Novel p21-activated kinase-dependent protrusions characteristically formed at the edge of transformed cells

During long-term culture, certain lines become neoplastic while accumulating changes in cell shape. Early and late cell populations have characteristic shape phenotypes that have been quantified by computerized assay. Phenotypes are determined from variables describing three-dimensional aspects of the subcellular distribution of mass. The features of cells can be recognized by use of latent factors, which are theoretical variables based on the covariance of the primary variables. Factor #7 represented a cell edge feature different from filopodia. We studied the morphological characteristics and morphogenesis of the feature. Brief exposure of cells from rat tracheal epithelium to phorbol 12-myristate 13-acetate (PMA) enhanced #7 values. The time to reach maximal #7 values was prolonged if PMA was administered with calcium ionophore or lysophosphatidic acid (LPA). Factor #7 was elevated during periods of ruffling suppression and stress fiber reorganization. Cells showing high #7 values were examined by scanning electron microscopy (SEM) and found to exhibit strap-shaped and cupola-shaped projections. Because RhoA regulates stress fiber formation, we sought to perturb #7 features by introducing dominant-acting negative and positive constructs of RhoA, RhoA-N19, and RhoA-V14. Neither affected #7 values. Although overexpression of the kinase inhibitory domain of p21-activated kinase 1 (PAK) had no effect on #7 values, they were affected by overexpression of a domain binding PAK-interacting guanine nucleotide exchange factor (PIX). Because a PAK-PIX complex is implicated in the remodeling of focal complexes (FCs) and recycling of PAK to the cytoplasm, the results implicate a component of FCs in the formation of #7 features. The data suggested that feature formation is driven by activated Cdc42-binding kinase (ACK) and Rac. Moreover, they suggested that the #7 protrusions are neurite-like structures and that their development involves FC regulation.