Paradoxical toxic and trophic osseous actions of aluminum: potential explanations

The osseous effects of aluminum are paradoxical. Aluminum displays toxic effects on bone metabolism in humans but in animal and cell culture models aluminum can exert both toxic and trophic effects. Clinical toxicity to aluminum is typically expressed as vitamin D-resistant osteomalacia and diminished bone remodeling activity. Animal models that mimic these toxic actions of aluminum, however, are difficult to establish. In this regard, aluminum deposition in bone of animals does not necessarily impair mineralization. When aluminum administration to animals results in defective mineralization, cure of osteomalacia can often be achieved by administration of vitamin D. Moreover, aluminum administration has been shown to stimulate de novo bone formation in dogs. Similar disparate actions of aluminum have also been observed in osteoblast culture systems. Aluminum is mitogenic in some and antiproliferative in other osteoblast cell lines. The mechanism(s) underlying these disparate effects of aluminum are unknown. The complexity of the aluminum osseous actions may result from differential responses of preosteoblasts and mature osteoblasts to aluminum and/or modulation of these direct cellular effects by interactions with systemic and/or local factors such as parathyroid hormone, vitamin D and/or other bone growth factors. In addition, differences in speciation of aluminum could contribute to its differential actions on bone. Regardless, understanding the basis for these differential osseous manifestations to aluminum may lead to a better knowledge of aluminum actions which could result in the development of pharmacologic agents to induce neoosteogenesis without adverse consequences as well as development of strategies to limit aluminum toxicity.