Ins and outs of ADF/cofilin activity and regulation.

The actin-binding proteins of the actin-depolymerisation factor (ADF)/cofilin family were first described more than three decades ago, but research on these proteins still occupies a front role in the actin and cell migration field. Moreover, cofilin activity is implicated in the malignant, invasive properties of cancer cells. The effects of ADF/cofilins on actin dynamics are diverse and their regulation is complex. In stimulated cells, multiple signalling pathways can be initiated resulting in different activation/deactivation switches that control ADF/cofilin activity. The output of this entire regulatory system, in combination with spatial and temporal segregation of the activation mechanisms, underlies the contribution of ADF/cofilins to various cell migration/invasion phenotypes. In this framework, we describe current views on how ADF/cofilins function in migrating and invading cells.

New blocking antibodies impede adhesion, migration and survival of ovarian cancer cells, highlighting MFGE8 as a potential therapeutic target of human ovarian carcinoma.

Milk Fat Globule--EGF--factor VIII (MFGE8), also called lactadherin, is a secreted protein, which binds extracellularly to phosphatidylserine and to αvß3 and αvß5 integrins. On human and mouse cells expressing these integrins, such as endothelial cells, phagocytes and some tumors, MFGE8/lactadherin has been shown to promote survival, epithelial to mesenchymal transition and phagocytosis. A protumoral function of MFGE8 has consequently been documented for a few types of human cancers, including melanoma, a subtype of breast cancers, and bladder carcinoma. Inhibiting the functions of MFGE8 could thus represent a new type of therapy for human cancers. Here, we show by immunohistochemistry on a collection of human ovarian cancers that MFGE8 is overexpressed in 45% of these tumors, and we confirm that it is specifically overexpressed in the triple-negative subtype of human breast cancers. We have established new in vitro assays to measure the effect of MFGE8 on survival, adhesion and migration of human ovarian and triple-negative breast cancer cell lines. Using these assays, we could identify new MFGE8-specific monoclonal antibodies, which efficiently blocked these three tumor-promoting effects of MFGE8. Our results suggest future use of MFGE8-blocking antibodies as new anti-cancer therapeutics in subgroups of ovarian carcinoma, and triple-negative breast carcinoma patients.

Unbalancing the phosphatidylinositol-4,5-bisphosphate-cofilin interaction impairs cell steering.

Cofilin is a key player in actin dynamics during cell migration. Its activity is regulated by (de)phosphorylation, pH, and binding to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)]. Here, we here use a human cofilin-1 (D122K) mutant with increased binding affinity for PI(4,5)P(2) and slower release from the plasma membrane to study the role of the PI(4,5)P(2)-cofilin interaction in migrating cells. In fibroblasts in a background of endogenous cofilin, D122K cofilin expression negatively affects cell turning frequency. In carcinoma cells with down-regulated endogenous cofilin, D122K cofilin neither rescues the drastic morphological defects nor restores the effects in cell turning capacity, unlike what has been reported for wild-type cofilin. In cofilin knockdown cells, D122K cofilin expression promotes outgrowth of an existing lamellipod in response to epidermal growth factor (EGF) but does not result in initiation of new lamellipodia. This indicates that, next to phospho- and pH regulation, the normal release kinetics of cofilin from PI(4,5)P(2) is crucial as a local activation switch for lamellipodia initiation and as a signal for migrating cells to change direction in response to external stimuli. Our results demonstrate that the PI(4,5)P(2) regulatory mechanism, that is governed by EGF-dependent phospholipase C activation, is a determinant for the spatial and temporal control of cofilin activation required for lamellipodia initiation.

A phage display-based method for determination of relative affinities of mutants. Application of the actin-binding motifs in thymosin beta 4 and the villin headpiece.

We propose phage display combined with enzyme-linked immunosorbent assay as a tool for the systematic analysis of protein-protein interactions by investigating the binding behavior of variants to a partner protein. Via enzyme-linked immunosorbent assay we determine both the amount of fusion protein presented at the phage surface and the amount of complex formed, the ratio of which is proportional to the affinity. Hence this method enables us to calculate the relative affinities of a large number of mutants. As model systems, we investigated actin-binding motifs conserved in a number of proteins binding monomeric or filamentous actin. The hexapeptide motifs LKKTET, present in thymosin beta4, and LKKEKG, present in the villin headpiece, were mutated, and the variants were analyzed. Study of the positional tolerance allows postulating that the motifs, although similar in primary structures adopt different conformations when bound to actin. In addition, our data show that the second and the fourth amino acid of the thymosin beta4 motif and the first three residues of the villin headpiece motif are most important for actin binding. The latter result challenges the charged crown hypothesis for the villin headpiece filamentous actin interaction.