To live an extended, healthy life is a human dream that scientists are striving to make come true through a better understanding of underlying mechanisms. Cheryl A. Conover, Ph.D., of the Division of Endocrinology, Diabetes, Metabolism and Nutrition at Mayo Clinic in Rochester, Minn., is one of those scientists. On the basis of recent studies from her laboratory, she proposes pregnancy-associated plasma protein-A (PAPP-A) as a therapeutic target for aging and age-related disease.
Dr. Conover explains: "PAPP-A was originally identified as one of four placentally derived proteins found at high concentrations in the plasma of pregnant women, hence the name pregnancy-associated plasma protein-A. However, the function of this protein remained elusive.
"Twenty-five years later, we discovered that PAPP-A was a novel zinc metalloprotease expressed by various nonreproductive cell types as well, including fibroblasts, vascular smooth muscle cells (SMCs) and osteoblasts. Our in vitro and in vivo studies indicated that PAPP-A functions to enhance the growth-stimulating effects of local insulin-like growth factors (IGFs) through cleavage of inhibitory IGF binding proteins (IGFBPs).
"PAPP-A is a secreted protein that tethers to the surface of cells through proteoglycan moieties. IGF bound to IGFBP-4 is unable to activate receptors. However, on cleavage of IGFBP-4 by PAPP-A, IGF is liberated from the complex in the pericellular environment and IGF signaling is initiated."
The IGFs are associated with cellular and chronological aging, and reduction of IGF-1 signaling has been shown to prolong life span in diverse species. Dr. Conover explains: "Since PAPP-A enhances local IGF-1 action, inhibition of PAPP-A expression or proteolytic activity represents an innovative approach to decreasing IGF availability with moderate restraint of IGF-1 receptor signaling. Thus, if reduction of IGF-1 signaling prolongs the life span, then inhibition of PAPP-A (and consequent reduction in bioavailable IGF-1) should promote longevity.
"To acquire the in vivo model to test this hypothesis, I took a sabbatical with Jan van Deursen, Ph.D., at Mayo Clinic in Minnesota and created a mouse with the PAPP-A gene deleted — the ultimate inhibition of PAPP-A. We showed that these PAPP-A knock-out (KO) mice live 30 to 40 percent longer than do their wild-type littermates. Histopathology indicated the maintenance of a healthy immune system, delayed occurrence of neoplasms, and reduced incidence and severity of degenerative diseases of aging in PAPP-A KO mice, which enabled them to maintain a better health status into old age than wild-type mice."
In 2001, Dr. Conover and colleagues published a paper in The New England Journal of Medicine with the first evidence for PAPP-A as a marker of acute coronary syndromes, which has now been confirmed in multiple clinical trials.
Dr. Conover highlights: "Using immunohistochemistry, PAPP-A was present in eroded and ruptured atherosclerotic plaques from human arterial specimens. The most intense staining for PAPP-A was at the inflammatory shoulder of the ruptured plaque containing activated SMCs and macrophages. There was little or no staining for PAPP-A in stable plaques. These findings suggested that PAPP-A may be synthesized in an autocrine andparacrine fashion by activated cells in unstable plaque.
"We found that primary cultures of human coronary artery SMCs and endothelial cells expressed PAPP-A, which was markedly stimulated by proinflammatory cytokines synthesized by activated macrophages. Although PAPP-A immunostaining colocalized with activated macrophages in unstable atherosclerotic plaque, we showed that macrophages do not express PAPP-A. However, PAPP-A secreted by SMCs readily binds to macrophages, as well as to SMCs. Cell-associated PAPP-A, which remains proteolytically active, appears to serve to effectively promote direct interaction of the freed IGF with receptors."
PAPP-A is clearly associated with vulnerable plaque, but does PAPP-A have an active role in the development of atherosclerosis or in plaque progression or both? Dr. Conover's laboratory addressed this question using the PAPP-A KO mice crossed with apolipoprotein E (ApoE) KO mice, the latter being an established murine model for studies of atherosclerosis.
Dr. Conover summarizes her findings: "We found that ApoE KO mice lacking PAPP-A had a 70 to 80 percent reduction in plaque area compared to ApoE KO mice expressing PAPP-A when fed a high-fat, Western-style diet. Lesion number was not different between the two groups. PAPP-A deficiency appeared to delay progression of lesion development from fatty streak to more advanced plaque. Thus, loss of PAPP-A was clearly beneficial for impedance of plaque development in this mouse model of atherosclerosis."
These and other findings in PAPP-A KO mice have led Dr. Conover to propose PAPP-A as a novel drug target for promoting an extended, healthy life span. Dr. Conover explains: "Of course, we would not be knocking out the PAPP-A gene in humans, but we are developing therapies to inhibit PAPP-A's proteolytic activity." The long-term goals of this laboratory include:
Bayes-Genis, et al. Pregnancy-associated plasma protein A as a marker of acute coronary syndromes. The New England Journal of Medicine. 2001;345:1022.