MCF-7 mammary tumour cells express the myeloid cell differentiation controlling factor, serine protease 3/myeloblastin

Our previous data indicated that HSP27 plays a role in MCF-7 cell different iation similar to that it has in HL-60 cells. In the latter case, this invo lves a control of its levels by proteinase 3/myeloblastin (PR3/Mbn), a seri ne proteinase hitherto considered specific of the myeloid lineage. Having o bserved that the treatment of MCF-7 cells with the serine protease inhibito r N-tosyl-1-phenylalaninechloromethyl ketone (TPCK) increased their content in HSP27 and induced them to acquire a secretory phenotype, we undertook t his work to test the assumption that an enzyme similar or identical to PR3/ Mbn might be expressed in this cell line. The data show that MCF-7 cells ex hibited specific immunopositivity for a monoclonal antibody against PR3/Mbn , Western blot analysis of immunoprecipitates from MCF-7 cell extracts, obt ained and checked with PR3/Mbn monoclonal antibodies, confirmed the presenc e of the 35 kDa glycosylated and 29 kDa mature forms of the protein. Finall y, Northern blot analysis confirmed the expression of the corresponding mRN A. Together with our data with TPCK, this substantiates our hypothesis that , as in HL-60 cells, regulation of MCF-7 cells differentiation might involv e a postranslation control on HSP27 levels by a serine protease.