An LRP5 receptor with internal deletion in hyperparathyroid tumors with implications for deregulated WNT/beta-catenin signaling.
BACKGROUND
Hyperparathyroidism (HPT) is a common endocrine disorder with incompletely understood etiology, characterized by enlarged hyperactive parathyroid glands and increased serum concentrations of parathyroid hormone and ionized calcium. We have recently reported activation of the Wnt signaling pathway by accumulation of beta-catenin in all analyzed parathyroid tumors from patients with primary HPT (pHPT) and in hyperplastic parathyroid glands from patients with uremia secondary to HPT (sHPT). Mechanisms that may account for this activation have not been identified, except for a few cases of beta-catenin (CTNNB1) stabilizing mutation in pHPT tumors.


RESULTS
Reverse transcription PCR and Western blot analysis showed expression of an aberrantly spliced internally truncated WNT coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) in 32 out of 37 pHPT tumors (86%) and 20 out of 20 sHPT tumors (100%). Stabilizing mutation of CTNNB1 and expression of the internally truncated LRP5 receptor was mutually exclusive. Expression of the truncated LRP5 receptor was required to maintain the nonphosphorylated active beta-catenin level, transcription activity of beta-catenin, MYC expression, parathyroid cell growth in vitro, and parathyroid tumor growth in a xenograft severe combined immunodeficiency (SCID) mouse model. WNT3 ligand and the internally truncated LRP5 receptor strongly activated transcription, and the internally truncated LRP5 receptor was insensitive to inhibition by DKK1.


CONCLUSIONS
The internally truncated LRP5 receptor is strongly implicated in deregulated activation of the WNT/beta-catenin signaling pathway in hyperparathyroid tumors, and presents a potential target for therapeutic intervention.
Björklund
Peyman
P
Department of Surgical Sciences, Uppsala University, Endocrine Unit, Uppsala University Hospital, Uppsala, Sweden.
Akerström
Göran
G
Westin
Gunnar
G
D000818:Animals; D015153:Blotting, Western; D005260:Female; D017353:Gene Deletion; D006801:Humans; D026502:LDL-Receptor Related Proteins / Q000096:biosynthesis* / Q000235:genetics*; D060492:Low Density Lipoprotein Receptor-Related Protein-5; D051379:Mice; D016513:Mice, SCID; D008954:Models, Biological; D009368:Neoplasm Transplantation; D010282:Parathyroid Neoplasms / Q000235:genetics* / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D051153:Wnt Proteins / Q000378:metabolism*; D060508:Wnt3 Protein; D051176:beta Catenin / Q000378:metabolism*
D026502:LDL-Receptor Related Proteins; C555482:LRP5 protein, human; D060492:Low Density Lipoprotein Receptor-Related Protein-5; C555483:Lrp5 protein, mouse; C555507:WNT3 protein, human; D051153:Wnt Proteins; D060508:Wnt3 Protein; C555508:Wnt3 protein, mouse; D051176:beta Catenin