Using sequential immunoprecipitation and mass spectrometry to identify methylation of NF-κB

Protein post-translational modifications in the regulation of cancer hallmarks

Posttranslational modifications (PTMs) of proteins, the major mechanism of protein function regulation, play important roles in regulating a variety of cellular physiological and pathological processes. Although the classical PTMs, such as phosphorylation, acetylation, ubiquitination and methylation, have been well studied, the emergence of many new modifications, such as succinylation, hydroxybutyrylation, and lactylation, introduces a new layer to protein regulation, leaving much more to be explored and wide application prospects. In this review, we will provide a broad overview of the significant roles of PTMs in regulating human cancer hallmarks through selecting a diverse set of examples, and update the current advances in the therapeutic implications of these PTMs in human cancer.

Mechanisms of NF-κB p65 and strategies for therapeutic manipulation

The transcription factor NF-κB is a critical regulator of immune and inflammatory responses. In mammals, the NF-κB/Rel family comprises five members: p50, p52, p65 (Rel-A), c-Rel, and Rel-B proteins, which form homo- or heterodimers and remain as an inactive complex with the inhibitory molecules called IκB proteins in resting cells. Two distinct NF-κB signaling pathways have been described: 1) the canonical pathway primarily activated by pathogens and inflammatory mediators, and 2) the noncanonical pathway mostly activated by developmental cues. The most abundant form of NF-κB activated by pathologic stimuli via the canonical pathway is the p65:p50 heterodimer. Disproportionate increase in activated p65 and subsequent transactivation of effector molecules is integral to the pathogenesis of many chronic diseases such as the rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, and even neurodegenerative pathologies. Hence, the NF-κB p65 signaling pathway has been a pivotal point for intense drug discovery and development. This review begins with an overview of p65-mediated signaling followed by discussion of strategies that directly target NF-κB p65 in the context of chronic inflammation.

p53-Pirh2 Complex Promotes Twist1 Degradation and Inhibits EMT

Epithelial-mesenchymal transition (EMT) is a critical process involved in cancer metastasis and chemoresistance. Twist1 is a key EMT-inducing transcription factor, which is upregulated in multiple types of cancers and has been shown to promote tumor cell invasiveness and support tumor progression. Conversely, p53 is a tumor suppressor gene that is frequently mutated in cancers. This study demonstrates the ability of wild-type (WT) p53 to promote the degradation of Twist1 protein. By forming a complex with Twist1 and the E3 ligase Pirh2, WT p53 promotes the ubiquitination and proteasomal degradation of Twist1, thus inhibiting EMT and maintaining the epithelial phenotype. The ability of p53 to induce Twist1 degradation is abrogated when p53 is mutated. Consequently, the loss of p53-induced Twist1 degradation leads to EMT and the acquisition of a more invasive cancer phenotype.Implication: These data provide new insight into the metastatic process at the molecular level and suggest a signaling pathway that can potentially be used to develop new prognostic markers and therapeutic targets to curtail cancer progression.

Chemical probes targeting epigenetic proteins: Applications beyond oncology

Epigenetic chemical probes are potent, cell-active, small molecule inhibitors or antagonists of specific domains in a protein; they have been indispensable for studying bromodomains and protein methyltransferases. The Structural Genomics Consortium (SGC), comprising scientists from academic and pharmaceutical laboratories, has generated most of the current epigenetic chemical probes. Moreover, the SGC has shared about 4 thousand aliquots of these probes, which have been used primarily for phenotypic profiling or to validate targets in cell lines or primary patient samples cultured in vitro. Epigenetic chemical probes have been critical tools in oncology research and have uncovered mechanistic insights into well-established targets, as well as identify new therapeutic starting points. Indeed, the literature primarily links epigenetic proteins to oncology, but applications in inflammation, viral, metabolic and neurodegenerative diseases are now being reported. We summarize the literature of these emerging applications and provide examples where existing probes might be used.

NF-κB: Regulation by Methylation

In normal cells exposed to stress, the central transcription factor NF-κB is activated only transiently, to modulate the activation of downstream immune responses. However, in most cancers, NF-κB is abnormally activated constitutively, contributing thus to oncogenesis and tumor progression. Therefore, downregulating NF-κB activity is an important goal of cancer treatment. In order to control NF-κB activity therapeutically, it is helpful to understand the molecular mechanisms that normally govern its activation and how dysregulated NF-κB activity may aid the development of disease. Recent evidence from our laboratories and others indicates that, in addition to various posttranslational modifications of NF-κB that have been observed previously, including phosphorylation, ubiquitination, and acetylation, NF-κB can be methylated reversibly on lysine or arginine residues by histone-modifying enzymes, including lysine and arginine methyl transferases and demethylases. Furthermore, these methylations are required to activate many downstream genes. Interestingly, amplifications and mutations of several such enzymes have been linked to cancer. We propose that some of these mutations may alter the methylation not only of histones but also of NF-κB, making them attractive therapeutic targets.