Abstract
Nitric oxide (NO) plays a critical role in acute and chronic inflammation. NO's contributions to cancer are of particular interest due to its context-dependent bioactivities. For example, immune cells initially produce cytotoxic quantities of NO in response to the nascent tumor. However, it is believed that this fades over time and reaches a concentration that supports the tumor microenvironment (TME). These complex dynamics are further complicated by other factors, such as diet and oxygenation, making it challenging to establish a complete picture of NO's impact on tumor progression. Although many activity-based sensing (ABS) probes for NO have been developed, only a small fraction have been employed in vivo, and fewer yet are practical in cancer models where the NO concentration is <200 nM. To overcome this outstanding challenge, we have developed BL660-NO, the first ABS probe for NIR bioluminescence imaging of NO in cancer. Owing to the low intrinsic background, high sensitivity, and deep tissue imaging capabilities of our design, BL660-NO was successfully employed to visualize endogenous NO in cellular systems, a human liver metastasis model, and a murine breast cancer model. Importantly, its exceptional performance facilitated two dietary studies which examine the impact of fat intake on NO and the TME. BL660-NO provides the first direct molecular evidence that intratumoral NO becomes elevated in mice fed a high-fat diet, which became obese with larger tumors, compared to control animals on a low-fat diet. These results indicate that an inflammatory diet can increase NO production via recruitment of macrophages and overexpression of inducible nitric oxide synthase which in turn can drive tumor progression.