Expression of smooth-muscle actin in cultured cells from human plasmacytoid myoepithelioma

Objective. The definition of plasmacytoid myoepithelioma, a neoplasm exhibiting myoepithelial differentiation, has been recently questioned. To better understand the histogenesis of this neoplasm, we searched for myoepithelial markers in histologic sections of plasmacytoid myoepithelioma and in a cell line (M1) derived from this same neoplasm. Study design. Expression of vimentin, pan-keratin (AE-3) and smooth-muscle actin was studied by immunohistochemistry in paraffin-embedded tissue and by immunofluorescence in M1 cells. Results. Plasmacytoid myoepithelioma tumor sections showed vimentin and AE-3 reactivity, but evidence of smooth-muscle actin was not seen. The cell line derived from this tumor also produced vimentin and cytokeratin. In addition, all cultured cells expressed smooth-muscle actin. Conclusion. We demonstrated that cells derived from a case of plasmacytoid myoepithelioma appear to show full myoepithelial differentiation in vitro. Thus, they are myoepithelial-like cells in nature. The lack of myogenous differentiation in vivo could be due to an inhibitory process mediated by the extracellular matrix.