Inhibiting myosin light chain kinase induces apoptosis in vitro and in vivo

Previous short-term studies have correlated an increase in the phosphorylation of the 20-kDa light chain of myosin II (MLC₂₀) with blebbing in apoptotic cells. We have found that this increase in MLC ₂₀ phosphorylation is rapidly followed by MLC₂₀ dephosphorylation when cells are stimulated with various apoptotic agents. MLC₂₀ dephosphorylation is not a consequence of apoptosis because MLC₂₀ dephosphorylation precedes caspase activation when cells are stimulated with a proapoptotic agent or when myosin light chain kinase (MLCK) is inhibited pharmacologically or by microinjecting an inhibitory antibody to MLCK. Moreover, blocking caspase activation increased cell survival when MLCK is inhibited or when cells are treated with tumor necrosis factor alpha. Depolymerizing actin filaments or detaching cells, processes that destabilize the cytoskeleton, or inhibiting myosin ATPase activity also resulted in MLC ₂₀ dephosphorylation and cell death. In vivo experiments showed that inhibiting MLCK increased the number of apoptotic cells and retarded the growth of mammary cancer cells in mice. Thus, MLC₂₀ dephosphorylation occurs during physiological cell death and prolonged MLC₂₀ dephosphorylation can trigger apoptosis.