Hepatic stellate cell protrusions couple platelet-derived growth factor-BB to chemotaxis

Hepatic stellate cells play an essential role in the liver's injury response. Although stellate cells are defined by the presence of cytoplasmic protrusions, the function of these characteristic structures has been obscure. We hypothesized that stellate cell protrusions act by coupling injury-associated stimuli to chemotaxis. To test this hypothesis, we developed an assay for directly visualizing the response of living stellate cells in early primary culture to local stimulation of the tips of protrusions with platelet-derived growth factor-BB (PDGF). Stellate cells exhibited elongate protrusions containing actin, myosin, and tubulin. PDGF, but not cytochrome C, localized at a protrusion tip induced a coordinated series of morphological events--cell spreading at the tip, movement of the cell body toward the PDGF, and retraction of trailing protrusions--that resulted in chemotaxis. Soluble PDGF and AG 1296, a receptor tyrosine kinase inhibitor, both reduced stellate cell chemotaxis. PDGF-induced chemotaxis was associated with an early and transient increase in myosin phosphorylation within the spreading lamella. We observed that blebbistatin, a myosin II inhibitor, completely and reversibly blocked protrusion-mediated lamella formation and chemotaxis. Moreover, blockade of MRLC phosphorylation with the myosin light chain kinase inhibitor, ML-7, or the rho kinase inhibitor, Y-27632, blocked lamella formation, myosin phosphorylation within the protrusion, and chemotaxis.

These results support a model in which protrusions permit stellate cells to promptly detect PDGF distant from their cell bodies and transduce this signal into mechanical forces that propel the cell toward the site of injury.