GM-CSF expression by tumor cells correlates with aggressivity and with stroma reaction formation

Granulation tissue involved in tissue repair and in the stroma reaction to epithelial tumors is characterized by the presence of myofibroblastic cells. It has been previously reported that granulocyte macrophage-colony stimulating factor (GM-CSF) induces a fibrotic reaction containing numerous myofibroblasts. This reaction results from a cascade of events, including stimulation of transforming growth factor-beta1 (TGF-beta1) production by macrophages which, in turn, promotes alpha-smooth muscle actin and collagen synthesis by fibroblasts. Moreover, GM-CSF is known to be expressed by many tumor cell types. In this study we have analyzed, by means of reverse transcription-polymerase chain reaction, GM-CSF mRNA expression in a progressive and a regressive rat colon carcinomas and in the corresponding cell lines, eliciting different degrees of desmoplastic reaction. We have also evaluated the expression of GM-CSF protein in selected cases. The expression of GM-CSF mRNA and, when tested, protein were higher in progressive compared to regressive cancer cells both in vivo and in vitro. We then investigated GM-CSF mRNA and protein expression in different human colon cancer cell lines known to exhibit different degrees of aggressivity in vivo. We found high levels of GM-CSF mRNA and protein in the most aggressive cell lines. Similar results were also obtained on human breast and cervical cancer cell lines. Our results are in agreement with the assumption that GM-CSF expression is correlated to tumor aggressivity. Conceivably, one of the GM-CSF actions affecting tumor progression is exerted through its influence on stroma reaction development.