Staurosporine induces formation of two types of extra-long cell protrusions: actin-based filaments and microtubule-based shafts.

Staurosporine (STS) has been known as a classic protein kinase C inhibitor and is a broad-spectrum inhibitor targeting over 250 protein kinases. In this study, we observed that STS treatment induced drastic morphologic changes, such as elongation of a very large number of nonbranched, actin-based long cell protrusions that reached up to 30 µm in an hour without caspase activation or PARP cleavage in fibroblasts and epithelial cells. These cell protrusions were elongated not only from the free cell edge but also from the cell-cell junctions. The elongation of STS-dependent protrusions was required for ATP hydrolysis and was dependent on myosin-X and fascin but independent of Cdc42 and VASP. Interestingly, in the presence of an actin polymerization inhibitor, namely, cytochalasin D, latrunculin A, or jasplakinolide, STS treatment induced excess tubulin polymerization, which resulted in the formation of many extra-long microtubule (MT)-based protrusions toward the outside of the cell. The unique MT-based protrusions were thick and linear compared with the STS-induced filaments or stationary filopodia. These protrusions, which were composed of microtubules, have been scarcely observed in cultured non-neuronal cells. Taken together, our findings revealed that STS-sensitive kinases are essential for the maintenance of normal cell morphology, and a common unidentified molecular mechanism is involved in the formation of the following two different types of protrusions: actin-based filaments and MT-based shafts.