Wnt‐3a is an intercellular signalling molecule that is involved in a variety of morphogenetic events. However, the molecular mechanisms underlying Wnt‐3a signalling are poorly understood. We have sought to establish in vitro systems to assay the activity of this protein and investigate its biological roles.

We prepared mouse L cells transfected with Wnt‐3a cDNA, and found that their β‐catenin protein level was up‐regulated. When conditioned medium (CM) was collected from cultures of the transfectants and added to nontransfected L cells, the β‐catenin level of the latter was also increased. Approximately 50% of the Wnt‐3a proteins synthesized by the transfectants were secreted into the CM in a soluble form. These secreted Wnt‐3a proteins formed an activity gradient in the environment surrounding the transfectants. Then, we studied whether Wnt‐3a had any effect on cellular behaviour in vitro. When the CM containing Wnt‐3a (W3a‐CM) was added to cultures of C57MG mammary epithelial cells, their morphology was altered to exhibit closer intercellular contacts. Immunostaining for various adhesion and cytoskeletal proteins showed that the actin–microfilamental system was re‐organized by the W3a‐CM treatment. It induced a directional alignment of actin stress fibres and other actin‐associated proteins. Moreover, villin, localized only at the perinuclear regions in untreated C57MG cells, was re‐distributed to the leading edges of the cells, co‐localizing with F‐actin, in the presence of Wnt‐3a.

Our findings suggest that Wnt‐3a protein, in the soluble form, can act to re‐organize cytoskeletal structures.