Accelerated coronary artery transplant arteriosclerosis is the major cause of sudden death or retransplantation three months post-transplant. Estrogen treatment appears to protect against cardiovascular disease in post menopausal women, in whom it is associated with a 50 percent decrease in cardiovascular mortality. Experimentally, the investigators find estradiol treatment protects against transplant arteriosclerosis. The mechanisms may be numerous and involve both growth factors and cytokines. However, this proposal will focus on insulin like growth factor I (IGF-I), since it is emerging as a crucial progression factor in the smooth muscle cell (SMC) cycle and is abundant in myointima in transplant arteriosclerosis. Furthermore, a decrease in vascular wall IGF-I is associated with a decrease in transplant arteriosclerosis. Preliminary studies show estradiol abolishes IGF-I induced cell proliferation in vascular explants of the allograft and in cultured SMC. The hypothesis is that estrogen inhibits transplant arteriosclerosis by its inhibition of IGF-I expression. The investigators postulate that rejection is accompanied by IGF-I upregulation by cytokines such as interleukin-6 (IL-6). Estrogens may in addition down regulate IL-6 and other cytokines expressed during rejection.
The specific aims are 1) to establish in a dose dependent manner that 17-beta estradiol inhibits transplant arteriosclerosis in the female rabbit heterotopic cardiac transplant model; 2) to establish that the protective effect is mediated through the nuclear estrogen receptor by using a specific estrogen receptor antagonist; 3) to confirm and extend that exogenous IGF-I will accelerate transplant arteriosclerosis and that estrogen will inhibit this IGF-I induced transplant arteriosclerosis; 4) to determine in the transplant model a direct effect of estradiol on a) IGF-I transcription and possible effects on translation by measuring mRNA IGF-I and IGF-I protein; and on b) IL-6 gene transcription and translation; and finally to explore whether the abrogation of mitogenic response of the graft to IGF-I and IL-6 following chronic estradiol treatment of the recipient is related to decreased IGF-I receptor expression or by inhibition of the IGF-I signalling pathway by determining the state of phosphorylation of IGF-I receptor and its proximal signalling molecule IRS-1. The long-term goal is to provide experimental data to support the benefit of estrogen replacement therapy in postmenopausal women receiving cardiac and other organ transplants. Rat and rabbit transplant models will be used.
