Tyrosine-phosphorylated STAT5 accumulates on podosomes in Hck-transformed fibroblasts and chronic myeloid leukemia cells.

In chronic myeloid leukemia (CML), the transforming activity of Bcr/Abl involves constitutive activation of the phagocyte specific Src-family tyrosine kinase Hck, which in turn directly activates the signal transducer and activator of transcription 5 (STAT5). The effect of Hck on STAT5 was first explored independently of Bcr/Abl by expressing the constitutively active Hck mutant (Hck(ca)) in MEF3T3-TetOff fibroblasts. As previously reported, Hck(ca)-expressing cells form podosomes which are actin-rich structures involved in trans-tissular cell migration and found in the few cell types able to cross anatomic boundaries. We demonstrated that in these cells, the tyrosine-phosphorylated form of STAT5 (PY-STAT5) increased and preferentially localized on podosomes together with Hck, instead of translocating to the nucleus as observed with conventional stimuli such as IFNgamma. To examine whether similar results were obtained in the presence of Bcr/Abl, the CML cell line K562 was used. We observed that (i) podosomal structures are present in these cells in contrast to Bcr/Abl-negative leukemic cells, (ii) podosome formation was inhibited by Bcr/Abl- and Src-kinase inhibitors, and (iii) PY-STAT5 mainly colocalized with Hck on these structures. The presence of podosomes was not sufficient to trap STAT5 since in normal macrophages which spontaneously form podosomes and express regulated Hck, PY-STAT5 is in the nucleus. In conclusion, this is the first report showing that PY-STAT5 associates to podosomes in a process dependent on constitutive activation of Hck. We propose that STAT5, previously classified as a transcription factor, could play another role outside the nucleus, elicited by the Bcr/Abl-Hck transforming pathway.