The CDK6-c-Jun-Sp1-MMP-2 axis as a biomarker and therapeutic target for triple-negative breast cancer

Triple-negative breast cancer (TNBC) has high metastatic, drug-resistance, and recurrence rates, and is characterized by an angiogenic and fibrotic microenvironment that favors cancer malignancy. However, details of the mechanisms underlying malignancy are still largely unknown. Our mouse model indicated that knockdown of CDK6 inhibited lung metastasis significantly compared to parental cells. Immunohistochemical analyses revealed that the levels of collagen and the angiogenic marker matrix metalloproteinase (MMP)-2 were much lower in CDK6-deficient cells. To examine mechanisms in the CDK6-mediated phenotype of cancer cells, we studied its role in MMP-2 expression. CDK6 mediated the recruitment of transcription factors including c-Jun and Sp1 to the MMP2 promoter. Knockdown of CDK6 significantly suppressed the expression of MMP2 mRNA. Consistent with the in vitro data, the expression of CDK6 was positively correlated with the angiogenic and fibrotic tumor microenvironment in TNBC patient tissues as shown by MMP-2 and fibronectin staining, respectively. More importantly, after screening a small molecule library of 31 protein kinase inhibitors, we found that the Raf inhibitor sorafenib displayed the highest cytotoxicity in CDK6-depleted cells. These data indicate that CDK6 serves as an anti-microenvironment target and affects the drug response in retinoblastoma-proficient TNBC, suggesting that combining a CDK6 inhibitor and sorafenib leads to a synthetic effect that may represent a personalized therapeutic approach for patients with TNBC.