oa Filamin A is Reduced and Contributes to CASR Sensitivity in Human Parathyroid Tumors

Background

Parathyroid tumors display reduced sensitivity to extracellular calcium ([Ca²⁺]o). The parathyroid cell sensitivity to [Ca²⁺]o is mediated by the calcium-sensing receptor (CASR), a G-protein-coupled receptor interacting with the scaffold protein filamin A (FLNA). We investigated: the FLNA expression in human parathyroid tumors, its effect on the CASR membrane stabilization, and on ERK signaling activation in HEK293 cells, the effect of the C-terminal CASR variants (R990G) on the interaction with FLNA.

Methods

FLNA expression was analyzed by immunohistochemistry and immunofluorescence in parathyroid tumors and in cells from parathyroid adenomas (PAds). Endogenous FLNA in HEK293 cells transfected with the CASR variants was silenced by siRNA technique. Results FLNA expression was down-regulated in human parathyroid tumors; In 74 PAds, FLNA mRNA levels positively correlated with CASR levels. In HEK293 cells transfected with 990R-CASR or 990G-CASR, FLNA silencing reduced both the total and membrane protein expression levels. However, in presence of endogenous FLNA, 990G-CASR expression in cell membrane was higher than that of the 990R-CASR. FLNA loss reduced significantly p-ERK levels induced both 990R-CaSR and 990G-CaSR. Treatment with the CASR agonist R568 delete the effect of FLNA loss and restoring 990G-CASR sensitivity to [Ca²⁺]o in absence of FLNA.

Conclusions

FLNA is down-regulated in parathyroid tumors and paralleled the CASR expression levels. Loss of FLNA reduced the CASR expression levels and the CASR-induced ERK phosphorylation. The CASR 990G allele was associated with increased sensitivity to [Ca²⁺]o, though FLNA was required to determine the 990G-CASR higher expression and activity than that of 990R-CASR protein.