Fibroblasts mediate induction of high mobility group box protein 1 in lung epithelial cancer cells by diffusible factors.

Cancer development is associated with the high mobility group box protein 1 (HMGB1), which modulates the transcriptional activity in the nucleus, but it is also present in the cytoplasm and outside the cell in certain conditions. As the progression of lung cancer is supported by mitogenic stimuli of stromal fibroblasts, we studied the impact of lung fibroblasts (WI-38) on the expression and localization of HMGB1 in lung epithelial cancer cells (H358). HMGB1 was mainly localized in the nucleus of non-mitotic H358 cells but highly distributed in the cytoplasm of mitotic cells. Conditioned medium (CM) from WI-38 fibroblasts enhanced the expression of HMGB1 at the mRNA and protein level compared to the control CM from H358 cells. In particular, the amount of cytoplasmic HMGB1 was elevated in response to fibroblast CM, which was reduced by inhibiting the basic fibroblast growth factor with blocking antibodies. Although cytoplasmic HMGB1 can be released from the cell, we did not determine a significant amount of extracellular HMGB1 in these conditions. This might partially be based on the sensitivity of HMGB1 to extracellular proteases as CM caused fast proteolysis of the cytoplasmic HMGB1 preparations. Our data suggest that diffusible factors of fibroblasts support the biological function of cancer cells via HMGB1 activation.