As noted by the National Research Council, “[p]erhaps the single clearest effect of fluoride on the skeleton is its stimulation of osteoblast proliferation.” (NRC 2006). Osteoblasts are bone-forming cells. “Stimulatory effects of fluoride on osteoblasts result in formation of osteoid, which subsequently undergoes mineralization.” (Fisher RL, et al. 1989). If the new osteoid tissue is subsequently mineralized, an increase in bone density will result. As numerous studies have shown, however, fluoride-induced osteoid does not always get mineralized, leading to an increased amount of unmineralized bone (“hypo-mineralization”) — which may make the bone more susceptible to fracture. When bone contains too much unmineralized osteoid tissue, a condition known as osteomalacia (in adults) and rickets (in children) results.

Some researchers speculate that the delay/failure of fluoride-induced osteoid to undergo mineralization is a demonstration that fluoride has a stimulatory effect on osteoblasts’ precursor cells (thus triggering an increased number of osteoblasts), but a toxic effect on the osteoblast cells themselves (thus resulting in less functional osteoblasts). According to one researcher:

“though the osteoblasts in fluoride-affected bone are active, there are fewer plump, cuboidal, highly secretory osteoblasts, suggesting that, whereas fluoride is mitogenic to osteoblastic precursors, it is toxic to individual osteoblasts at the same concentration.”
SOURCE: Chachra D, et al. (1999). The effect of fluoride treatment on bone mineral in rabbits. Calcified Tissue International 64:345-351.

The mechanism by which fluoride stimulates osteoblast proliferation is not yet understood, although several theories have been proposed (see below). Whatever the mechanism, fluoride’s capacity to stimulate cell proliferation in bone tissues increases the likelihood that fluoride, a known mutagen, could induce a cancerous change. As noted by the National Research Council, “Because fluoride stimulates osteoblast proliferation, there is a theoretical risk that it might induce a malignant change in the expanding cell population. This has raised concerns that fluoride exposure might be an independent risk factor for new osteosarcomas.”

General Background

“The effect on osteoblasts was surmised from clinical trials in the early 1980s documenting an increase in vertebral bone mineral density that could not be ascribed to any effect of fluoride on bone resorption. Biopsy specimens confirmed the effect of fluoride on increasing osteoblast number in humans. . . . The demonstration of an effect of fluoride on osteoblast growth in vitro was first reported in 1983 in avian osteoblasts.”
SOURCE: National Research Council. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. National Academies Press, Washington D.C. p 109.

“Studies in vitro suggested that the increased (bone) formation may be due to a direct effect of fluoride in stimulating osteoblast proliferation.”
SOURCE: Lundy MW, et al. (1995). Histomophometric analysis of iliac crest bone biopsies in placebo-treated versus fluoride-treated subjects. Osteoporosis International 5:115-129.

“The anabolic effect of fluoride therapy is dependent on its cellular effects on the osteoblast lineage.”
SOURCE: Boivin G, et al. (1993). Relationship between bone fluoride content and histological evidence of calcification defects in osteoporotic women treated long term with sodium fluoride. Osteoporosis International 3:204-208.

“Fluoride is one of the most potent but least well understood stimulators of bone formation in vivo. Bone formation was shown to arise from direct effects on bone cells. Treatment with sodium fluoride increased proliferation and alkaline phosphatase activity of bone cells in vitro and increased bone formation in embryonic calvaria at concentrations that stimulate bone formation in vivo.”
SOURCE: Farley JR, et al. (1983). Fluoride directly stimulates proliferation and alkaline phosphatase activity of bone-forming cells. Science 222: 330-332

Fluoride Has Toxic Effect on Osteoblasts (which May Explain Failure of osteoid to Properly Mineralize)

“The increased amount of trabecular bone in fluoride therapy is claimed to be the morphologic expression for fluoride as a stimulus for bone formation. We propose that the increased amount of trabecular bone results from pathological bone formation by injured osteoblasts and decreased bone resorption by resorbing osteocytes and osteoclasts.”
SOURCE: Krook L, Minor RR. (1998). Fluoride and alkaline phosphatase. Fluoride 31: 177-182.

“though the osteoblasts in fluoride-affected bone are active, there are fewer plump, cuboidal, highly secretory osteoblasts, suggesting that, whereas fluoride is mitogenic to osteoblastic precursors, it is toxic to individual osteoblasts at the same concentration.”
SOURCE: Chachra D, et al. (1999). The effect of fluoride treatment on bone mineral in rabbits. Calcified Tissue International 64:345-351.

In fluoride-treated rats there was a “tendency for the mineral apposition rate to decrease and for the osteoid maturation time to increase, suggesting that a toxic effect on osteoblast function or on mineralization had occurred…. At the tissue level mineralized bone formation rate tended to fall, indicating that the inhibition of osteoblast function had outweighed the stimulatory effect on osteoblast proliferation.”
SOURCE: Ittel TH, et al. (1992). Effect of fluoride on aluminum-induced bone disease in rats with renal failure. Kidney International 41: 1340-1348.

“Long-term effects of fluoride may include decreases in the life span and number of bone cells rather than a persistent hypercellurarity of bone forming cells.”
SOURCE: Ittel TH, et al. (1992). Effect of fluoride on aluminum-induced bone disease in rats with renal failure. Kidney International 41: 1340-1348.

“Skeletal fluorosis is thus characterized by an unbalanced coupling in favor of bone formation, and a great number of osteoblasts with a high proportion of flat osteoblasts. This may explain the mineralization impairment proven by thick osteoid seams and reduced mineral apposition rate, and supports the view that fluoride may have a dual effect on osteoblasts: a probable increased birthrate at the tissue-level due to a mitogenic effect of fluoride on precursors of osteoblasts, and a toxic effect at the individual cell-level. The addition of these two effects represents, however, a marked increase of bone formation at the organ level.”
SOURCE: Boivin G, et al. (1989). Skeletal fluorosis: histomorphometric analysis of bone changes and bone fluoride content in 29 patients. Bone 10:89-99.

“There is some evidence that fluoride at high concentrations may exert a toxic effect on osteoblast function. Following a long-term exposure to fluoride, especially at high doses, osteoblasts assume a flat, inactive appearance… Following long-term exposure to fluoride, the amount of osteoid surfaces covered by osteoblasts are decreased, suggestive of reduced osteoblastic activity. ”
SOURCE: Pak CY. (1989). Fluoride and osteoporosis. Proceedings of the Society for Experimental Biology and Medicine 191: 278-86.

“one possible mode of action of NaF may be that, initially, its administration may stimulate osteoblasts to produce more bone in relation to bone removed by osteoclasts. However, within a short time interval, toxic effects may begin to appear and thus result in the observed decreases in bone formative activity. Since osteoclasts are also affected by NaF administration, with a longer passage of time, further increases in bone mass may not occur. The preservation of existing bone mass may be attributable to decreases in numbers of bone cells, in their functional efficiencies, and in their individual life-spans resulting presumably from cellular toxic effect of NaF.”
SOURCE: Snow GR, Anderson C. (1986). Short-term chronic fluoride administration and trabecular bone remodeling in beagles: a pilot study. Calcified Tissue International 38: 217-221.

“osteoblasts that survive as osteocytes are visibly abnormal.”
SOURCE: Riggs BL. (1983). Treatment of osteoporosis with sodium fluoride: An appraisal. Bone and Mineral Research 2: 366-393.

“The osteoblasts were atrophic in the present material.”
SOURCE: Krook L, Maylin GA. (1979). Industrial fluoride pollution. Chronic fluoride poisoning in Cornwall Island cattle. Cornell Veterinarian 69(Suppl 8): 1-70.

“cytologic abnormalities found in osteoblasts.”
SOURCE: Baylink DJ, Bernstein DS. (1967). The effects of fluoride therapy on metabolic bone disease. Clinical Orthopaedics and Related Research 55: 51-85.

“Mottling and osteomalacia were associated with changes in the osteoblasts. On moderate levels, osteoblasts that survived were abnormal, as seen in mottling. On a high fluoride levels, the osteoblasts were devoid of demilunes, spindled or very pyknotic.”
SOURCE: Johnson LC. (1965). Histogenesis and mechanisms in the development of osteofluorosis. In: H.C.Hodge and F.A.Smith, eds : Fluorine chemistry, Vol. 4. New York, N.Y., Academic press (1965) 424-441.

proposed Mechanisms by which Fluoride Stimulates Osteoblast Proliferation

“Understanding the subcellular signaling mechanisms by which fluoride affects osteoblasts is of paramount importance. Information in this area has the potential to determine whether the fluoride effects are specific, whether toxicity is an issue, and what concentration may influence bone cell function. Moreover, as the pathways become more clearly defined, other targets might emerge. Two hypotheses in the literature describe the effect of fluoride. Both state that the concentration of tyrosine phosphorylated signal pathway intermediates is elevated after fluoride exposure. However, the means by which this occurs differs in the hypotheses. One view is that fluoride blocks or inhibits the activity of a phosphotyrosine phosphatase, thereby increasing the pool of tyrosine-phosphorylated proteins. The other view supports an action of fluoride (along with aluminum) on the stimulation of tyrosine phosphorylation that would also increase the pool of tyrosine-phosphorylated proteins. In the first hypothesis, growth factor activation of the Ras-Raf-MAP kinase pathway would involve stimulation of phosphotyrosine kinase activity. This is mediated by a family of cytosolic G proteins with guanosine triphosphate acting as the energy source. In the presence of fluoride, a sustained high concentration of tyrosine-phosphorylated proteins would be maintained because of the inability of the cell to dephosphorylate the proteins. This theory implicates the existence of a fluoride-sensitive tyrosine phosphatase in osteoblasts. Such an enzyme has been identified and purified. It appears to be a unique osteoblastic acid phosphatase-like enzyme that is inhibited by clinically relevant concentrations of fluoride (Lau et al. 1985, 1987, 1989; Wergedal and Lau 1992). The second hypothesis supports the belief that an AlFx complex activates tyrosine phosphorylation directly. Data from this viewpoint indicate that fluoride alone does not stimulate tyrosine phosphorylation but rather that it requires the presence of aluminum (Caverzasio et al. 1996). The purported mechanism is that the MAP kinase pathway is activated by AlFx, which triggers the proliferation response. A novel tyrosine kinase, Pyk2, has been identified that is known to be activated by AlFx through a G-protein-coupled response and might be responsible for this effect (Jeschke et al. 1998). Two key pieces of evidence that support a G-protein-regulated tyrosine kinase activation step in the fluoride effect are that the mitogenic effect of fluoride can be blocked by genistein (a protein tyrosine kinase inhibitor) and pertussis toxin (a specific inhibitor of heterotrimeric G proteins) (Caverzasio et al. 1997; Susa et al. 1997).
At least two other potential mechanisms deserve mention. Kawase and Suzuki (1989) suggested that fluoride activates protein kinase C (PKC), and Farley et al. (1993) and Zerwekh et al. (1990) presented evidence that calcium influx into the cells might be a signal for the fluoride-mediated stimulation of proliferation.
In summary, the in vitro effects of fluoride on osteoblast proliferation appear to involve, at the least, a regulation of tyrosine-phosphorylated proteins. Whether this occurs through activation of MAP kinases, G proteins, phosphatases, PKC, or calcium (or a combination) remains to be determined. Whatever the mechanism, however, it is evident that fluoride has an anabolic activity on osteoblasts and their progenitors.”
SOURCE:  National Research Council. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. National Academies Press, Washington D.C. p 110.

“The mechanism whereby NaF acts to stimulate bone cell proliferation and differentiation is controversial in that there are currently two major competing models, both of which involve activation of mitogen-activated protein kinase (MAPK). On the one hand, we postulated that NaF, at osteogenic concentrations, inhibits a fluoride-sensitive protein-tyrosine phosphatase in osteoblasts, resulting in the sustained increase of protein-tyrosine phosphorylation of key signaling proteins of the MAPK mitogenic pathway. This would then lead to the potentiation of bone cell proliferation initiated by growth factors. On the other hand, Caverzasio et al. proposed an alternate model in which NaF acts through the formation of the fluoroaluminate ion (AlF4) with the aluminum ion. The AlF4 ion would then activate a pertussis toxin-sensitive heterotrimeric Go/i protein, leading to an activation of certain cellular protein-tyrosine kinases. This, in turn, would increase the tyrosine phosphorylation of key signaling proteins of the MAPK signaling pathway, subsequently leading to increased osteoblast proliferation.”
SOURCE: Lau KH, et al. (2002). Bone cell mitogenic action of fluoroaluminate and aluminum fluoride but not that of sodium fluoride involves upregulation of the insulin-like growth factor system. Bone 30: 705–711.

“The first observation that fluoride can directly influence the activity of osteoblastic cells in culture was provided by Farley and coworkers in 1983. They showed that micromolar concentrations of fluoride increased the proliferation and alkaline phosphatase activity of bone cells dervied from chick embryonic calvaria… Similar observations were then reported by the same group in human bone cells derived from the trabecular bone of femoral head samples obtained during hip replacement. Following these initial observations, the in vitro effects of fluoride on osteoblastic cells were investigated in several laboratories. However, it became apparent that the in vitro effects of fluoride on cultured osteoblasts were difficult to reproduce in similar or other cultured osteoblast-like systems. Two laboratories reported their negative results on the direct effect of fluoride on human osteoblastic proliferation. From these negative results… it was concluded that, in vivo, fluoride may either act indirectly through the local synthesis of some growth factors or exerts a preferential action on a subpopulation of osteoblastic cells. In favor of the latter hypothesis, it has been suggested that osteoblast precursors are more sensitive to fluoride action than mature osteoblasts. The possibility that in vivo the effect of fluoride would be mediated by a cofactor has also been evoked.”
SOURCE: Caverzasio J, et al. (1998). Fluoride: mode of action. Bone 22: 585-589.

“It was found that aluminum is an important cofactor for the expression of the mitogenic effect of fluoride in osteoblast-like cells… In the presence of traces of aluminum, fluoride concentrations close to those measured in plasma of osteoporotic patients treated with fluoride salts reproducibly enhanced cell proliferation. In vivo studies indicated that the combination of fluoride and aluminum was also more efficient than administration of each ion separately in increasing tibia bone mineral mass in the adult ovariectomized rats. These observations strongly suggested that a fluoroalumino complex is probably the active fluoride molecule responsible for the enhancement of the proliferation of bone-forming cells and the change in bone mineral mass in vivo.”
SOURCE: Caverzasio J, et al. (1998). Fluoride: mode of action. Bone 22: 585-589.

“Mode of action as well as toxic events may be related to the inhibitory or activating effects of fluoride on a variety of cellular enzymes. It is of particular interest that aluminum forms strong complexes with fluoride which may participate in the spectrum of effects previously ascribed to fluoride. Since both ions are mainly excreted by the kidney, an impairment of excretory renal function should facilitate retention of fluoride and aluminum and may thus aggravate mutual effects.”
SOURCE: Ittel TH, et al. (1992). Effect of fluoride on aluminum-induced bone disease in rats with renal failure. Kidney International 41: 1340-1348.

“These findings… provide additional support for our hypothesis that F stimulates osteoblast-line cell proliferation by potentiating the mitogenic actions of growth factors.”
SOURCE: Lau KH, et al. (1989). A proposed mechanism of the mitogenic action of fluoride on bone cells: inhibition of the activity of an osteoblastic acid phosphatase. Metabolism 38:858-68.

“The mechanism by which fluoride stimulates osteoblasts is unclear. Both cellular and noncellular mechanisms have been suggested… Clearly, however, noncellular and cellular mechanisms are not mutually exclusive.”
SOURCE: Riggs BL. (1983). Treatment of osteoporosis with sodium fluoride: An appraisal. Bone and Mineral Research 2: 366-393.