Abstract
Chronic inflammation leads to the formation of a pro-tumorigenic microenvironment that can promote tumor development, growth and differentiation through augmentation of tumor angiogenesis. Prostate cancer (CaP) risk and prognosis are adversely correlated with a number of inflammatory and angiogenic mediators, including Toll-like receptors (TLRs), NF-κB and vascular endothelial growth factor (VEGF). Peroxiredoxin 1 (Prx1) was recently identified as an endogenous ligand for TLR4 that is secreted from CaP cells and promotes inflammation. Inhibition of Prx1 by CaP cells resulted in reduced expression of VEGF, diminished tumor vasculature and retarded tumor growth. The mechanism by which Prx1 regulates VEGF expression in normoxic conditions was investigated in the current study. Our results show that incubation of mouse vascular endothelial cells with recombinant Prx1 caused increases in VEGF expression that was dependent upon TLR4 and required hypoxia inducible factor-1 (HIF-1) interaction with the VEGF promoter. The induction of VEGF was also dependent upon NF-κB; however, NF-κB interaction with the VEGF promoter was not required for Prx1 induction of VEGF suggesting that NF-κB was acting indirectly to induce VEGF expression. The results presented here show that Prx1 stimulation increased NF-κB interaction with the HIF-1α promoter, leading to enhanced promoter activity and increases in HIF-1α mRNA levels, as well as augmented HIF-1 activity that resulted in VEGF expression. Prx1 induced HIF-1 also promoted NF-κB activity, suggesting the presence of a positive feedback loop that has the potential to perpetuate Prx1 induction of angiogenesis. Strikingly, inhibition of Prx1 expression in CaP was accompanied with reduced expression of HIF-1α. The combined findings of the current study and our previous study suggest that Prx1 interaction with TLR4 promotes CaP growth potentially through chronic activation of tumor angiogenesis.
Highlights
Numerous studies have shown that inflammatory conditions increase the risk of cancer [1,2,3]
To determine whether the induction of vascular endothelial growth factor (VEGF) in endothelial cells by rPrx1 was dependent upon Toll-like receptor 4 (TLR4) expression or signaling endothelial cells were transfected with plasmids encoding either a shRNA specific for TLR4 or a dominant negative mutant of MyD88 (MyD88DN), that abrogates TLR4 signaling [29]
The results described above show that peroxiredoxin 1 (Prx1) enhancement of endothelial cell expression of VEGF in normoxic conditions occurs via TLR4 dependent increases in HIF-1a transcription
Summary
Numerous studies have shown that inflammatory conditions increase the risk of cancer [1,2,3]. In established tumors inflammation is a key component of the tumor microenvironment that contributes to tumor survival, proliferation and invasion [4]. Hallmarks of inflammation in cancer include persistent NF-kB activation and increased angiogenesis as a result of elevated levels of vascular endothelial growth factor (VEGF). Prostate tumors express constitutively activated NF-kB [5,6] and elevated VEGF levels [7] that are associated with disease progression [8,9,10,11,12]. The factors that promote chronic inflammation in established prostate cancer (CaP) are largely unknown. We have recently identified peroxiredoxin 1 (Prx1) as a mediator of inflammation in CaP [13]
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