Atypical fracture of the tibial diaphysis associated with bisphosphonate therapy: a case report.

Atypical subtrochanteric femoral shaft fractures (AFFs) have recently emerged as a potential long-term complication of bisphosphonate therapy. In 2010, the American Society for Bone and Mineral Research (ASBMR) Task Force published a definition for AFF consisting of 5 major and 7 minor features. Little attention has so far been paid to the possibility that bisphosphonate-associated atypical fractures may also involve the diaphysis of other long bones. We report here the case of a patient on long-term bisphosphonate therapy who presented a diaphyseal tibial insufficiency fracture fulfilling all the major criteria (except for the location), and a number of the minor criteria of an atypical fracture. Our case report suggests the need for greater awareness of the possibility of atypical fractures at other sites, particularly in weight-bearing long bones other than the femur, and suggests that long-term bisphosphonate therapy may also contribute to the occurrence of these atypical fractures.

Identification of furin pro-region determinants involved in folding and activation.

The pro-region of the subtilisin-like convertase furin acts early in the biosynthetic pathway as an intramolecular chaperone to enable proper folding of the zymogen, and later on as an inhibitor to constrain the activity of the enzyme until it reaches the trans -Golgi network. To identify residues that are important for pro-region function, we initially identified amino acids that are conserved among the pro-regions of various mammalian convertases. Site-directed mutagenesis of 17 selected amino acids within the 89-residue pro-region and biosynthetic labelling revealed that I60A-furin and H66A-furin were rapidly degraded in a proteasome-dependent manner, while W34A-furin and F67A-furin did not show any autocatalytic activation. Intriguingly, the latter mutants proteolytically cleaved pro-von Willebrand factor precursor to the mature polypeptide, suggesting that the mutations permitted proper folding, but did not allow the pro-region to exercise its role in inhibiting the enzyme. Homology modelling of furin's pro-region revealed that residues Ile-60 and His-66 might be crucial in forming the binding interface with the catalytic domain, while residues Trp-34 and Phe-67 might be involved in maintaining a hydrophobic core within the pro-region itself. These results provide structural insights into the dual role of furin's pro-region.

Ectodomain shedding of furin: kinetics and role of the cysteine-rich region.

Furin, a member of the subtilisin-like pro-protein convertase family, is a type I membrane protein that undergoes ectodomain shedding. Metabolic labeling of cells stably expressing furin demonstrated that the shed form of furin is detected after 30 min. Moreover, sequence analysis revealed that specific residues of the cysteine-rich region of furin aligned with those of tumor necrosis factor receptor, which is also shed. Introduction within furin's cysteine-rich region of mutations that impair TNFR1 shedding also abolished furin shedding. Our results show that shedding of furin occurs rapidly and further suggest that specific cysteine residues may impart a conformation to the enzyme, thereby affecting its susceptibility to proteolysis.