Osteopontin inhibits macrophage nitric oxide synthesis to enhance tumor proliferation

Interactions between tumor cells and their host environment can play a major role in regulating survival programs required for tumor progression. Osteopontin (OPN) is a glycophosphoprotein overexpressed by tumors, and is a key molecule for tumor progression and metastasis. OPN also inhibits expression of autocrine and paracrine inducible nitric oxide synthase (iNOS). Given the cytotoxic effects of macrophage NO expression, we hypothesized that tumor-derived OPN inhibits expression of local macrophage iNOS to potentiate tumor survival.

We used a coculture system of murine CT26 colorectal cancer cells with RAW264.7 murine macrophage cells. CT26 expresses OPN at high levels. RNA interference was utilized to produce long-term specific silencing of OPN in CT26.

Inhibition of constitutive OPN synthesis in CT26 upregulates local NO production with inhibition of CT26 proliferation and promotion of CT26 apoptosis. Macrophage iNOS expression is accompanied by increased binding activity of nuclear factor-kappaB DNA. When the CT26 culture media were examined for a panel of proinflammatory cytokines, elevated concentrations of granulocyte colony-stimulating factor (G-CSF) were found. Subsequently, in CT26 cells treated with antisense-G-CSF, NO levels in CT26-RAW cocultures were significantly decreased.

In our system of CT26-RAW264.7 coculture, we conclude that inhibition of OPN synthesis in CT26 results in G-CSF-mediated induction of macrophage iNOS expression with resultant inhibition of CT26 proliferation via increased apoptosis. Our results suggest that tumor-derived OPN may enhance tumor survival by down regulating expression of NO in the local microenvironment. This is one mechanism by which OPN may potentiate cancer survival and progression.