Fifty percent of women more than the age of 50 are projected to break a bone at some point during their life, and that statistic is part of what puts the osteoporosis drug Fosamax (alendronate sodium, Merck & Co. Inc.) among the top 20 selling drugs in the U.S. The drug works by inhibiting osteoclasts, a type of macrophage involved in regulating bone density, from breaking down bone cells.
Earlier this week, Merck announced that it has been granted FDA approval for a combination regimen of Fosamax and vitamin D; Fosamax itself has been on the market since 1995.
Why vitamin D? Well, vitamin D helps increase the absorption of dietary calcium. Calcium is a highly popular dietary supplement; an estimated 20 percent of U.S. adults take calcium-containing supplements daily. There also is the cottage industry of fortification, which has added calcium to everything from orange juice to goldfish snack crackers.
Meanwhile, basic research continues to unravel the mechanisms of bone formation and turnover. In the March 31, 2005, issue of the Journal of Biological Chemistry, scientists from the Van Andel Research Institute in Grand Rapids, Mich.; the University of Alabama at Birmingham; the University of Kentucky at Lexington; and Boston's Beth Israel Deaconess Medical Center reported an unexpected new way to regulate osteoclasts: by targeting osteoblasts.
Osteoblasts have the opposite function of osteoclasts - they are precursor cells responsible for laying down bone. From individuals with a defective wnt co-receptor, it's known that the wnt pathway can influence bone formation, but the molecular mechanisms were unclear. However, the wnt pathway also is known for its importance in development.
"Basically, it is conserved in all multicellular organisms, and during development, activation of the wnt pathway is necessary for the development of just about any organ," Bart Williams, scientific investigator at the Van Andel Research Institute, told BioWorld Today. The pathway also is known to play a role in cancer; the current focus of Williams' laboratory, in fact, is the study of whether aberrant wnt signaling plays any role in the predisposition of some tumor types to metastasize in the bone.
The wnt receptor, also known as Frizzled, seems to have dozens of activators and dozens of effectors, but one of the most-studied intracellular consequences of wnt pathway activation is turning on the protein beta-catenin. To see whether that pathway is the one affecting bone formation, the researchers constructed osteoblast-specific knockouts for either beta-catenin or Apc, a protein with the main function of degrading beta-catenin.
Global beta-catenin knockouts die during early embryonic development, and even in osteoblast-specific knockouts, the phenotype was pretty striking.
"Basically, beta-catenin knockouts lived for four to five weeks and had too little bone, and Apc knockouts lived for 10 to 14 days and had too much bone," Williams said.
In vitro experiments with knockout osteoblast cells showed that the extreme effects of knocking out beta-catenin and Apc were unlikely to be due to effects on the osteoblasts themselves, as they proliferated fairly normally. Several cellular markers for their maturation were decreased, suggesting that the cells do not proceed along their developmental pathway in an entirely normal fashion; but the cellular effects of knocking out either beta-catenin or Apc were fairly mild, at least given the animals' drastically reduced life expectancy.
The researchers found a possible reason for that discrepancy by looking at the effects that knocking out beta-catenin or Apc might have on osteoclasts. Williams and his colleagues found that osteoclasts were increased in beta-catenin and decreased in Apc knockouts. Further experiments revealed that the osteoblast wnt pathway regulates osteoclasts via the production of two proteins: RANKL and OPG, both of which are secreted by osteoblasts and bind to osteoclasts. The levels of both proteins were altered in the beta-catenin and Apc knockouts.
"OPG is essentially a dummy receptor; if the osteoclast binds OPG it can't bind RANKL," Williams said. "So OPG protects bone, and together, the ratio [of OPG and RANKL] control the amount of bone."
Neither RANKL's nor OPG's ability to regulate osteoclasts is news; in fact, Amgen Inc., of Thousand Oaks, Calif., is in Phase III trials with AMG 162, a monoclonal antibody that inhibits RANKL.
"What's unexpected, and what surprised us," Williams said, "is that osteoblasts are feeding into this process via the wnt pathway."
The research has several possible applications in drug discovery. For one thing, Williams said he and his collaborators are in discussions with biotech and pharmaceutical companies to use the mice to test compounds for possible therapeutic effects in osteoporosis.
Also, the mechanism described targets both a different cell type and a different pathway than current drugs such as Fosamax. So, Williams said, "if it pans out in the clinic, it would be a complementary approach."