Comparative angiotoxic responses of avian and rodent species in vivo: implications in atherogenesis

Modulation of alpha4 integrin mRNA levels is coupled to deficits in vasomotor function in rat arterioles by allylamine

Allylamine, a selective cardiovascular toxin that induces oxidative stress, is known to alter expression of extracellular matrix and cell adhesion proteins that are central to arterial remodeling. Our goals were to determine whether AAM treatment in rats modulates integrin/matrix-dependent arteriolar function, and to what extent integrin expression correlated to these alterations. Integrins are transmembrane proteins that facilitate mechanical and molecular signaling between the extracellular matrix and cytoskeleton, and so are suitable candidates for involvement in phenotypic and functional alterations of smooth muscle in response to oxidative stress. Arg-Gly-Asp (RGD) and Leu-Asp-Val (LDV), two integrin-binding motifs found in ECM proteins such as collagens and fibronectin, are known to interact with integrins alphavbeta3 and alpha4beta1, respectively. Previously, we found that RGD containing peptides induce vasodilation through alphavbeta3, while LDV containing peptides induce vasoconstriction through alpha4beta1 of normal rat cremasteric arterioles. In allylamine-treated rats (AAM), the vasomotor response to LDV, but not RGD, was attenuated in a dose-dependent manner. To determine whether changes in integrin subunit mRNA levels correlated with these functional changes, we performed reverse transcription and Real-time PCR for alpha4 and beta3 integrin subunits on RNA isolated from single, first-order cremasteric arterioles. AAM treatment caused a dose-dependent decrease in alpha4 mRNA expression, but not beta3 mRNA expression, suggesting that the changes in vasomotor activity to LDV peptides may be attributable in part to reduced alpha4 expression upon exposure to AAM. These data are supported by similar decreases in alpha4integrin cell surface protein expression in cultured vascular smooth muscle cells treated either in vivo and in vitro with AAM.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Differential processing of osteopontin characterizes the proliferative vascular smooth muscle cell phenotype induced by allylamine

Repeated cycles of vascular injury by allylamine induce vascular lesions similar to those seen in atherosclerotic vessels, or following balloon catheterization. Vascular (aortic) smooth muscle cells harvested from allylamine-treated animals (i.e., allylamine cells) acquire a proliferative advantage relative to control counterparts that is associated with differential secretion and extracellular matrix sequestration of several proteins. In the present study, we have characterized two of these proteins (M(r) 52 and 36 kDa; pl 5.6 and 5.2, respectively) and their putative role in the expression of a proliferative phenotype. Because the physical properties of these proteins were comparable to those of osteopontin (OPN) and its thrombin-generated fragment(s), initial experiments were conducted to examine the expression and processing of OPN in this cell system. OPN mRNA expression was enhanced during early G1 cell cycle progression in allylamine cells relative to control counterparts. However, comparable amounts of OPN (M(r) 56, 52, and 50 kDa) were detected by Western analysis in media conditioned by both cell types using the OP-199 or B77-Rat1 antibodies to OPN. Allylamine cells, however, produced increased amounts of a 36 kDa protein recognized by the OP-199 antibody. Incubation of conditioned media from [35S]methionine-labeled allylamine cells with thrombin decreased the intensity of the 52 kDa protein, while increasing the intensity of a 36 kDa protein. RT-PCR analysis demonstrated expression of a 1.2 kb OPN band in both cell types consistent with the predicted size of OPN mRNA, suggesting that the 36 kDa fragment recognized by OP-199 in allylamine cells was likely not due to altered splicing of the OPN transcript. To determine if OPN and/or the 36 kDa fragment played a central role in the proliferative capacity of allylamine cells, the effect of an antibody to an alpha v integin subunit was examined. An antibody to the alpha v subunit, but not alpha 4, nullified the proliferative advantage of allylamine cells relative to control counterparts, suggesting that integrin-mediated signaling is a key feature of the proliferative phenotype of allylamine cells. We conclude that enhanced proteolytic cleavage of OPN may characterize the modulation of vascular SMCs to a more proliferative phenotype following chemical injury by allylamine.

Comparative cytotoxic responses of cultured avian and rodent aortic smooth muscle cells to allylamine

The present studies were designed to compare the acute cytotoxic responses of cultured avian and rodent aortic smooth muscle cells (SMCs) to allylamine (AAM), a selective vascular toxin. SMCs were isolated from male Japanese quail or Sprague-Dawley rats and established in culture by standard procedures. Cellular glutathione (GSH) content and lactate dehydrogenase (LDH) leakage were used as indices of cytotoxicity. Exposure of avian and rodent SMCs in primary culture to AAM (0.2-200 microM) for 4 h was associated with a significant reduction in cellular GSH and enzyme leakage in cultures of both cell types. Increased exposure time to 24 h further depleted cellular GSH levels and enhanced the leakage of LDH in primary cultures of avian SMCs. In contrast, enhanced LDH leakage occurred without further GSH depletion in primary cultures of rodent SMCs upon exposure to AAM for 24 h. Removal of serum did not modulate the cytotoxic response profile of primary cultures of avian SMCs treated with 200 microM AAM, but was associated with marked elevation in cellular GSH levels and significant LDH leakage in rodent SMC cultures. The cytotoxic responses to 0.2-200 microM AAM in secondary cultures of avian SMCs were comparable to those observed in primary culture. In contrast, AAM-induced enzyme leakage did not consistently correlate with changes in GSH content in subcultured rodent SMCs. Challenge with 200 microM acrolein (ACR) for 4 h reduced the GSH content in avian, but not rodent, subcultures of SMCs. However, significant LDH leakage occurred in subcultures of both cell types upon exposure to ACR. Although hydrogen peroxide (H2O2) did not modulate GSH levels in avian or rodent cultures, leakage of LDH was observed in rat SMCs challenged with 200 microM H2O2. Removal of serum did not alter the cytotoxic responses of avian subcultures to 200 microM AAM for 24 h, but fully prevented cytotoxicity in rodent subcultures. These data suggest that potentially significant variations in the sequence of events leading to injury may exist between quail and rat aortic SMCs. These differences may contribute to the enhanced avian susceptibility to AAM-induced aortic injury in vivo.

Altered protein secretion and extracellular matrix deposition is associated with the proliferative phenotype induced by allylamine in aortic smooth muscle cells

Repeated cycles of allylamine-induced aortic injury in vivo modulate the proliferative potential of smooth muscle cells (SMCs) during serial propagation in vitro. This modulation may be partly mediated by disturbances in polyphosphoinositide metabolism which afford allylamine-treated cells a growth advantage over control cells [Cox, Murphy and Ramos (1990) Exp. Mol. Pathol. 53, 52-63]. The present studies were conducted to further evaluate the mechanisms which mediate the enhanced proliferative potential of allylamine cells. Cellular growth and/or [3H]thymidine incorporation into DNA were evaluated in control and allylamine cells seeded on plastic culture dishes or glass coverslips in the presence of 0.1, 1 or 10% fetal bovine serum (FBS). On either substrate, incubation in 0.1% FBS for 48 h inhibited DNA synthesis in cultures of both cell types, but the inhibitory response was more pronounced in allylamine cells. Subsequent challenge with 10% FBS increased thymidine incorporation to a greater extent in allylamine cells. Interestingly, enhanced DNA synthesis of allylamine cells was associated with increased cell numbers only when seeded on a glass surface. The enhanced growth rate on glass was not due to increased plating efficiency since comparable attachment rates were observed for both cell types. Reseeding of control cells on glass substrates pre-coated by allylamine cells afforded control cells a growth advantage comparable with that observed for allylamine cultures. Conditioned media from growth-arrested, as well as cycling cultures, of allylamine cells stimulated DNA synthesis in cultures of either cell type to a greater extent than conditioned media from control counterparts. In addition, the responsiveness of allylamine cells to secreted products was enhanced relative to that of control cells. Metabolic labelling studies revealed that the synthesis and/or secretion of 52, 46, 33 and 28 kDa proteins was enhanced in allylamine cultures relative to controls, and that the expression of two proteins of 30 and 31 kDa only occurred in allylamine cultures. We conclude that the enhanced growth response of allylamine cells is associated with both altered protein secretion and differential extracellular matrix deposition.

Modulation of phosphoinositide metabolism in aortic smooth muscle cells by allylamine

Aortic smooth muscle cells (SMC) modulate from a contractile to a proliferative phenotype upon subchronic exposure to allylamine. The present studies were designed to determine if this phenotypic modulation is associated with alterations in the metabolism of membrane phosphoinositides. 32P incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA) was lower by 31, 35, and 22%, respectively, in SMC from allylamine-treated animals relative to controls. In contrast, incorporation of [3H]myoinositol into inositol phosphates did not differ in allylamine cells relative to control cells. Exposure to dibutyryl (db) cAMP (0.2 mM) and theophylline (0.1 mM) reduced 32P incorporation into PIP and PIP2 in SMC from both experimental groups. Under these conditions, a decrease in [3H]myoinositol incorporation into inositol 1-phosphate was only observed in allylamine cells. The effects of db cAMP and theophylline in allylamine and control SMC correlated with a marked decrease in cellular proliferation. These results suggest that alterations in phosphoinositide synthesis and/or degradation contribute to the enhanced proliferation of SMC induced by allylamine. To further examine this concept, the effects of agents which modulate protein kinase C (PKC) activity were evaluated. Sphingosine (125-500 ng/ml), a PKC inhibitor, decreased SMC proliferation in allylamine, but not control cells. 12-O-Tetradecanoylphorbol-13-acetate (1-100 ng/ml), a PKC agonist, stimulated proliferation in control cells, but inhibited proliferation in cells from allylamine-treated animals. We conclude that allylamine-induced phenotypic modulation of SMC is associated with alterations in phosphoinositide metabolism.