstream regulatory sequences using 5' deletion mutants of a promoter-CAT construct containing germ line sequences from nucleotide position -887 to +151 showed that IFN-gamma and RA act through different sites on the N-myc promoter. In addition to its transcriptional effect, IFN-gamma was also found to shorten the half-life of N-myc mRNA. Taken together, these findings provide a mechanistic basis for the synergistic action of IFN-gamma and RA in inducing neuroblastoma differentiation and a rationale for the possible development of combination differentiation therapy for clinical use.

Interferon-gamma-stimulated and GTP-binding-proteins-mediated phospholipase A2 activation in human neuroblasts

Interferon-gamma (IFN-gamma) is a potent growth-inhibitory cytokine also endowed with differentiating activity on neural cells. Binding of IFN-gamma to its high-affinity receptor induces a rapid and transient activation of phospholipase A2 (PLA2). The mechanism coupling the IFN-gamma receptor (IFN-gamma-R) to PLA2 activation is not clearly defined, and no information is available on this mechanism in neuroblast cells. We have tested the hypothesis that GTP-binding proteins (G-proteins) may couple the IFN-gamma-R to PLA2 in the human neuroblastoma (NB) cell line LAN-5. Incubation of NB cells with IFN-gamma resulted in a rapid increase in [3H]arachidonic acid (AA) release, and this effect was blocked by pretreatment with anti-IFN-gamma antibodies. IFN-gamma-stimulated AA release was still observed in permeabilized cells that were blocked by pretreatment with anti-IFN-gamma-R antibodies. Exposure of permeabilized LAN-5 cells to guanosine 5'-[gamma-thio]triphosphate (GTP[S]), a non-hydrolysable GTP analogue, induced a dose-dependent release of [3H]AA. A non-specific nucleotide effect was excluded, since similar stimulatory effects on AA mobilization were not observed by GTP, ATP, CTP, ADP and GDP. IFN-gamma-stimulated AA release was completely blocked by the guanine nucleotide analogue that inhibits G-protein function, guanosine 5'-[beta-thio]diphosphate (GDP[S]). A role for G-proteins in IFN-gamma-R coupling to PLA2 was further supported by the inhibition of IFN-gamma-induced [3H]AA release by treatment of permeabilized cells with pertussis toxin and with the antiserum against the common alpha-subunits of G-proteins. To determine a possible contribution to AA mobilization by the phospholipase C and diacyglycerol lipase pathway or by protein kinase C activation, the effects of neomycin, a phospholipase C inhibitor, and PMA (phorbol 12-myristate 13-acetate), a direct activator of protein kinase C, were investigated. Neither neomycin nor PMA affected either basal or IFN-gamma-stimulated AA release. Ca2+ concentration, which has been shown to regulate the activity of some PLA2s, does not appear to play an important role in the regulation of the IFN-gamma-stimulated PLA2 activity, since incubating permeabilized cells in different concentrations of Ca2+ induced AA release without affecting the IFN-gamma response. Altogether, these findings suggest the existence of IFN-gamma-R, which couples a Ca(2+)-independent PLA2 activation via pertussis-toxin-sensitive G-proteins.

Do second messengers play a role in interferon signal transduction?

The signalling pathway by which the binding of interferons (IFNs, alpha and beta) to their receptor elicits its biological activity, the activation of the transcription of a distinct set of genes called the IFN-stimulated genes (ISG), is far from clear. Debate continues about whether interferon-receptor interaction results directly in gene activation or if second messengers are involved. In this article, we will discuss the evidence that rapid and transient changes in lipid metabolism and the activation of specific isoforms of protein kinase C (PKC) are involved in the initial signalling of interferon activation.

Effects of interferon-gamma and its interaction with retinoic acid on human neuroblastoma differentiation

Differentiation-promoting effects of interferon-gamma (IFN-gamma), both alone and in combination with retinoic acid (RA), were studied on the human neuroblastoma cell line, LA-N-5. The results show that IFN-gamma inhibited the growth and induced morphological differentiation in a dose- and time-dependent manner with measurable effects appearing at 20-40 IU/ml after 3 to 4 days of treatment in vitro. Acetylcholinesterase activity, used as a biochemical index of neuroblastoma differentiation, increased up to 2.5-fold in the presence of IFN-gamma with a half maximal concentration of approximately 100 IU/ml. Concomitantly, modest IFN-induced increases (less than or equal to 2-fold) in choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) activities were seen. Combination treatment of cells with IFN-gamma and RA resulted in synergistic effects on morphological differentiation, growth inhibition and induction of ChAT. Reversal of IFN-gamma's ability to influence neuroblastoma cell growth as well as potentiate the anti-tumor effects of RA was obtained in the presence of an antibody against the IFN-gamma receptor, implying receptor-mediated physiological events. Taken together, these data confirm the differentiating effects of IFN-gamma on human neuroblastoma cells and suggest that combination therapy with RA may be beneficial in the treatment of this disease.