Serum deprivation induces a unique hypercontractile phenotype of cultured smooth muscle cells

Chronic asthma is characterized by hypertrophyand hyperplasia of airway smooth muscle cells (SMC) that limit airflowby a geometric effect. Whether contractility of airway SMC is altered is not clear. Cultured cells were used as a model of hyperplasia. Phenotypic changes seen indicated conversion to a synthetic, weakly contractile type. At confluence, although limited reversal of proteinchanges was seen, no restoration in contractility occurred. Phenotypicmodulation of postconfluent cultured airway SMC under prolonged serumdeprivation (arrested cells) is reported here. Two phenotypicallydistinct groups of cells were identified in primary airway SMCcultures: 1) elongatedspindle-shaped cells, which expressed large amounts of smooth musclecontractile and regulatory proteins, and2) flat and stellate cells, whichexpressed very little. The first group showed a surprising shorteningcapacity and a velocity that was even greater than that of the freshly isolated cells, whereas the second group became spherical and noncontractile. Even more surprising was that the myosin heavy chain(MHC) isoform (SM-B) generally said to be associated with the highershortening velocity disappeared from the cell, while the content of thekey rate-limiting regulating enzyme, myosin light chain kinase (MLCK),increased 30-fold. We conclude that a functional, contractile phenotypeof airway SMC can be obtained by prolonged serum deprivation. Wespeculate that the increased contractility could be the result ofincreased phosphorylation of the 20-kDa myosin light chain resultingfrom increased content of smooth muscle MLCK rather than any increasein endogenous MHC ATPase activity. This model may be useful for studyof SMC differentiation and contraction.