Chronic hypoxia and inflammatory cytokines are hallmarks of inflammatory joint diseases like rheumatoid arthritis (RA), suggesting a link between this microenvironment and central pathological events. Because TACE/ADAM17 is the predominant protease catalyzing the release of tumor necrosis factor ␣ (TNF␣), a cytokine that triggers a cascade of events leading to RA, we examined the regulation of this metalloprotease in response to hypoxia and TNF␣ itself. We report that low oxygen concentrations and TNF␣ enhance TACE mRNA levels in synovial cells through direct binding of hypoxia-inducible factor-1 (HIF-1) to the 5 promoter region. This is associated with elevated TACE activity as shown by the increase in TNF␣ shedding rate. By the use of HIF-1-deficient cells and by obliterating NF-B activation, it was determined that the hypoxic TACE response is mediated by HIF-1 signaling, whereas the regulation by TNF␣ also requires NF-B activation. As a support for the in vivo relevance of the HIF-1 axis for TACE regulation, immunohistological analysis of TACE and HIF-1 expression in RA synovium indicates that TACE is up-regulated in both fibroblast-and macrophage-like synovial cells where it localizes with elevated expression of both HIF-1 and TNF␣. These findings suggest a mechanism by which TACE is increased in RA-affected joints. They also provide novel mechanistic clues on the influence of the hypoxic and inflammatory microenvironment on joint diseases.Tumor necrosis factor-␣ converting enzyme (TACE) 2 or ADAM17 was initially described as the predominant enzyme responsible for the physiological cleavage of membrane-anchored tumor necrosis factor-␣ (TNF␣), releasing it in soluble form (1, 2). This enzyme belongs to the ADAM (a disintegrin and metalloprotease domain) family of transmembrane, multidomain zinc metalloproteinases (3) and is expressed in a wide variety of cell types, including TNF␣ non-producing cells (1). Beside TNF␣, TACE was also shown to solubilize a wide variety of proteins including the receptors TNFR-I and TNFR-II (4), interleukin-1RII (5), interleukin-6R (6), and macrophage/colony-stimulating factor-R (7), the cytokine transforming growth factor-␣ (4), members of the membrane-bound epidermal growth factor family (4), the Notch receptor (8), the chemokine fractalkine (9), L-selectin (4), and the ␤-amyloid precursor protein (10). The importance of TACE substrates in a variety of physiological functions, including development, is underscored by the fact that in vivo inhibition of TACE or disruption of the TACE gene results in the death of mice between embryonic day 17.5 and the first day after birth, due to a number of developmental defects. In addition, the implication of TACE substrates in immunoregulation has made this enzyme an efficient therapeutic target in the treatment of a number of pathological conditions including airway inflammation, cancer, and arthritis.Because of the pathophysiological importance of TACE-mediated shedding, several studies have addressed the mechanism of TACE regulation. Sur...

The hypoxic and acidic microenvironments in tumors are strongly associated with malignant progression and metastasis, and have thus become a central issue in tumor physiology and cancer treatment. Despite this, the molecular links between acidic pH- and hypoxia-mediated cell invasion/metastasis remain mostly unresolved. One of the mechanisms that tumor cells use for tissue invasion is the generation of invadopodia, which are actin-rich invasive plasma membrane protrusions that degrade the extracellular matrix. Here, we show that hypoxia stimulates the formation of invadopodia as well as the invasive ability of cancer cells. Inhibition or shRNA-based depletion of the Na+/H+ exchanger NHE-1, along with intracellular pH monitoring by live-cell imaging, revealed that invadopodia formation is associated with alterations in cellular pH homeostasis, an event that involves activation of the Na+/H+ exchange rate by NHE-1. Further characterization indicates that hypoxia triggered the activation of the p90 ribosomal S6 kinase (p90 RSK), which resulted in invadopodia formation and site-specific phosphorylation and activation of NHE-1. This study reveals an unsuspected role of p90RSK in tumor cell invasion and establishes p90RS kinase as a link between hypoxia and the acidic microenvironment of tumors.

The ability of cancer cells to invade and metastasize is the major cause of death in cancer patients. Autotaxin (ATX) is a secreted lysophospholipase whose level of expression within tumors correlates strongly with their aggressiveness and invasiveness. ATX is the major enzyme involved in the production of lysophosphatidic acid (LPA), a phospholipid that is known to act mostly through its three first characterized receptors (LPA 1 , LPA 2 , and LPA 3 ). Tumor cell invasion across tissue boundaries and metastasis are dependent on the capacity of invasive cancer cells to breach the basement membrane. This process can be initiated by the formation of the actin-rich cell protrusions, invadopodia. In this study, we show that ATX is implicated in the formation of invadopodia in various cancer cells types and this effect is dependent on the production of LPA. We further provide evidence that LPA 4 signaling in fibrosarcoma cells regulates invadopodia formation downstream of ATX, a process mediated through the activation of EPAC by cyclic AMP and subsequent Rac1 activation. Results using LPA 4 shRNA support the requirement of the LPA 4 receptor for cell invasion and in vivo metastasis formation. This work presents evidence that blocking the LPA receptor, LPA 4 , in fibrosarcoma cells could provide an additional tool to improve the efficacy of treatment of metastasis in patients. Because LPA receptors and ATX are currently being targeted in preclinical trials, the current findings should stimulate future studies to evaluate the expression pattern and clinical outcome of LPA 4 , together with other LPA receptors, in various cancer patients. Cancer Res; 70(11); 4634-43. ©2010 AACR.

The pH-dependent partitioning of chemotherapeutic drugs is a fundamental yet understudied drug distribution mechanism that may underlie the low success rates of current approaches to counter multidrug resistance (MDR). This mechanism is influenced by the hypoxic tumour microenvironment and results in selective trapping of weakly basic drugs into acidified compartments such as the extracellular environment. Here we report that hypoxia not only leads to acidification of the tumour microenvironment but also induces endosome hyperacidification. The acidity of the vesicular lumen, together with the alkaline pH of the cytoplasm, gives rise to a strong intracellular pH gradient that drives intravesicular drug trapping and chemoresistance. Endosome hyperacidification is due to the relocalization of the Na+/H+ exchanger isoform 6 (NHE6) from endosomes to the plasma membrane, an event that involves binding of NHE6 to the activated protein kinase C–receptor for activated C kinase 1 complex. These findings reveal a novel mechanism of hypoxia-induced MDR that involves the aberrant intracellular distribution of NHE6.

Fibroblast-like synoviocytes (FLS) play a major role in invasive joint destruction in rheumatoid arthritis (RA). This prodestructive phenotype has been shown to involve autocrine TGF-β that triggers formation of matrix-degrading invadosomes through molecular mechanisms that are not fully elucidated. The platelet-derived growth factor (PDGF) receptor (PDGFR) family of receptor tyrosine kinases (RTK) has been shown to cooperate with TGF-β in various pathological conditions. We therefore sought to determine whether RTK activity played a role in invadosome biogenesis. We demonstrated that, among the common RTKs, PDGFR-αβ was specifically phosphorylated in FLS from RA patients. Phosphorylation of PDGFR-αβ was also elevated in RA synovial tissues. Interference with PDGFR activation or PDGF neutralization inhibited invadosome formation in RA synoviocytes, indicating the presence of an autocrine PDGFR activation loop that involved endogenous PDGF. Among the PDGF-A–D isoforms, only PDGF-B was found both significantly elevated in FLS lines from RA patients, and related to high-invadosome forming cells. Addition of TGF-β upregulated invadosome formation, PDGF-B mRNA expression, and phosphorylation of PDGFR. All of these functions were efficiently suppressed by TGF-β neutralization or interference with the Smad/TβR1or PI3K/Akt pathway. Among the class 1 PI3K family proteins known to be expressed in RA synoviocytes, PI3Kα was selectively involved in PDGF-B expression, whereas both PI3Kα and PI3Kδ participated in invadosome formation. Our findings demonstrate that PDGFR is a critical RTK required for the prodestructive phenotype of RA synovial cells. They also provide evidence for an association between autocrine TGF-β and PDGFR-mediated invadosome formation in RA synoviocytes that involves the production of PDGF-B induced by TGF-β.