Sustained inhibition of PKCα reduces intravasation and lung seeding during mammary tumor metastasis in an in vivo mouse model

Metastasis is the major reason for breast cancer-related deaths. Although there is a host of indirect evidence for a role of protein kinase C (PKC) alpha in primary breast cancer growth, its role in the molecular pathways leading to metastasis has not been studied comprehensively. By treating mice with alpha V5-3, a novel peptide inhibitor selective for PKC alpha, we were able to determine how PKC alpha regulates metastasis of mammary cancer cells using a syngeneic and orthotopic model. The primary tumor growth was not affected by alpha V5-3 treatment. However, the mortality rate was reduced and metastasis in the lung decreased by more than 90% in the alpha V5-3-treated mice relative to the control-treated mice. alpha V5-3 treatment reduced intravasation by reducing matrix metalloproteinase-9 activities. alpha V5-3 treatment also reduced lung seeding of tumor cells and decreased cell migration, effects that were accompanied by a reduction in nuclear factor kappa B activity and cell surface levels of the CXCL12 receptor, CXCR4. alpha V5-3 treatment caused no apparent toxicity in non-tumor-bearing naive mice. Rather, inhibiting PKC alpha protected against liver damage and increased the number of immune cells in tumor-bearing mice. Importantly, alpha V5-3 showed superior efficacy relative to anti-CXCR4 antibody in reducing metastasis in vivo. Together, these data show that pharmacological inhibition of PKC alpha effectively reduces mammary cancer metastasis by targeting intravasation and lung seeding steps in the metastatic process and suggest that PKC alpha-specific inhibitors, such as alpha V5-3, can be used to study the mechanistic roles of PKC alpha specifically and may provide a safe and effective treatment for the prevention of lung metastasis of breast cancer patients. Oncogene (2011) 30, 323-333; doi:10.1038/onc.2010.415; published online 20 September 2010