Effects of immunosuppressants on platelet-derived growth factor-A chain mRNA expression and coronary arteriosclerosis in rat cardiac allografts

PDGF-induced proliferation in human arterial and venous smooth muscle cells: molecular basis for differential effects of PDGF isoforms

Platelet-derived growth factor (PDGF) has been implicated in the pathogenesis of arterial atherosclerosis and venous neointimal hyperplasia. We examined the effects of PDGF isoforms on smooth muscle cells (SMCs) from arterial and venous origins in order to further understand the differential responsiveness of these vasculatures to proliferative stimuli. Serum-starved human arterial and venous SMCs exhibited very different proliferative responses to PDGF isoforms. Whereas, proliferation of arterial SMCs was strongly stimulated by PDGF-AA, venous SMCs showed no proliferative response to PDGF-AA, but instead demonstrated a significantly greater proliferative response to PDGF-BB than arterial SMCs. Part of this difference could be attributed to differences in PDGF receptors expression. There was a 2.5-fold higher (P < 0.05) density of PDGF receptor-α (PDGF-Rα) and a 6.6-fold lower (P < 0.05) density of PDGF-Rβ expressed on arterial compared to venous SMCs. Concomitant with an increased proliferative response to PDGF-AA in arterial SMCs was a marked PDGF-Rα activation, enhanced phosphorylation of ERK1/2 and Akt, a transient activation of c-Jun NH2-terminal kinase (JNK), and a significant reduction in expression of the cell-cycle inhibitor p27(kip1). This pattern of signaling pathway changes was not observed in venous SMCs. No phosphorylation of PDGF-Rα was detected after venous SMC exposure to PDGF-AA, but there was enhanced phosphorylation of ERK1/2 and Akt in venous SMCs, similar to that seen in the arterial SMCs. PDGF-BB stimulation of venous SMC resulted in PDGF-Rβ activation as well as transactivation of epidermal growth factor receptor (EGF-R); transactivation of EGF-R was not observed in arterial SMCs. These results may provide an explanation for the differential susceptibility to proliferative vascular diseases of arteries and veins.

Immunosuppression and transplant vascular disease: benefits and adverse effects

Cardiac allograft vasculopathy (CAV) occurs within 5 years of transplantation surgery and represents the main cause of death in long-term heart transplant survivors. The detailed pathogenesis of CAV is unknown, but there are strong indications that immunologic mechanisms, which are regulated by nonimmunologic factors, are the major cause of this phenomenon. Cyclosporine A (CsA) is a frequently used immunosuppressive agent in transplant medicine to prevent rejection. The mechanism of action of CsA involves initial binding to cyclophilin to form a complex that then inhibits calcineurin (CN), leading to reduced interleukin (IL)-2 production as part of the signal transduction pathway for the activation of B-lymphocytes and T-lymphocytes. Based on this proposed mechanism, it was expected that CsA should be an effective strategy in attenuating the host immune response against transplanted allograft tissue; however, CsA has not changed the outcome of CAV. Several mechanisms have been suggested for the ineffectiveness of CsA in long-term prevention of CAV. For example, routine therapeutic doses of CsA may block CN incompletely (50%), whereas complete blockade requires doses that are not clinically tolerable. Another explanation is the possible activation of T-cell receptors directly (CN independent) by the immune response, which induces protein kinase C theta (PKCtheta) and leads to IL-2 production and immune rejection. Moreover, there may be a role for nonimmunologic mechanisms, such as complement, which cannot be controlled by CsA, or CsA may cause hypercholesterolemia or induce overexpression of transforming growth factor-beta (TGF-beta). This review also compares the effect of CsA with other immunosuppressants in allograft artery preservation and their clinical efficacy.