Lack of TNFα expression protects anaplastic lymphoma kinase-positive T-cell lymphoma (ALK plus TCL) cells from apoptosis

Here we report that T-cell lymphomas characterized by the expression of anaplastic lymphoma kinase (ALK+ TCL) fail to express the TNF alpha and frequently display DNA methylation of the TNF alpha gene promoter. While only a subset of the ALK+ TCL-derived cell lines showed a high degree of the promoter methylation, all 6 showed low to nondetectable expression of the TNF alpha mRNA, and none expressed the TNF alpha protein. All 14 ALK+ TCL tissue samples examined displayed some degree of the TNF alpha promoter methylation, which was the most prominent in the distal portion of the the promoter. Treatment with a DNA methyltransferase inhibitor, 5'-aza-2'-deoxy-cytidine (5-ADC), reversed the promoter methylation and led to the expression of TNF alpha mRNA and protein. Furthermore, in vitro DNA methylation of the promoter impaired its transcriptional activity in the luciferase reporter assay. This impairment was seen even if only either distal or proximal portion were methylated, with methylation of the former exerting a more profound inhibitory effect. Notably, the ALK+ TCL cell lines uniformly expressed the type 1 TNF alpha receptor (TNF-R1) protein known to transduce the TNF alpha-induced pro-apoptotic signals. Moreover, exogeneous TNF alpha inhibited growth of the ALK+ TCL cell lines in a dose-dependent manner and induced activation of the members of the cell apoptotic pathway: Caspase 8 and caspase 3. These findings provide additional rationale for the therapeutic inhibition of DNA methyltransferases in ALK+ TCL. They also suggest that treatment with TNF alpha may be highly effective in this type of lymphoma.