Filopodia and adhesion in cancer cell motility

Abstract

Slender bundled actin containing plasma membrane protrusions, called filopodia, are important for many essential cellular processes like cell adhesion, migration, angiogenesis and the formation of cell-cell contacts. In migrating cells, filopodia are the pioneers at the leading edge which probe the environment for cues. Integrins are cell surface adhesion receptors critically implicated in cell migration and they are transported actively to filopodia tips by an unconventional myosin, myosin-X. Integrin mediated adhesion stabilizes filopodia and promotes cell migration even though integrins are not essential for filopodia initiation. Myosin-X binds also PIP3 and this regulates its activation and localization to filopodia. Filopodia stimulate cell migration in many cell types and increased filopodia density has been described in cancer. Furthermore, several proteins implicated in filopodia formation, like fascin, are also relevant for cancer progression. To investigate this further, we performed a meta-analysis of the expression profiles of 10 filopodia-linked genes in human breast cancer. These data implicated that several different filopodia inducing genes may contribute in a collective manner to cancer progression and the high metastasis rates associated with basal-type breast carcinomas.

Figure 1

Actin cytoskeleton, myosin-X and β1-integrins in filopodia formation. (1) β1-integrins are endocytosed from the plasma-membrane and recycled back to cell surface via tubulating actin-dependent recycling endosomes. (2) Monomeric PIP₃-unbound myosin-X is in a closed conformation. PIP₃-unbound myosin-X is transported via microtubule tracks to the plasma-membrane in small GTPase Rab7 positive vesicles. (3) Dimerized PIP₃-bound myosin-X promotes actin fiber convergence during filopodia initiation. (4) Lateral movement of β1-integrins along the leading edge could be a myosin-X driven process. (5) β1-integrins are transported to filopodia tips in a myosin-X dependent manner. Myosin-X and β1-integrins serve as a link between the cellular cytoskeleton and the extracellular matrix.

Figure 2

Phosphoinositides and filopodia. (1) N-WASP is activated by PI (4,5)P₂ and GTP-Cdc42 binding. The resulting open-conformation of N-WASP with an exposed VCA-domain interacts with and activates the Arp2/3-complex and increases the rate of actin polymerization. The binding of the SH3-domain of IR Sp53 to N-WASP can also result in activation of N-WASP. (2) WAVE 2 is localized to the leading edge by binding to PI (3,4,5)P₃ and IR Sp53. GTP-Rac and IR Sp53 both enhance WAVE 2 mediated Arp2/3 activation.

Figure 3

Filopodia-associated genes are upregulated in breast carcinomas with a poor prognosis. In silico transcriptomics analysis of a set of filopodia related genes discussed in this review. Unsupervised hierarchical clustering of the expression levels of 10 filopodia related genes in 251 breast tumors. Each cell in the cluster (middle part) shows the log2 expression ratio for the particular gene in separate tumor samples divided by the median expression of that gene in all samples. Red indicates expression above the median; green, below the median. Upper part: color-coded tumor-type classification (see the legend) of each sample. Lower part: clinicopathological parameters related to each sample. Black bars indicate high ERBB2 expression, high K_i-67 expression, high PCNA expression, lymph node positivity, presence of p53 mutation and PrR and ER negativity. Tumor grade is indicated with white (grade 1), gray (grade 2) or black (grade 3) bars in the row adjacent to the label grade. Patient survival is blotted in the bottom part. Numbers on the right indicate survival in years. The survival time panel displays for each sample the documented survival time. In addition a curve is fitted through all the available survival dots. The genes included encode for the following proteins: RHOF (Rif), FMNL1, VASP, HAS3 (hyaluronan synthase 3), FMN2, DIAPH2 (Dia2), FCSN1 (fascin1), FMNL2, MYO10 (Myosin-X) and ACTR2 (Arp2).