The proteolytic capacity of cells is determined by their expression profile of proteases and their inhibitors. There is little information on the proteolytic potential of tumorigenic (TG) cells.
Primary TG cells were isolated from the MMTV-PyMT mouse model of invasive breast cancer using surface markers CD24+CD90+CD45-. The high tumorigenicity of this cell population was confirmed by stem cell assays in vitro and in vivo. An RNA-Seq analysis of primary TG cells vs. non-tumorigenic (NTG) cancer cells revealed differential gene regulation of several proteases, their inhibitors, and ECM proteins. Moreover, proteolytic activity, as well as enhanced sphere formation of these TG cells could be shown. Both effects were reduced upon treatment with broad range protease inhibitors.
To further characterize proteolysis in TG cells, three stable TG cell lines were established from the MMTV-PyMT and MMTV-Wnt1 breast cancer mouse models. These lines differ among each other morphologically, in their gene expression profile, and can be maintained in a stem cell state under hypoxic conditions. Transfer of these cells into normoxic conditions induced changes in morphology, gene expression profiles, and major cellular functions including the induction of different degrees of epithelial-to-mesenchymal transition, as well as differential gene regulation of proteases like matrix metalloproteinases, cathepsins, and a-disintegrin-and-metalloproteinases (ADAMs). This was further addressed by testing the effects of various protease inhibitors on growth and invasion of TG cell lines.
In summary, murine TG cells possess a high proteolytic capacity that can change in response to changed environmental conditions. Furthermore, the differential gene expression and differing cellular behaviors among the three BCSC lines suggest heterogeneity within the TG cells populations of different tumors.