NTAL Potentiates Glucocorticosteroid-Induced Apoptosis in T-ALL – in Vivo Observations and Experimental Model

PAG, LAT, NTAL and LIME belong to category of transmembrane adaptor proteins (TRAPs). They do not possess an enzymatic or kinase function, but they are involved in mediation of signal transmission from surface receptors to cell nucleus. We propose that some of them are engaged in development or maintenance of leukaemia.

We have previously demonstrated that expression status of TRAPs at mRNA level is specific in some subgroups of childhood ALL, particularly in B-cell precursor acute lymphoblastic leukaemia (BCP-ALL) with the TEL/AML1 fusion gene. Furthermore, we have described that variable expression of NTAL mRNA is related to the response to initial glucocorticosteroid pre-phase in the treatment of childhood T-ALL; patients with high NTAL levels show better treatment response compared to the low-NTAL cases.

In the current study, we aimed to prove experimentally that different levels of NTAL protein influence response of leukaemic T cells to glucocorticosteroids.

In the wild type Jurkat cells (human T-cell leukaemia cell line) the NTAL protein is undetectable. For the in-vitro experiments we created a derivative Jurkat cell line transfected with the NTAL construct. In the derivative cell line, the NTAL positivity at both mRNA and protein level was verified using RT-PCR and Western blotting. The derivative cell line in a cell culture behaves similar as wild type Jurkat cell line. Transfectants and wild type Jurkat cells were incubated with methylprednisolone, dexamethasone or with solvent alone (H₂O) as a negative control. Using flow cytometry we determined a percentage of surviving cells after 24, 48 and 72 hours of treatment. Cells were stained with Annexin V and DAPI and living cells were defined as Annexin V, DAPI negative. At each time point, the number of living cells in the negative control was set to 100%. After 48 hours of dexamethasone treatment the number of surviving cells in the Jurkat wild-type was higher by 12% compared to the Jurkat cells expressing NTAL. After 72 hours this difference was even more prominent reaching 46%. The same effect of methylprednisolone treatment was less pronounced (non-significant difference at 48 hours and 5% difference at 72 hours). Based on our experimental data we propose that NTAL acts in T-cells as a putative tumour suppressor. A plausible mechanism of its function is that NTAL competitively inhibits another adaptor LAT, which is required for signal propagation to the nucleus in T-cells. NTAL and LAT can compete for its localisation in proper “lipid rafts” or for palmitoylation. Our data can be used for subsequent functional analysis of signaling pathways in leukaemic blasts as well as in physiological lymphoid cells.