Comparison of the effects of smokeless tobacco extract with the effects of prolyl hydroxylase inhibitors on collagenous and noncollagenous protein synthesis by osteoblasts

Calcium hydroxide ameliorates tobramycin toxicity in cultured chick tibiae

Results from this laboratory have shown that bone metabolism is directly related to extracellular pH and that high concentrations of tobramycin released from impregnated polymethylmethacryrate (PMMA) beads has pH-dependent toxic effects on bone. In the present study, beneficial effects of calcium hydroxide-impregnated PMMA were investigated regarding tobramycin toxicity and bone metabolism in chick embryo tibiae in vitro. Also using Ca(OH)2 as a pH regulator, the antibiotic efficacy of tobramycin-impregnated PMMA was evaluated with respect to inhibition of Staphylococcus aureus growth. When Ca(OH)2 was added to PMMA beads containing tobramycin, the beads released hydroxyl and calcium ions into the culture medium and released more antibiotic than beads containing only tobramycin. Bone metabolism (glycolysis, total protein synthesis, and collagen synthesis) was enhanced by Ca(OH)2-impregnated beads with or without tobramycin. Additionally, bacterial growth was inhibited more strongly when S. aureus was incubated with tobramycin- and Ca(OH)2-impregnated PMMA disks than with disks containing only tobramycin. This study demonstrates the feasibility of adding Ca(OH)2 to tobramycin-impregnated PMMA beads as a regulator of local pH and a promoter of bone metabolism for protection of bone when high concentrations of tobramycin are used to treat osteomyelitis. It also suggests that lower concentrations of antibiotic may be effective if Ca(OH)2 and tobramycin are administered simultaneously.

Interaction of tobramycin and pH in cultured chick tibiae

The toxicity of tobramycin at concentrations released from antibiotic-impregnated polymethylmethacrylate beads was determined using cultured embryonic chick tibiae. Because previous results from this laboratory have shown that osteoblast metabolism is inhibited at low pH and because of the potential for a low local pH in infected bone, the antibiotic was tested in medium with pH values from 6.8 to 7.4. Bone metabolism was evaluated by measuring the rates of glycolysis (lactate production), protein synthesis ([3H]proline uptake), and collagen synthesis ([3H]proline hydroxylation). Tobramycin at the concentrations released from the beads (1.0-1.5 mg/ml) contributed to lowering the pH of the medium. At pH 7.4, the antibiotic produced as much as a 30, 39, and 48% decrease in glycolysis, protein synthesis, and collagen synthesis, respectively. Tibiae exposed to tobramycin for 3 days, followed by 2 days without the antibiotic, showed only a partial recovery from its toxic effects. Although tobramycin was relatively less toxic in an acidic environment, the overall metabolic activity of the bones was poorest when the tobramycin concentration was high (2.0 mg/ml) and pH was low (6.8). The results of this study support the following conclusions: (a) tobramycin at high concentrations is toxic to bone, (b) a combination of high tobramycin concentration and low pH in the bone microenvironment may greatly inhibit bone metabolism, and (c) treatment and prevention of osteomyelitis by means of tobramycin-impregnated beads may be augmented by preventing the pH from decreasing in traumatized areas.

The effects of smokeless tobacco extract on bone nodule formation and mineralization by chick osteoblasts in vitro

Short-term exposure to smokeless tobacco extracts (STE) reportedly inhibits osteoblast metabolism. The objective of this study was to determine the effects of serial dilutions of a water-soluble extract of smokeless tobacco on osteoblast proliferation and their potential to form and mineralize bone nodules. STE significantly stimulated cell proliferation when diluted 10(2)-10(4) times; 10(3) and 10(4) dilutions produced the greatest effect. 10(2)-10(4) STE dilutions significantly increased alkaline phosphatase activity at day 7 but 10(6) STE dilutions significantly decreased it. 10(3) and 10(4) dilutions significantly increased bone nodule formation, but inhibited their mineralization. In contrast, 10(5) and 10(6) dilutions significantly decreased bone nodule formation, but increased their mineralization. Stimulation of in vitro bone nodule formation by STE was similar to that produced by 10(-7) M insulin-like growth factor 1 (IGF-1) in vivo. Heat and acid treatment of STE significantly reduced its beneficial effect on cell proliferation, suggesting that a peptide within STE may be responsible for enhancement of osteogenic cell proliferation. Thus, STE may contain a peptide capable of significantly stimulating osteoblast proliferation, differentiation and metabolism, similar to the effects of IGF-1. This peptide could have potential therapeutic benefits.

In vitro inhibition of collagen formation by 2,4-pyridine dicarboxylate and minoxidil in rabbit corneal fibroblasts

Two anti-fibrotic agents were evaluated in vitro for potential application in trabeculectomy. The rate of collagen formation by cultured rabbit corneal fibroblasts was determined by [3H]proline uptake and hydroxylation assay. Incubation of fibroblasts, for 96 hours, with 16 mM 2,4-pyridine dicarboxylic acid (2,4-PDCA), a competitive inhibitor of prolyl 4-hydroxylase, decreased [3H]OH-proline formation from control by 28.2 +/- 3.9%; (average +/- S.E.M), whereas 10 mM minoxidil, an antihypertensive pyrimidine oxide and a lysyl hydroxylase inhibitor, induced a decrease of 17.3 +/- 4.5%. Incubating fibroblasts with a mixture of the two inhibitors, 16 mM 2,4-PDCA and 10 mM minoxidil, resulted in a further decrease in [3H]proline incorporation of 40.7 +/- 5.1%. After 96 hours of incubation with the inhibitors, rabbit corneal fibroblast growth was decreased from control by 17.2 +/- 4.2% and 10.5 +/- 4.5% for 16 mM 2,4-PDCA and 10 mM minoxidil, respectively. These effects were dose dependent. The results suggest that both 2,4-PDCA and minoxidil have an inhibitory effect on collagen formation and may be useful in delaying surgical wound healing.

Smokeless tobacco contains an inhibitor of prolyl hydroxylase activity

The effects of smokeless tobacco extract (STE) and various constituents of STE on prolyl hydroxylase activity were determined using enzyme extracted from chick embryos. STE inhibited prolyl hydroxylase activity in a concentration-dependent manner, but nicotine and anabasine had essentially no effect. Enzymatic activity was inhibited by zinc, but not by the other inorganic elements in STE; however, the zinc concentration in STE was not high enough to produce the observed inhibition. The inhibition by STE was diminished by increasing concentrations of 2-oxoglutarate, but not by increasing concentrations of other cofactors. Thus, STE contains an inhibitor of prolyl hydroxylase which may be competitive with 2-oxoglutarate.