Aim. To illuminate the reason of inactivity of the TGFB-SMAD2/3 pathways in CLL cells. Methods. CLL cells were isolated from peripheral blood of CLL patients, using gradient centrifugation at the ficoll. Expression and cellular localization of SMAD2, 3 and 4 proteins were analyzed by fluorescence microscopy, using specific antibodies. Results. The SMAD2 protein was basically not expressed in CLL cells, in contrast to B cells, isolated from the peripheral blood of a healthy donor. Moreover, the SMAD3 and SMAD4 proteins were localized exclusively in the cytoplasm (a proportion of SMAD3 was detected in the membrane) of CLL cells. Conclusions. The TGFB-SMAD2/3 signaling pathway is not active in CLL cells. We have found that SMAD2 is not expressed. Also, the nuclear heterodimers that consisted of SMAD3 and SMAD4 proteins, were not detected. Keywords: chronic lymphocytic leukemia (CLA), acute myeloid leukemia (AML), peripheral blood B-cells, SMAD, the TGFB-SMAD2/3 signaling pathway.

To find out the cause of inhibition of the IL2-STAT5 signaling pathway in chronic lymphocytic leukemia (CLL) cells. Methods. CLL cells were isolated from peripheral blood, using gradient centrifugation on a ficoll-verografin mixture. Expression of the STAT1-6 genes at the mRNA level was analyzed, using the Oncomine database. Expression, phosphorylation status and cellular localization of the STAT5 protein were studied by fluorescence microscopy, using specific antibodies. Results. Unlike in B-cells of healthy donors, expression of the STAT5A protein was low in the patient CLL cells. As we have previously shown, the IL-2-STAT5 (JAK-STAT5) signaling pathway is inhibited in CLL cells. Now we demonstrated a low level of phosphorylation of the STAT5 protein, or a complete lack of phosphorylation in CLL cells. The STAT5A protein shows cytoplasmic localization, indicating the absence of complexes in the nucleus that activate/repress transcription of the STAT5-dependent genes. Conclusions. Inhibition of the IL-2-STAT5 pathway in CLL cells is caused by a lack of the STAT5 proteins phosphorylation and/or the absence of the active STAT5A transcription complexes in the nucleus of CLL cells.