Cancer is an interactive multi-tissue lesion dynamically linked to the stroma by signaling networks that regulate gene and protein expression, and post-translational modifications within the tumor and its microenvironment. Using computational biology analyses of human breast cancer patients, MMP12 was demonstrated to be up regulated in ERBB2 and triple negative tumors, yet unchanged in luminal A and luminal B tumors.
4T1 shMMP12 and 4T1 shControl cell lines were implanted in BALB/c and B10 wild type mice. No differences in tumor growth were observed, but the 4T1 shMMP12 tumors resulted in a lower incidence of lung metastasis and an increased survival post-primary tumor resection at 21 days. MMP12 expressed by the tumor enhances carcinogenesis and metastasis despite MMP12 expression by tumor associated macrophages in the host stroma. In contrast, in Mmp12-/- B10 mice, differential roles for MMP12 were observed depending on cellular origin, with MMP12 expression by macrophages showing host protection regardless of the MMP12 status of the tumor cells.
Using TAILS, we discovered novel in vivo substrates of MMP12 including the intracellular tyrosine-protein kinase Syk and PLCG2, both tumor suppressors involved in dampening the migration and the metastatic potential of breast cancer cells. 4T1 shMMP12 cells had higher levels of Syk protein than 4T1 shControl cells that was associated with lower migratory potential and metastasis. Treatment of the MMP12 knockdown cells with a Syk inhibitor (BAY 61-3606) or rescue experiments by addition of recombinant murine MMP12, both enhanced cell migration to the same extent.
Thus, tumor cell derived MMP12 enhances the cell migration and metastatic potential of breast cancer cells in vivo through the intracellular processing and inactivation of two tumor suppressors, SYK and PLCG2, thus decreasing the survival of mice post-tumor resection. In contrast, tumor associated macrophage MMP12 has host protective effects, potentially through enhanced macrophage anti-tumor functions.