CHANGES IN THE PHOSPHORYLATION STATUS OF THE 27 KDA HEAT-SHOCK-PROTEIN (HSP27) ASSOCIATED WITH THE MODULATION OF GROWTH AND OR DIFFERENTIATION IN MCF-7 CELLS/

We have used human mammary cells of the MCF-7 strain, which constituti vely express high levels of the small heat shock protein HSP27 and we have compared the changes in the phosphorylation status of this protei n together with changes in cell growth and/or morphology induced by th e action of one of the following agents: (1) TPA (12-O-tetradecanoylph orbol-13-acetate), known as a differentiation inducer in MCF-7 cells; (2) OH-TAM (hydroxytamoxifen), which exerts a cytostatic and cytotoxic action; or (3) TNF chi (tumour necrosis factor), which induces apopto tic cell death in this cell line. Our data show that TPA and TNF stimu late an immediate and massive phosphorylation of HSP27, whereas OH-TAM affect the phosphorylation status of the protein only after a 3 day d elay. In the case of TPA, high levels of HSP27 phosphorylation were ma intained for at least 4 days, along with growth inhibition and acquisi tion by the cells of a secretory phenotype. TPA and OH-TAM exerted sim ilar immediated effects on cell growth, despite the different time cou rse of their action on HSP27 phosphorylation. This excludes the possib ility that the latter is a necessary consequence of, or an absolute re quisite to, growth inhibition. With OH-TAM and TNF the increase in HSP 27 phosphorylation was concomitant with the appearance of apoptosis, n ot observed with TPA. This indicates that increased phosphorylation of HSP27 is not specifically associated with the triggering or the execu tion of apoptosis in these cells. Altogether, our data support the con cept that phosphorylated HSP27 is involved (and might then be rate lim iting in some instances) in the execution of vital cell programmes (in cluding resistance to stress, proliferation and differentiation), as w ell as in that of cell death. This is consistent with its role in acti n polymerization and its position downstream of the p38/RK-type MAPkin ase, itself a point of convergence for diverse signal transduction pat hways.