The Role of Tristetraprolin in Head and Neck Squamous Cell Carcinoma.

Abstract

Head and neck cancer (HNSCC) is the sixth most common cancer in the world but its treatment has not significantly improved in several decades. Pro-inflammatory mediators such as interleukin-6 (IL-6), vascular endothelial growth factor (VEGF), prostaglandin 2 (PGE2) and matrix metalloproteinases (MMPs) promote tumor invasion and angiogenesis. However, targeted inhibition of individual pro-inflammatory mediators or their receptors had limited success in the treatment of HNSCC. This is likely due to redundancy in function between pro-inflammatory mediators. Therefore, an attractive treatment target would be a common mechanism that regulates multiple pro-inflammatory mediators in HNSCC. The objective of this dissertation is to identify molecular mechanisms such as Tristetraprolin (TTP), an RNA-binding protein that mediates the decay of multiple pro-inflammatory mediators in macrophages. We determined that TTP expression was reduced in HNSCC and inversely correlated with the secretion of IL-6, VEGF, and PGE2. Immunohistochemical staining of tissue microarrays for IL-6 demonstrated that staining intensity is prognostic for poor disease-specific survival, tumor recurrence and development of second primary tumors, and poor overall survival. Moreover, low TTP and high IL-6 are prognostic for poor disease specific survival in HNSCC. Upstream regulators of TTP were identified using complementary approaches. Rap1B, a small GTPase with a critical role in HNSCC, induces p38 MAPK-mediated phosphorylation (i.e. suppression) of TTP. Loss of TTP promotes mRNA stability and secretion of IL-6, MMP-9, and MMP-2. Using in vitro and in vivo strategies, we demonstrate that suppression of TTP increases invasion of HNSCC via IL-6, MMP-9 or MMP-2. Moreover, high IL-6 and high MMP-9 are prognostic for poor clinical outcomes in HNSCC patients. In exploring the role of tumor derived ligands and angiogenesis, we determined that galanin activates p38 MAPK. siRNA-mediated knockdown of p38 MAPK decreases VEGF and PGE2 secretion in HNSCC. p38 MAPK knockdown also decreases the length and number of endothelial cell sprouts, whereas TTP knockdown has the reverse effect. Knockdown of IL-6 in HNSCC cells transduced with shTTP reduces angiogenesis. In conclusion, targeting upstream regulators of pro-inflammaotry mediators, such as p38 and TTP, may improve treatment response of HNSCC by inhibiting multiple pro-inflammatory mediators that promote invasion and angiogenesis.