Inhibition of transforming growth factor-β signaling potentiates tumor cell invasion into collagen matrix induced by fibroblast-derived hepatocyte growth factor

• Tumor cells actively invade into collagen gel in three-dimensional co-culture with fibroblasts.
• HGF was identified to be a major pro-invasion factor secreted from fibroblasts.
• TGF-β directly stimulated tumor cell invasion but down-regulated HGF expression in fibroblasts.
• Most notably, TGF-β signaling inhibitors enhanced tumor cell invasion by increasing HGF expression in fibroblasts.
• These results suggest the possibility that TGF-β inhibitors may enhance tumor progression in their clinical application.

Interaction between tumor cells and stromal fibroblasts plays essential roles in tumor progression. However, its detailed molecular mechanism remains unclear. To understand the mechanism, we investigated molecules mediating this interaction using the three-dimensional (3D) co-culture system of Panc-1 pancreatic carcinoma cells with normal fibroblasts. When the two kinds of cells were placed on the top of collagen gel, the tumor cells scattered into the fibroblast layer, apparently undergoing epithelial‐mesenchymal transition. When fibroblasts were placed within collagen gel, Panc-1 cells actively invaded into the collagen gel, extending a microtubule-based long protrusion. Although transforming growth factor-β (TGF-β) and hepatocyte growth factor (HGF) individually stimulated the tumor cell invasion into collagen gel without fibroblasts, TGF-β signaling inhibitors (SB431542 and LY2157299) significantly enhanced the Panc-1 cell invasion in the 3D co-culture with fibroblasts. Experiments with HGF/Met signaling inhibitors or with the fibroblast conditioned medium revealed that HGF was a major invasion-promoting factor secreted from fibroblasts and SB431542 increased the HGF secretion by blocking the HGF-suppressing activity of cancer cell-derived TGF-β. These results indicate that HGF and TGF-β are critical regulators for both tumor–stroma interaction and tumor invasion. The results also suggest that TGF-β signaling inhibitors may promote tumor progression under some pathological conditions.

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