Ras-transformation reduce FAM20C expression and osteopontin phosphorylation.

Gitte N Schytte,Ida Bregenov,Brian Christensen,Esben S Sørensen

doi:10.1042/bsr20194378

Abstract

Family with sequence similarity 20, member C (FAM20C) is the main kinase of secreted phosphoproteins, including the multifunctional protein and cytokine, osteopontin (OPN). The phosphorylation of OPN varies greatly among cell types, tissues and species, and the different phospho-isoforms contribute to the multifunctionality of the protein. Expression of OPN is increased in human malignancies, and less phosphorylated isoforms of the protein have been associated with this phenotype. Here, we compared OPN from ras-transformed fibroblasts with that from their non-transformed parental cells, and found that OPN was less phosphorylated after ras-transformation. Furthermore, we demonstrated that expression of FAM20C mRNA was reduced five-fold in ras-transformed fibroblasts compared with non-transformed fibroblasts. Transfection with FAM20C of the ras-transformed fibroblasts restored the FAM20C mRNA expression but the phosphorylation of OPN was not increased proportionally. Likewise, the mRNA level of FAM20C was reduced in the malignant ras-transformed mammary cell line MCF10ACA1a compared with its non-transformed parental cell line MCF10A. These results suggest that expression of the FAM20C kinase is reduced after oncogenic ras-transformation, which potentially affects the phosphorylation of secreted phosphoproteins.

Highlights

With sequence similarity 20, member C (FAM20C) was recently identified as the long-sought Golgi kinase that phosphorylates secreted proteins [1,2]
We demonstrate for the first time that the expression of the FAM20C kinase is significantly reduced in ras-transformed cells, and that transfection with FAM20C re-establishes the mRNA expression levels, whereas it does not restore the impaired OPN phosphorylation observed in ras-transformed cells
Upon dephosphorylation with alkaline phosphatase (ALP), OPN from murine embryo fibroblast cell (fOPN) was recognized by the 3D9 antibody (Figure 1A)

Summary

Introduction

With sequence similarity 20, member C (FAM20C) was recently identified as the long-sought Golgi kinase that phosphorylates secreted proteins [1,2]. FAM20C is the main kinase of secreted proteins and it phosphorylates proteins participating in a wide variety of physiological processes [4]. Several of these processes are impaired by FAM20C loss-of-function mutations. Abrogated FAM20C phosphorylation of fibroblast growth factor 23 inhibits its proteolytic inactivation which results in a pathological hypophosphatemia condition [5]. FAM20C-mediated phosphorylation of oocyte secreted bone morphogenetic protein 15 and growth differentiation factor 9 is essential for regulation of their activity in folliculogenesis and ovulation [6]. FAM20C-mediated phosphorylation of von Willebrand factor increased platelet adhesion [7], and lack of FAM20C phosphorylation of histidine-rich calcium binding protein causes ventricular arrhythmia [8]

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Conclusion