YAF2 exerts anti-apoptotic effect in human tumor cells in a FANK1- and phosphorylation-dependent manner

Highlights

• Human YAF2 protein is phosphorylated at Serine 167.

• Phosphorylated YAF2 up-regulates FANK1 protein level through ubiquitin-proteasome system.

• FANK1 is a novel binding partner of YAF2.

• Phosphorylated form of YAF2 inhibits tumor cell apoptosis in a FANK1-dependent manner.

Abstract

YY1-associated factor 2 (YAF2) was frequently reported to modulate target gene transcription through both epigenetic and non-epigenetic means. However, other mechanisms were also utilized by YAF2 to carry out its biological functions. Here, we demonstrated that YAF2 from human tumor and non-tumor cells were mainly expressed as Serine 167 phosphorylated form. Further studies showed that the phosphorylated YAF2 up-regulated while its knockdown by specific siRNAs reduced fibronectin type III and ankyrin repeat domains 1 (FANK1) protein level. Mechanistic exploration disclosed that phosphorylated YAF2 inhibit proteasomal degradation of polyubiquitinated FANK1, leading to its increased stability. We then validated their interaction, and displayed that the FN3 domain of FANK1 binds to amino-terminal of YAF2. Functional studies showed that phosphorylated YAF2 inhibits tumor cell apoptosis in a FANK1-dependent manner. Taken together, our current findings demonstrated that phosphorylated YAF2 exhibits anti-apoptotic activity through targeting FANK1 expression in human tumor cells.

Introduction

The polycomb group (PcG) proteins play crucial roles in epigenetic regulation by using its histone modifying activities. PcG proteins mainly fall into two multisubunit complexes, namely the Polycomb repressive complex 1 (PRC1) and 2 (PRC2), which act synergistically to control gene expressions during organism development [1]. PRC1 consists of two subtypes: canonical PRC1 (CBX–PRC1) and non-canonical PRC1 (nc-PRC1, including RYBP–PRC1 and YAF2–PRC1) [2]. The former is recruited to H3K27me3 by CBXs to further catalyze monoubiquitination of adjacent histone H2A at lysine 119 (H2AK119Ub1) and target gene repression, whereas nc-PRC1 itself exhibits strong catalytic activity toward H2AK119 monoubiquitination and this modification is further recognized by RYBP or YAF2 to enroll other members of the nc-PRC1 [3,4].

In addition to modulate gene expression as a critical nc-PRC1 member, YAF2 as a RYBP family member was also reported to interact with a number of transcriptional factors to modulate target gene transcriptions. As a result, YAF2 plays vital roles during embryonic development, organogenesis, stem cell maintenance, and so on [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14]].

Contradicting results have been reported regarding to apoptosis-inducing activity of YAF2. During early zebrafish embryo development and organogenesis, YAF2 was reported to inhibit caspase-8-mediated apoptosis [12]. However, evidence also showed that YAF2 activates p53 and induces cell apoptosis by stabilizing PDCD5 [15]. To date, in spite of some progresses, we still know little about YAF2, especially its posttranslational modifications and functions.

We previously found that FANK1 is a binding partner of RYBP, and RYBP up-regulates FANK1 protein stability, activates AP-1 signaling pathway and induces tumor cell apoptosis [16]. Since YAF2 has high protein sequence homology to RYBP, we wonder whether YAF2 also interacts with and regulates tumor cell apoptosis through targeting FANK1 expression. Here, we provide evidences to support that YAF2 was phosphorylated at serine 167 and its phosphorylated form induced the accumulation of FANK1 protein through acting on ubiquitin-proteasome system. We finally demonstrated that YAF2 inhibits FANK1-mediated tumor cell apoptosis in a S167 phosphorylation-dependent manner. Thus, our current findings provide the basis for the discovery of novel anti-cancer drugs which target the S167 phosphorylation of YAF2.