The Role of Transforming Growth Factor β1 in the Fibroblastic Reaction Associated with Rat Colorectal Tumor Development

Many tumors are surrounded by a highly fibrous stroma composed of fibroblasts and extracellular matrix. This desmoplastic response has been suggested to both inhibit and favor tumor progression. The present study deals with the effects of tumor cells on the fibroblastic reactions they cause and relates this to progression or regression of tumors. Two rat colon carcinoma cell lines, one which develops progressive tumors when injected s.c. in syngeneic animals (PROb cell line) and the other which develops regressive tumors in similar conditions (REGb cell line), were compared by the fibroblastic reaction which they cause. Comparative histological analysis of progressive and regressive tumors developed by the two cell lines showed a significant but opposite response of fibroblastic compartment. The progressive tumor nodules were observed to grow within a loose tissue, whereas the regressive tumor cells were surrounded by a fibrous capsule. Immunohistological labelings revealed the presence of α-smooth muscle actin-positive myofibroblats during the tumor expansion, while these specific cells disappeared during the tumor regression. Immunostainings of transforming growth factor β1 showed an increasing staining of the progressive tumor cells during tumor development but a slight expression by tumor cells and stroma during the tumor regression. This growth factor was demonstrated to facilitate initial steps of the tumor progression by addition of active transforming growth factor β1 at the time of s.c. injection of PROb cells in syngeneic rat models. In vitro experimental analysis with the use of neutralizing antibody showed that active transforming growth factor β produced by the progressive cells inhibited fibroblast proliferation and facilitated their differentiation into myofibroblasts. Since the number of myofibroblasts increased with time in progressive tumors, their presence may constitute a potential growth advantage for tumor growth. In contrast, our results indicated involvement of platelet-derived growth factor-like protein(s) in fibroblast proliferation under the control of regressive cells and the presence of an important sheath of α-smooth muscle actin-negative fibroblasts in regressive tumors may support a role for this growth factor in vivo . Thus, the ability of tumor cells to produce or induce the production of transforming growth factor β or platelet-derived growth factor may give rise to a specific fibroblast reaction, which in turn may determine consequent tumor evolution.