Abstract
We previously showed that inducible nitric oxide synthase (iNOS) protein expression in melanoma tumor cells is associated with poor patient prognosis. Here, we analyzed the association between iNOS and the oncogenic PI3K-AKT pathway. TCGA data show that iNOS and phospho-Akt Ser473 expression were associated significantly only in the subset of tumors with genetically intact PTEN. Employing a stage III melanoma TMA, we showed that iNOS protein presence is significantly associated with shorter survival only in tumors with PTEN protein expression. These findings led to our hypothesis that the iNOS product, nitric oxide (NO), suppresses the function of PTEN and stimulates PI3K-Akt activation. Melanoma cells in response to NO exposure in vitro exhibited enhanced AKT kinase activity and substrate phosphorylation, as well as attenuated PTEN phosphatase activity. Biochemical analysis showed that NO exposure resulted in a post-translationally modified S-Nitrosylation (SNO) PTEN, which was also found in cells expressing iNOS. Our findings provide evidence that NO-rich cancers may exhibit AKT activation due to post-translational inactivation of PTEN. This unique activation of oncogenic pathway under nitrosative stress may contribute to the pathogenesis of iNOS in melanoma. Significance: Our study shows that iNOS expression is associated with increased PI3K-AKT signaling and worse clinical outcomes in melanoma patients with wt (intact) PTEN. Mutated PTEN is already inactivated. We also demonstrate that NO activates the PI3K-AKT pathway by suppressing PTEN suppressor function concurrent with the formation of PTEN-SNO. This discovery provides insight into the consequences of inflammatory NO produced in human melanoma and microenvironmental cells. It suggests that NO–driven modification provides a marker of PTEN inactivation, and represents a plausible mechanism of tumor suppressor inactivation in iNOS expressing subset of cancers.
Highlights
Melanoma is the deadliest form of skin cancer and its incidence has been increasing in the United States for the last 30 years
While previous studies have implicated loss of PTEN expression as a significant event in melanoma, our data supports that post-translational modification of the intact PTEN protein by nitric oxide (NO) is another likely mechanism of tumor suppressor inactivation, which may play an important role in this disease
As activation of the phosphatidylinositol 3-kinase (PI3K)-AKT pathway has been implicated in resistance to both targeted and immune therapies, these results provide further evidence for a inducible nitric oxide synthase (iNOS) Associates With AKT Activation in Melanoma significant role for inflammation-driven nitric oxide, and indicate that such inflammation could be considered as a potential target in this disease [9]
Summary
Melanoma is the deadliest form of skin cancer and its incidence has been increasing in the United States for the last 30 years. The molecular pathogenesis of melanoma is strongly associated with activation of the phosphatidylinositol 3-kinase (PI3K) –AKT pathway, promoting cellular growth, proliferation, and survival. Activation of the PI3K-AKT pathway generally occurs in the setting of concurrent oncogenic RAS-RAF-MEKERK signaling [1]. Previous studies showed that the constitutive activation of the PI3K-AKT pathway occurred in the setting of PTEN loss of expression and function, which was detected in up to 30% of cutaneous melanomas [2]. The analyses of human melanoma samples and cell lines showed that low PTEN levels were associated with elevated phospho(active)-AKT, which remained significant even in brain metastasis [3]. Supporting the clinical significance of activation of the PI3K-AKT pathway, PTEN loss was shown to promote resistance to both targeted and immune therapies for melanoma [4]
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