Hypoxia-Induced Invadopodia Formation Involves Activation of NHE-1 by the p90 Ribosomal S6 Kinase (p90RSK)

Abstract

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.

Regulation of invadopodia by the tumor microenvironment

“…In hypoxic cancer cells, NHE-1 localizes to invadopodia, and its expression and activation by p90 ribosomal S6 kinase (p90RSK) is required for invadopodia formation. 98,99 In breast cancer cells, NHE-1 promotes invadopodia formation by acidifying the extracellular invadopodia environment and alkalinizing the invadopodia cytosol to release cofilin from cortactin ( Fig. 1).…”

“…In HT1080 fibrosarcoma cells, exposure to acidic pH increases ECM degradation, which is augmented by hypoxia. 98 Alteration of the tumor microenvironmental pH affects the expression, secretion, and activation of proteases, including MT1-MMP, MMP-9, and MMP-2, which are essential for the ECM-degrading activity of invadopodia. Acidic extracellular pH increases MMP-9 expression, and pharmacological inhibition of NHE-1 downregulates MT1-MMP, a key activator of MMP-2.…”

Invadopodia and basement membrane invasion in vivo

“…and metabolism. 28,[59][60][61] Despite intensive study, one of the greatest gaps in the field has been establishing whether these structures actually exist in vivo, and if so, how they facilitate basement membrane transmigration. 21,62,63 In this review, we present recent work from our lab and others that have indentified invadopodia in vivo.…”

Role of pHi, and proton transporters in oncogene-driven neoplastic transformation

“…There is now ample evidence that in addition to these above stimuli tumour cell NHE1 is further activated by the components of the TMM previously described [29]: low serum [47,47], acidic pH e [48] and hypoxia [8,[59][60][61], which links these components into a dynamic, reciprocal system that drives further microenvironmental acidification and malignant progression starting from the first moments of neoplastic transformation. Altogether, these data lead to the recognition of a synergistic, positive feedback interaction between the tumour cell and both the metabolic and stromal microenvironments in tumours and suggests that NHE1 may have an important role in integrating these interactions.…”