The Role of Nitric Oxide Dysregulation in Tumor Maintenance

The inflammatory nature of the tumor microenvironment provides a cytokine and chemokine rich proliferative environment. Much of the responsibility of this environment is due to the production of Reactive Oxygen Species (ROS). These studies examined the proliferative rich tumor environment from a new perspective of Nitric Oxide Synthase (NOS) dysregulation. NOS’s have the ability to become uncoupled and generate superoxide in lieu of nitric oxide (NO). A requirement of NOS for the production of NO is the cofactor tetrahydrobiopterin (BH4) and when it is missing NOS becomes uncoupled and turns into a peroxynitrite synthase. Here I demonstrate that NOS is uncoupled in tumor cells due to depleted BH4 levels. This uncoupling leads to decreased NO signaling and increased pro-inflammatory, pro-survival, signaling as a result of the increased generation of ROS/RNS from uncoupled NOS activity. I was able to recouple NOS through exogenous BH4 both in vitro and in vivo, reducing ROS/RNS and reestablishing NO signaling through cGMP protein associated kinase. Reduction of ROS/RNS resulted in the reduced activity of two major constitutively active transcription factors in breast cancer cells, NFκB and STAT3. In MCF-7 and MDA231 cells I found that increased NO-dependent PKG signaling led to tumor cell toxicity mediated by downregulation of β-catenin. Downregulation of β-catenin led to increased protein levels of p21 in MCF-7 and p27 in MDA 231cells, ultimately resulting in cell death. These results suggest that there is potential for BH4 as a therapeutic agent since exogenous dietary BH4 ameliorates chemically induced colitis, and reduced azoxymethane (AOM) induced colon and spontaneously developing mammary carcinogenesis.