Aguiari, Gianluca

Cell proliferation and apoptosis are typical hallmarks of autosomal dominant polycystic kidney disease (ADPKD) and cause the development of kidney cysts that lead to end‐stage renal disease (ESRD). Many factors, impaired by polycystin complex loss of function, may promote these biological processes, including cAMP, mTOR, and EGFR signaling pathways. In addition, microRNAs (miRs) may also regulate the ADPKD related signaling network and their dysregulation contributes to disease progression. However, the role of miRs in ADPKD pathogenesis has not been fully understood, but also the function of p53 is quite obscure, especially its regulatory contribution on cell proliferation and apoptosis. Here, we describe for the first time that miR501‐5p, upregulated in ADPKD cells and tissues, induces the activation of mTOR kinase by PTEN and TSC1 gene repression. The increased activity of mTOR kinase enhances the expression of E3 ubiquitin ligase MDM2 that in turn promotes p53 ubiquitination, leading to its degradation by proteasome machinery in a network involving p70S6K. Moreover, the overexpression of miR501‐5p stimulates cell proliferation in kidney cells by the inhibition of p53 function in a mechanism driven by mTOR signaling. In fact, the downregulation of this miR as well as the pharmacological treatment with proteasome and mTOR inhibitors in ADPKD cells reduces cell growth by the activation of apoptosis. Consequently, the stimulation of cell death in ADPKD cells may occur through the inhibition of mTOR/MDM2 signaling and the restoring of p53 function. The data presented here confirm that the impaired mTOR signaling plays an important role in ADPKD.

ABSTRACT Polycystin‐2 (PC2), encoded by the PKD2 gene, mutated in 10‐15% of autosomal‐dominant polycystic kidney disease (ADPKD) patients, is a Ca 2+ ‐permeable cation channel present in kidney epithelia and other tissues. As PC2 was found expressed in B‐lymphoblastoid cells (LCLs) and Ca 2+ signaling pathways are important regulators of B cell function activities, we investigated whether PC2 plays some role in B‐LCLs. In LCLs, PC2 was found mainly in ER membranes but ~8 times less than in kidney HEK293 cells. The same reductions were found in PKD2 and PKD1 RNA; thus, PKD genes maintained, in LCLs, the same reciprocal proportion as they do in kidney cells. In LCLs obtained from subjects carrying PKD2 mutations (PKD2‐LCLs) and showing reduced PC2 levels, intracellular Ca 2+ concentrations evoked by platelet‐activating factor (PAF), were significantly lower than in non‐PKD‐LCLs. This reduction was also found in PKD1‐ LCLs but without PC2 reductions. Likewise, cell proliferation, which is controlled by Ca 2+ , was reduced in PKD2‐ and PKD1‐LCLs. Moreover, in LCLs with PKD2 nonsense mutations, aminoglycoside antibiotics reduced the PC2 defect by promoting readthrough of stop codons. Therefore, PC2 and PC1 are functionally expressed in LCLs, which provide a model, easily obtainable from ADPKD patients, to study PKD gene expression and function.