Tumor Cell Invadopodia: Invasive Protrusions that Orchestrate Metastasis

Abstract

Invadopodia are a subset of invadosomes that are implicated in the integration of signals from the tumor microenvironment to support tumor cell invasion and dissemination. Recent progress has begun to define how tumor cells regulate the plasticity necessary for invadopodia to assemble and function efficiently in the different microenvironments encountered during dissemination in vivo. Exquisite mapping by many laboratories of the pathways involved in integrating diverse invadopodium initiation signals, from growth factors, to extracellular matrix (ECM) and cell-cell contact in the tumor microenvironment, has led to insight into the molecular basis of this plasticity. Here, we integrate this new information to discuss how the invadopodium is an important conductor that orchestrates tumor cell dissemination during metastasis.

Keywords: Mena; TMEM; common invadopodium core; invadopodium-related prognostics.

Copyright © 2017 Elsevier Ltd. All rights reserved.

Figures

Figure 1. Temporal model of precursor core assembly and maturation common to invadopodium initiation signals

Invadopodium precursors initiate with the arrival of cofilin, Arp2/3, and N-WASp to an actin-cortactin complex. The proportion of G- and F-actin in the precursor at these early stages is unknown. Tks5 arrives ~20s later, and this stabilizes the precursor complex by linking it to basal levels of PI(3,4)P2 at the plasma membrane. Arg phosphorylation of cortactin at Y421 activates the Nck1-N-WASp complex to stimulate the initial Arp2/3-dependent nucleation of actin polymerization which is limited. This leads to recruitment of the moesin-NHE1 complex resulting in the local increase in pH which activates cofilin. The following 2–3 minutes show both a rapid increase in cofilin-dependent actin polymerization which synergizes with Arp2/3 activity to cause a large increase in new actin polymerization and invadopodium protrusion. Also during precursor stabilization the Mena-lamellipodin complex delivers SHIP2 at the precursor core. SHIP2 results in local PI(3,4)P2 production at the precursor, and this elevated PI(3,4)P2 production further stabilizes the invadopodium precursor by lamellipodin-dependent recruitment of MenaINV. MenaINV inhibits PTP1B thereby maintaining the phosphorylation of cortactin at Y421, which pushes the reactions to the right resulting in actin polymerization as described in Figure 2 leading to invadopodium maturation and matrix degradation.

Figure 2. Spatial model showing the pathways involved in initiation of invadopodia from growth factor receptors, ECM and cell-cell contact

Growth factor stimulation and/or ECM-integrin engagement (Table 1), initiates the co-assembly of a complex of Tks5, cortactin, N-WASp, cofilin, Arp2/3 and actin forming the initial invadopodium precursor core (red box). Lamellipodin binds PI(3,4)P2 at invadopodium precursor, a lipid that is uniquely localized to the core compared to other phosphoinositides, and recruits Mena isoforms, SHIP2 and Arg kinase to the precursor. SHIP2 produces additional PI(3,4)P2 to which Tks5 binds thereby stabilizing the precursor by binding it to the plasma membrane. Mena and Arg kinase are also delivered to the core by α5 and β1 integrins, respectively. Arg phosphorylates cortactin at Y421 in the core leading to talin-moesin-NHE1 complex recruitment that produces a local increase in pH to activate cofilin-initiated actin polymerization. CD44 also interacts with components of the ECM, such as fibronectin and hyaluronan, and promotes α5β1 integrin activation and cortactin phosphorylation at invadopodia. Cortactin tyrosine phosphorylation also recruits Nck1. Nck1 and Cdc42 activate the N-WASp-Arp2/3 complex which nucleates actin polymerization from the cofilin-generated F-actin resulting in invadopodium protrusion and maturation. Both Cdc42 and RhoA also promote recruitment of MT1-MMP to the plasma membrane at invadopodia. MenaINV inhibits PTP1B dephosphorylation of cortactin Y421 thereby supporting the above events and stabilizing invadopodium maturation. In addition, Mena inhibits Rac1-induced invadopodium disassembly by PAK1 phosphorylation of cortactin at S113. Tumor hypoxia has also been shown to promote invadopodium maturation in a Notch-dependent manner via HIF1α-mediated upregulation of ADAM12, which cleaves of HB-EGF from the plasma membrane to activate the EGFR. Macrophage-induced Notch signaling in the tumor cell promotes MenaINV expression. MenaINV may form tetramers with other Mena isoforms at RTKs, α5 integrin and in the invadopodium core. MenaINV stimulates invadopodium maturation by inhibiting the dephosphorylation of RTKs and cortactin Y421 thereby synergizing with growth factor and ECM signals. FAK activation of Src kinase activates Arg kinase acts as described above for growth factor and ECM signaling. FAK activation induces PI3K-dependent generation of PI(3,4,5)P3 from PI(4,5)P2, which is then dephosphorylated by SHIP2 to form PI(3,4)P2 which binds to Tks5 to stabilize the precursor core. Note that the overlap of the integrin and receptor tyrosine signaling pathways relative to the common invadopodium core structure can also define the crosstalk between growth factor receptors and ECM signals leading to more nuanced invadopodium initiation and function. The models in Figures 1 and 2 are derived from work with rat, mouse and human tumor cell lines and primary breast tumor cells derived from patient biopsies all of which are adenocarcinoma cells. The extent to which these models apply to other types of cancer is unknown.