Use of Small Molecule Manipulation of the Wnt/ β‐catenin and Smad/BMP Pathways for Promotion of Bone Growth

Clark Roth

 Introduction. Non-union is a serious complication of a fracture characterized by an arrest in the fracture repair process. It is estimated that 10% percent of long bone fractures progress to non-union and that a total of 100,000 fractures go on to non-union yearly in the United States. 1 Non-unions are difficult to treat and have a high financial impact due to their propensity to cause long term disability. 2 Risk factors known to increase incidence of non-union include: complicated or open fractures, use of nicotine in any form, anemia, diabetes, poor nutrition, hypovitaminosis D, use of anti-inflammatory drugs, and old age. 1,2,7 It is generally accepted that the progression to a non-union in most cases represents a multifactorial process but dysregulation of the Wnt, BMP, and TGF-b pathways has been implicated. 1-7  We studied the potential for small molecule manipulation of these pathways in order to stimulate bone growth to aid in the treatment of fracture nonunion. Methods. The small molecules we found to show promise for promotion of osteogenesis were Betulinic acid, mollugin, berberine, and tricin. 8-11 Alizarin red assay was used to assess the ability of each molecule to promote calcium deposition. Luciferase assay was performed on the molecules acting on the Wnt/b-catenin pathway (Tricin, Berberine) to assess their ability to modulate the downstream transcription factor Lef1. Betulinic acid and Mollugin were each tested in vivo (Zebrafish & mouse models respectively). Results. Berberine was shown to enhance Run2x expression by activating the canonical Wtn/b-catenin pathway.8 Mollugin enhanced  BMP-2 mediated osteoblast differentiation and was able to promote ectopic bone formation in a mouse spine.9 Tricin promoted proliferation and mineralization of MSCs via activation of the Wnt/b-catenin pathway.10 Betulinic acid stimulated the Smad 1/5/8 and p38 pathway resulting in ectopic bone formation in zebrafish.11 Conclusions. Each of the four small molecules showed the potential to promote bone growth via the activation of endogenous growth pathways. Much remains to be studied concerning the potential of adverse effects with the use of pharmacological manipulation of bone growth though animal studies have thus far shown minimal side effects.

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