Once the cancer has begun, NO seems to play a protumoral role rather than antitumoral one as the concentration required to cause tumor cell cytotoxicity cannot be achieved by cancer cells.
The mechanistic roles of nitric oxide (NO) during cancer progression have been important considerations since its discovery as an endogenously generated free radical. Nonetheless, the impacts of this signaling molecule can be seemingly contradictory, being both pro-and antitumorigenic, which complicates the development of cancer treatments based on the modulation of NO fluxes in tumors. At a fundamental level, low levels of NO drive oncogenic pathways, immunosuppression, metastasis, and angiogenesis, while higher levels lead to apoptosis and reduced hypoxia and also sensitize tumors to conventional therapies. However, clinical outcome depends on the type and stage of the tumor as well as the tumor microenvironment.
Nitric oxide is generated by three main nitric oxide synthase isoforms: neuronal (nNOS), endothelial (eNOS), and inducible (iNOS).
– In many cancers, especially under inflammatory conditions, iNOS expression is upregulated. In contrast, eNOS levels may also be altered in cancers such as breast or prostate cancer.
• Expression Patterns in Tumors:
– Elevated iNOS expression is commonly observed in various tumor types (e.g., colon, breast, lung, and melanoma) and is often associated with an inflammatory microenvironment.
– Changes in eNOS and nNOS expression have also been reported and may contribute to angiogenesis and tumor blood flow regulation.
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