Anti-sense inhibition of small-heat-shock-protein (HSP27) expression in MCF-7 mammary-carcinoma cells induces their spontaneous acquisition of a secretory phenotype.

This work was aimed at testing the hypothesis (hitherto supported only by indirect evidence) that, besides contributing to resistance to stress, the small heat-shock-protein HSP27 might be involved in the control of growth and differentiation in mammary-tumour cells, where it is known to be oestrogen-regulated. Therefore, MCF-7 cells were transfected with a modulatable human hsp27 anti-sense cDNA. Clones of transfectants (designated alphahsp27) were selected which, upon expression of the anti-sense, exhibited a decline in HSP27 accumulation, associated with a decrease in resistance to heat shock and in proliferation rate, the degree of the latter reflecting their respective reduction in HSP27 content. The effects of anti-sense inhibition of HSP27 production were similar to those exerted on parental cells by phorbol myristate (TPA). Both resulted in growth inhibition, accumulation of lipid droplets in the cytoplasm, formation of secretory microvesicles with internal microvilli and increased release of several proteins, including the isoforms of a 52-kDa protein, which we identified as the oestrogen-regulated protein cathepsin D, all this without noticeable change in actin organization. These data constitute the first direct support for the hypothesis that, at least in some cell types, HSP27 might play a modulatory role in cell differentiation and (perhaps by this) in proliferation. While allowing dissociation of this role from the known action of HSP27 on actin polymerization, they suggest similar modulation of the function of some protein(s) implicated in the acquisition of the secretory phenotype by MCF-7 cells, with HSP27 also exerting an inhibitory action that can be alleviated either by its phosphorylation (as occurs with TPA) or by inhibition of its production.