IL-1β suppresses nephronectin expression in osteoblasts via ERK1/2 and JNK

Phorbol-12-myristate 13-acetate inhibits Nephronectin gene expression via Protein kinase C alpha and c-Jun/c-Fos transcription factors

Nephronectin (Npnt) is an extracellular matrix protein and ligand of integrin α₈β₁ known to promote differentiation of osteoblasts. A search for factors that regulate Npnt gene expression in osteoblasts revealed that phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), had a strong effect to suppress that expression. Research was then conducted to elucidate the signaling pathway responsible for regulation of Npnt gene expression by PMA in osteoblasts. Treatment of MC3T3-E1 cells with PMA suppressed cell differentiation and Npnt gene expression. Effects were noted at a low concentration of PMA, and were time- and dose-dependent. Furthermore, treatment with the PKC signal inhibitor Gö6983 inhibited down-regulation of Npnt expression, while transfection with small interfering RNA (siRNA) of PKCα, c-Jun, and c-Fos suppressed that down-regulation. The present results suggest regulation of Npnt gene expression via the PKCα and c-Jun/c-Fos pathway.

Nephronectin as a Matrix Effector in Cancer

Simple Summary

The extracellular matrix provides an important scaffold for cells and tissues of multicellular organisms. The scaffold not only provides a secure anchorage point, but also functions as a reservoir for signalling molecules, sequestered and released when necessary. A dysregulated extracellular matrix may therefore modulate cellular behaviour, as seen during cancer progression. The extracellular matrix protein nephronectin was discovered two decades ago and found to regulate important embryonic developmental processes. Loss of either nephronectin or its receptor, integrin α8β1, leads to underdeveloped kidneys. Recent findings show that nephronectin is also dysregulated in breast cancer and plays a role in promoting metastasis. To enable therapeutic intervention, it is important to fully understand the role of nephronectin and its receptors in cancer progression. In this review, we summarise the literature on nephronectin, analyse the structure and domain-related functions of nephronectin and link these functions to potential roles in cancer progression.

Abstract

The extracellular matrix protein nephronectin plays an important regulatory role during embryonic development, controlling renal organogenesis through integrin α8β1 association. Nephronectin has three main domains: five N-terminal epidermal growth factor-like domains, a linker region harbouring two integrin-binding motifs (RGD and LFEIFEIER), and a C-terminal MAM domain. In this review, we look into the domain-related functions of nephronectin, and tissue distribution and expression. During the last two decades it has become evident that nephronectin also plays a role during cancer progression and in particular metastasis. Nephronectin is overexpressed in both human and mouse breast cancer compared to normal breast tissue where the protein is absent. Cancer cells expressing elevated levels of nephronectin acquire increased ability to colonise distant organs. In particular, the enhancer-motif (LFEIFEIER) which is specific to the integrin α8β1 association induces viability via p38 MAPK and plays a role in colonization. Integrins have long been desired as therapeutic targets, where low efficiency and receptor redundancy have been major issues. Based on the summarised publications, the enhancer-motif of nephronectin could present a novel therapeutic target.

Nephronectin Expression is Inhibited by Inorganic Phosphate in Osteoblasts

Nephronectin (Npnt), an extracellular matrix protein, is known to be a ligand of integrin α8β1, and it has also been known to play critical roles as various organs. In the present study, elevated extracellular inorganic phosphate (Pi) strongly inhibited the expression of Npnt in MC3T3-E1 cells, while the existence of extracellular calcium (Ca) was indispensable for its effect. Furthermore, Pi-induced inhibition of Npnt gene expression was recovered by inhibitors of both sodium-dependent Pi transporter (Pit) and fibroblast growth factor receptors (Fgfrs). These results demonstrated that Npnt gene expression is regulated by extracellular Pi via Pit and Fgfrs.