Regulation of hairy-cell survival through constitutive activation of mitogen-activated protein kinase pathways.

The hairy cells (HCs) of hairy-cell leukemia are intrinsically activated mature clonal B cells. The aims of this study were to gain further insights into the nature of this activation and to assess its importance for the prolonged HC survival in this chronic disease. We show that HCs contain phosphorylated/activated p38 MAPK, JNK and ERK1/ERK2 (ERK1/2). PKC inhibitors increased the activation of p38 and JNK, but reduced the phosphorylation of ERK1/2. Moreover, PKC inhibition resulted in cell death; cell death was also observed when the activation of ERK1/2 in HCs was abrogated with an inhibitor of MEK1/2 activation. In addition to PKC, active Src kinase was also shown to be involved in the maintenance of Raf-independent ERK activation in HCs. During cell culture on a nonadherent surface, ERK phosphorylation was sustained, while phosphorylation of p38 and JNK decreased. This decrease was not observed in HCs cultured on vitronectin (VN), indicating that p38/JNK activation is probably a consequence of in vivo HC interaction with VN present in abundance in the red pulp of the spleen. Taken together, these results suggest that active p38/JNK make HCs susceptible to apoptosis, but the cells are effectively rescued by ERK activation involving constitutively active PKC and Src. These findings are relevant for the understanding of the prolonged cell survival of HCs and their selective sensitivity to some chemotherapeutic agents.

Response of hairy cells to IFN-alpha involves induction of apoptosis through autocrine TNF-alpha and protection by adhesion.

Although hairy cell leukemia is uniquely sensitive to interferon-alpha (IFN-alpha), the biologic basis for this phenomenon remains unclear. Here we examine the effects of IFN-alpha on cultured hairy cells (HCs), taking into account the possible modifying influence of cell adhesion. We make the novel observation that therapeutic concentrations of IFN-alpha kill nonadherent HCs by inducing apoptosis. In keeping with the persistence of HCs in tissues during therapy, such killing was inhibited by integrin-mediated adhesion to vitronectin or fibronectin. Exposure of HCs to IFN-alpha resulted in a marked increase in tumor necrosis factor-alpha (TNF-alpha) secretion. Furthermore, blocking antibodies to TNF-RI or TNF-RII protected HCs from IFN-alpha-induced apoptosis, demonstrating that such killing was mediated by TNF-alpha. In the absence of IFN-alpha, exogenous TNF-alpha did not induce HC apoptosis, showing that IFN-alpha sensitized HCs to the proapoptotic effect of autocrine TNF-alpha. This sensitization to TNF-alpha-induced killing was attributable to suppression of IAP (inhibitors of apoptosis) production known to be regulated by the cytoprotective nuclear factor-kappaB-dependent arm of TNF-alpha signaling. Moreover, engagement of the receptors for fibronectin or vitronectin prevented this IFN-alpha-induced down-regulation of IAPs. Understanding of the signals involved in the combined effects of IFN-alpha and TNF-alpha and abrogation of those induced by integrin engagement offers the possibility of sensitizing other malignant cells to IFN-alpha-induced killing and thereby extending the therapeutic use of this cytokine.