Distinct Effects of Voltage- and Store-dependent Calcium Influx on Stretch-induced Differentiation and Growth in Vascular Smooth Muscle

Stretch of the vascular wall stimulates smooth muscle hypertrophy by activating the MAPK and Rho/Rho kinase (ROK) pathways. We investigated the role of calcium in this response. Stretch-stimulated expression of contractile and cytoskeletal proteins in mouse portal vein was inhibited at mRNA and protein levels by blockade of voltage-dependent Ca²⁺ entry (VDCE). In contrast, blockade of store-operated Ca²⁺ entry (SOCE) did not affect smooth muscle marker expression but decreased global protein synthesis. Activation of VDCE caused membrane translocation of RhoA followed by phosphorylation of its downstream effectors LIMK-2 and cofilin-2. Stretch-activated cofilin-2 phosphorylation depended on VDCE but not on SOCE. VDCE was associated with increased mRNA expression of myocardin, myocyte enhancer factor (MEF) -2A and -2D, and smooth muscle marker genes, all of which depended on ROK activity. SOCE increased ERK1/2 phosphorylation and c-Fos expression but had no effect on phosphorylation of LIMK-2 and cofilin-2 or on myocardin and MEF2 expression. Knockdown of MEF2A or -2D eliminated the VDCE-induced activation of myocardin expression and increased basal c-Jun and c-Fos mRNA levels. These results indicate that MEF2 mediates VDCE-dependent stimulation of myocardin expression via the Rho/ROK pathway. In addition, SOCE activates the expression of immediate-early genes, known to be regulated by MEF2 via Ca²⁺-dependent phosphorylation of histone deacetylases, but this mode of Ca²⁺ entry does not affect the Rho/ROK pathway. Compartmentation of Ca²⁺ entry pathways appears as one mechanism whereby extracellular and membrane signals influence smooth muscle phenotype regulation, with MEF2 as a focal point.