LPP expression during in vitro smooth muscle differentiation and stent-induced vascular injury

Lipoma preferred partner (LPP) has been identified as a protein highly expressed in smooth muscle (SM) tissues. The aim of the present study was to determine mechanisms that regulate LPP expression in an in vitro model of SM cell (SMC) differentiation and in stent-induced pig coronary vessel injury. All trans-retinoic acid treatment of A404 cells induced a strong increase in LPP, as well as SM alpha-actin, SM myosin heavy chain, and smoothelin mRNA levels, in a Rho kinase (ROK)-dependent manner. Adenovirus mediated overexpression of myocardin in A404 cells significantly increased LPP mRNA expression. Interestingly, inactivation of RhoA with C3-exoenzyme or treatment with ROK inhibitors strongly inhibited myocardin mRNA expression in retinoic acid-treated A404 cells or human iliac vein SMCs. LPP silencing with short interfering RNA significantly decreased SMC migration. LPP expression was also markedly decreased in focal adhesion kinase (FAK)-null cells known to have impaired migration but rescued with inducible expression of FAK. LPP expression in FAK-null fibroblasts enhanced cell spreading. In stented pig coronary vessels, LPP was expressed in the neointima of cells lacking smoothelin and showed expression patterns identical to those of SM alpha-actin. In conclusion, LPP appears to be a myocardin-, RhoA/ROK-dependent SMC differentiation marker that plays a role in regulating SMC migration.     

Atorvastatin inhibits myocardin expression in vascular smooth muscle cells

Atorvastatin (ATV), an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, is widely prescribed as a lipid-lowering drug. It also inhibits the RhoA-Rho-associated kinase pathway in vascular smooth muscle (SM) cells and critically inhibits SM function. Myocardin is a coactivator of serum response factor, which upregulates SM contractile proteins. The RhoA-Rho-associated kinase pathway, which directly triggers SM contraction, also increases myocardin gene expression. Therefore, we investigated whether ATV inhibits myocardin gene expression in SM cells. In mice injected with ATV (IP 20 μg/g per day) for 5 days, myocardin gene expression was significantly downregulated in aortic and carotid arterial tissues with decreased expression of myocardin target genes SM α-actin and SM22. Correspondingly, the contractility of aortic rings in mice treated with ATV or the Rho-associated kinase inhibitor Y-27632 was reduced in response to treatment with either KCl or phenylephrine. In cultured mouse and human aortic SM cells, KCl treatment stimulated the expression of myocardin, SM α-actin, and SM22. These stimulatory effects were prevented by ATV treatment. ATV-induced inhibition of myocardin expression was prevented by pretreatment with either mevalonate or geranylgeranylpyrophosphate but not farnesylpyrophosphate. Treatment with Y-27632 mimicked ATV effects on the gene expression of myocardin, SM α-actin, and SM22, further suggesting a role for the RhoA-Rho-associated kinase pathway in ATV effects. Furthermore, ATV treatment inhibited RhoA membrane translocation and activation; these effects were prevented by pretreatment with mevalonate. We conclude that ATV inhibits myocardin gene expression in vivo and in vitro, suggesting a novel mechanism for ATV inhibition of vascular contraction.     

Similarities and differences in smooth muscle alpha-actin induction by TGF-beta in smooth muscle versus non-smooth muscle cells.

Transforming growth factor-beta (TGF-beta) has been shown to stimulate smooth muscle (SM) alpha-actin expression in smooth muscle cells (SMCs) and non-SMCs. We previously demonstrated that the 2 CArG boxes A and B and a novel TGF-beta control element (TCE) located within the first 125 bp of the SM alpha-actin promoter were required for TGF-beta inducibility of SM alpha-actin in SMCs. The aims of the present study were (1) to determine whether the TCE exhibits SMC specificity or contributes to TGF-beta induction of SM alpha-actin expression in non-SMCs (ie, endothelial cells and fibroblasts) and (2) to determine whether TGF-beta can induce expression of multiple TCE-containing SMC differentiation marker genes, such as SM22alpha, h(1) calponin, and SM myosin heavy chain (SM MHC) in non-SMCs. Results of transient transfection assays demonstrated that mutation of CArG A, CArG B, or the TCE within a 125-bp promoter context completely abolished TGF-beta inducibility of SM alpha-actin in endothelial cells and fibroblasts. However, in contrast to observations in SMCs, inclusion of regions upstream from (-155) completely repressed TGF-beta responsiveness in non-SMCs. Electrophoretic mobility shift assays showed that TGF-beta enhanced binding of a serum response factor to the CArG elements and the binding of an as-yet-unidentified factor to the TCE in endothelial cells and fibroblasts, but to a much lesser extent compared with SMCs. TGF-beta also stimulated expression of the SMC differentiation marker SM22alpha in non-SMCs. However, in contrast to SMCs, TGF-beta did not induce expression of h(1) calponin and SM MHC in non-SMCs. In summary, these results suggest a conserved role for CArG A, CArG B, and the TCE in TGF-beta-induced expression of SM alpha-actin in SMCs and non-SMCs that is modified by a complex interplay of positive- and negative-acting cis elements in a cell-specific manner. Furthermore, observations that TGF-beta stimulated expression of several early but not late differentiation markers in non-SMCs indicate that TGF-beta alone is not sufficient to induce transdifferentiation of non-SMCs into SMCs.