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Endocrine Research, Unit Division of Endocrinology and Metabolism, Department of Internal Medicine, Mayo Clinic College of Medicine, Rochester, Minnesota 55905
Address all correspondence and requests for reprints to: Dr. Sundeep Khosla, Endocrine Research Unit, Mayo Clinic, 200 First Street SW, 5-194 Joseph, Rochester, Minnesota 55905. E-mail: khosla.sundeep{at}mayo.edu.
In contrast to the age-old tradition of trying empiric, often unproven remedies for various maladies (which, regrettably, is still practiced today with alarming frequency in some segments of the "health care" industry), the holy grail of modern investigative medicine is to identify physiological pathways, define how these are altered in various disease states, and then develop drugs that specifically target, and hopefully reverse, the abnormality causing illness. In this issue, Morony and colleagues (1) describe the use of a molecule, osteoprotegerin (OPG), that illustrates precisely this route of drug development. Using an experimental model for the syndrome of hypercalcemia of malignancy (HHM), they compare OPG to a traditional remedy, bisphosphonates, which, although certainly an effective treatment for this disorder, were developed the other way around; they were first found to work in certain skeletal disorders, but their mechanisms of action remain an area of active investigation. As perhaps a validation for those of us who base our careers on trying to understand disease pathways, the new, mechanism-based drug did better than the drugs discovered empirically.
The disorder in question is HHM, which, depending on the prognosis of the underlying tumor, can either lead to a peaceful coma and perhaps comfortable death or be a severe impediment to maximizing the quality of remaining life. Like most pivotal concepts in the field of bone and calcium metabolism, it was Fuller Albright in 1941 (2) who hypothesized that a malignant tumor might release a systemically active factor, resulting in hypercalcemia. This hypothesis was unequivocally proven some 40 yr later in a landmark study by Stewart et al. (3), who demonstrated that 41 of 50 consecutive patients with cancer-associated hypercalcemia had elevated nephrogenous cAMP excretion (often used as a marker for PTH or PTH-like activity on the kidney), similar to patients with primary hyperparathyroidism; however, unlike patients with primary hyperparathyroidism, HHM patients had low levels of circulating PTH. This led to the inescapable conclusion that, as usual, Fuller Albright had been correct. However, the offending agent was not PTH per se, but, rather, something in the circulation of patients with malignancy-associated hypercalcemia that acted like PTH on kidney and bone. In a surprisingly short period of time, given that modern cloning techniques were still evolving, PTHrP was identified as the factor causing hypercalcemia in a significant proportion of patients with hypercalcemia of malignancy (4, 5).
Both PTH and PTHrP bind and activate the classic G protein-coupled PTH receptor (PTH1R) (6) present on renal tubular cells and osteoblasts. It had been known for a long time that osteoblastic cells were necessary for providing a signal to osteoclasts to resorb bone (7), but the factor(s) produced by osteoblasts that appeared to be critical for osteoclast development remained largely unknown until a somewhat accidental finding (even the new paradigm of drug discovery requires, it seems, a bit of luck) by the gene discovery group at Amgen. Using an admittedly "brute force" approach, these investigators were making transgenic mice using cDNAs encoding different TNF receptor-related molecules, and one of the transgenic lines had a marked increase in skeletal radiodensity (osteopetrosis) (8). Additional analysis of these mice revealed that the osteopetrosis was due to a profound decrease in osteoclasts, indicating that the particular protein encoded by that cDNA, which they termed OPG (short for osteoprotegerin, i.e. to protect bone) played a decisive role in regulating osteoclastogenesis. The rest, as is often said, is history, and the entire OPG/receptor activator for nuclear factor-
B (RANK) ligand (RANKL)/receptor activator for nuclear factor-B system was soon unraveled, solving a long-standing mystery plaguing bone biologists, namely, just how osteoblastic cells controlled the formation of osteoclasts.
These findings also shed additional insights into the pathogenesis of HHM. Figure 1
depicts a working model of the mechanisms causing HHM mediated by PTHrP, which appears to be involved in the majority of cases of malignancy-associated hypercalcemia (9). A variety of tumors either produce and release PTHrP systemically into the circulation or, when present in the bone marrow, at least into the bone microenvironment. Circulating PTHrP stimulates renal tubular reabsorption of calcium and both circulating and locally produced PTHrP, by activating the PTH/PTHrP receptor on osteoblast lineage cells, stimulates RANKL, and suppresses OPG production by these cells (10, 11, 12). RANKL binds its cognate receptor, RANK, on preosteoclastic cells (13), leading to osteoclastogenesis and increased bone resorption. Because OPG binds and neutralizes RANKL (14), the concomitant stimulation of RANKL and suppression of OPG after activation of the PTH/PTHrP receptor leads to rampant osteoclast development. As shown in Fig. 3B of the paper by Morony and colleagues in this issue (1), these osteoclasts are truly nasty cells, with a voracious appetite for destroying bone.
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Things in life and in science are, however, never quite as good as they seem. Although clinical trials using OPG-Fc were initiated in patients with multiple myeloma, they were subsequently suspended due, it appears, to a patient developing antibodies to the OPG-Fc. Amgen has now moved on to develop a human monoclonal antibody to RANKL (AMG162) (15) that appears to be equally effective, and this is a promising new therapy for osteoporosis as well as HHM and other osteoclast-mediated disorders.
In summary, although the paper of Morony and colleagues (1) is interesting and important in its own right, the scientific underpinnings of this work can be traced back over 60 yr to the prophecies of Fuller Albright. Perhaps that, in the end, is what makes the entire process of scientific discovery so fascinating. It is probably not an overstatement to say that if examined closely, virtually any work published in this issue of Endocrinology could be shown to rest on the shoulders of myriad investigators who came before with their flashes of insight combined, as always, with a bit of luck.
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Footnotes |
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B; RANKL, receptor activator for nuclear factor-B ligand. Received May 6, 2005.
Accepted for publication May 10, 2005.
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