SMYD3: a regulator of epigenetic and signaling pathways in cancer

Chromatin modifiers and their implications in oncogenesis have been an exciting area of cancer research. These are enzymes that modify chromatin via post-translational modifications such as methylation, acetylation, sumoylation, phosphorylation, in addition to others. Depending on the modification, chromatin modifiers can either promote or repress transcription. SET and MYN-domain containing 3 (SMYD3) is a chromatin modifier that has been implicated in the development and progression of various cancer types. It was first reported to tri-methylate Histone 3 Lysine 4 (H3K4), a methylation mark known to promote transcription. However, since this discovery, other histone (H4K5 and H4K20, for example) and non-histone (VEGFR, HER2, MAP3K2, ER, and others) substrates of SMYD3 have been described, primarily in the context of cancer. This review aims to provide a background on basic characteristics of SMYD3, such as its protein structure and tissue expression profiles, discuss reported histone and non-histone substrates of SMYD3, and underscore prognostic and functional implications of SMYD3 in cancer. Finally, we briefly discuss ongoing efforts to develop inhibitors of SMYD3 for future therapeutic use. It is our hope that this review will help synthesize existing research on SMYD3 in an effort to propel future discovery.

Lysine methylation signaling of non-histone proteins in the nucleus

Lysine methylation, catalyzed by protein lysine methyltransferases (PKMTs), is a central post-translational modification regulating many signaling pathways. It has direct and indirect effects on chromatin structure and transcription. Accumulating evidence suggests that dysregulation of PKMT activity has a fundamental impact on the development of many pathologies. While most of these works involve in-depth analysis of methylation events in the context of histones, in recent years, it has become evident that methylation of non-histone proteins also plays a pivotal role in cell processes. This review highlights the importance of non-histone methylation, with focus on methylation events taking place in the nucleus. Known experimental platforms which were developed to identify new methylation events, as well as examples of specific lysine methylation signaling events which regulate key transcription factors, are presented. In addition, the role of these methylation events in normal and disease states is emphasized.

Histone methyltransferase SMYD2 selective inhibitor LLY-507 in combination with poly ADP ribose polymerase inhibitor has therapeutic potential against high-grade serous ovarian carcinomas

Dysfunction of histone methylation is known to be related to cancer progression. The histone methyltransferase SMYD2 methylates histone protein H3 and non-histone proteins, including poly ADP ribose polymerase 1 (PARP1). There have been reports of SMYD2 overexpression in several types of cancers. However, there are no reports regarding its role in high-grade serous ovarian carcinomas (HGSOCs). Therefore, we investigated the expression profile and conducted functional analysis on SMYD2 in HGSOC cells. In addition, we verified whether SMYD2 inhibition increases the susceptibility of HGSOC cells to PARP inhibitors. We analyzed the expression of histone methyltransferase SMYD2 by quantitative real-time polymerase chain reaction and immunohistochemistry using HGSOC clinical tissues (n = 35). We performed functional analyses, including cell proliferation assay, cell cycle analysis, and immunoblotting, after treatment with SMYD2 siRNAs and SMYD2 selective inhibitor LLY-507 in HGSOC cells. We also performed colony-formation assay after combination treatment with LLY-507 and PARP inhibitor olaparib in HGSOC cells. The expression profiles of SMYD2 showed significant overexpression of SMYD2 in HGSOC clinical tissues. The knockdown or inhibition of SMYD2 by siRNAs or LLY-507, respectively, suppressed cell growth by increasing the proportion of apoptotic cells. LLY-507 showed additive effect with olaparib in the colony-formation assay. These findings suggest that LLY-507 can be used alone or in combination with a PARP inhibitor for the treatment of patients with HGSOC.

Tri-methylation of ATF7IP by G9a/GLP recruits the chromodomain protein MPP8

BACKGROUND

G9a and the related enzyme GLP were originally identified as histone lysine methyltransferases and then shown to also methylate several other non-histone proteins.

RESULTS

Here, we performed a comprehensive screen to identify their substrates in mouse embryonic stem cells (mESCs). We identified 59 proteins, including histones and other known substrates. One of the identified substrates, activating transcriptional factor 7-interacting protein 1 (ATF7IP), is tri-methylated at a histone H3 lysine 9 (H3K9)-like mimic by the G9a/GLP complex, although this complex mainly introduces di-methylation on H3K9 and DNA ligase 1 (LIG1) K126 in cells. The catalytic domain of G9a showed a higher affinity for di-methylated lysine on ATF7IP than LIG1, which may create different methylation levels of different substrates in cells. Furthermore, we found that M-phase phosphoprotein 8 (MPP8), known as a H3K9me3-binding protein, recognizes methylated ATF7IP via its chromodomain. MPP8 is also a known component of the human silencing hub complex that mediates silencing of transgenes via SETDB1 recruitment, which is a binding partner of ATF7IP. Although the interaction between ATF7IP and SETDB1 does not depend on ATF7IP methylation, we found that induction of SETDB1/MPP8-mediated reporter-provirus silencing is delayed in mESCs expressing only an un-methylatable mutant of ATF7IP.

CONCLUSIONS

Our findings provide new insights into the roles of lysine methylation in non-histone substrates which are targeted by the G9a/GLP complex and suggest a potential function of ATF7IP methylation in SETDB1/MPP8-mediated transgene silencing.

An unexpected journey: lysine methylation across the proteome

The dynamic modification of histone proteins by lysine methylation has emerged over the last decade as a key regulator of chromatin functions. In contrast, our understanding of the biological roles for lysine methylation of non-histone proteins has progressed more slowly. Though recently it has attracted less attention, ε-methyl-lysine in non-histone proteins was first observed over 50 years ago. In that time, it has become clear that, like the case for histones, non-histone methylation represents a key and common signaling process within the cell. Recent work suggests that non-histone methylation occurs on hundreds of proteins found in both the nucleus and the cytoplasm, and with important biomedical implications. Technological advances that allow us to identify lysine methylation on a proteomic scale are opening new avenues in the non-histone methylation field, which is poised for dramatic growth. Here, we review historical and recent findings in non-histone lysine methylation signaling, highlight new methods that are expanding opportunities in the field, and discuss outstanding questions and future challenges about the role of this fundamental post-translational modification (PTM).