Newly identified aspects of tumor suppression by RB

The retinoblastoma (RB) tumor suppressor belongs to a cellular pathway that plays a crucial role in restricting the G1-S transition of the cell cycle in response to a large number of extracellular and intracellular cues. Research in the last decade has highlighted the complexity of regulatory networks that ensure proper cell cycle progression, and has also identified multiple cellular functions beyond cell cycle regulation for RB and its two family members, p107 and p130. Here we review some of the recent evidence pointing to a role of RB as a molecular adaptor at the crossroads of multiple pathways, ensuring cellular homeostasis in different contexts. In particular, we discuss the pro- and anti-tumorigenic roles of RB during the early stages of cancer, as well as the importance of the RB pathway in stem cells and cell fate decisions.     

Epigenetic drugs as pleiotropic agents in cancer treatment: biomolecular aspects and clinical applications

In the last three decades huge efforts have been made to characterize genetic defects responsible for cancer development and progression, leading to the comprehensive identification of distinct cellular pathways affected by the alteration of specific genes. Despite the undoubtable role of genetic mechanisms in triggering neoplastic cell transformation, epigenetic modifications (i.e., heritable changes of gene expression that do not derive from alterations of the nucleotide sequence of DNA) are rapidly emerging as frequent alterations that often occur in the early phases of tumorigenesis and that play an important role in tumor development and progression. Epigenetic alterations, such as modifications in DNA methylation patterns and post-translational modifications of histone tails, behave extremely different from genetic modifications, being readily revertable by "epigenetic drugs" such as inhibitors of DNA methyl transferases and inhibitors of histone deacetylases. Since epigenetic alterations in cancer cells affect virtually all cellular pathways that have been associated to tumorigenesis, it is not surprising that epigenetic drugs display pleiotropic activities, being able to concomitantly restore the defective expression of genes involved in cell cycle control, apoptosis, cell signaling, tumor cell invasion and metastasis, angiogenesis and immune recognition. Prompted by this emerging clinical relevance of epigenetic drugs, this review will focus on the large amount of available data, deriving both from in vitro experimentations and in vivo pre-clinical and clinical studies, which clearly indicate epigenetic drugs as effective modifiers of cancer phenotype and as positive regulators of tumor cell biology with a relevant therapeutic potential in cancer patients.     

Functional interactions between retinoblastoma and c-MYC in a mouse model of hepatocellular carcinoma

Inactivation of the RB tumor suppressor and activation of the MYC family of oncogenes are frequent events in a large spectrum of human cancers. Loss of RB function and MYC activation are thought to control both overlapping and distinct cellular processes during cell cycle progression. However, how these two major cancer genes functionally interact during tumorigenesis is still unclear. Here, we sought to test whether loss of RB function would affect cancer development in a mouse model of c-MYC-induced hepatocellular carcinoma (HCC), a deadly cancer type in which RB is frequently inactivated and c-MYC often activated. We found that RB inactivation has minimal effects on the cell cycle, cell death, and differentiation features of liver tumors driven by increased levels of c-MYC. However, combined loss of RB and activation of c-MYC led to an increase in polyploidy in mature hepatocytes before the development of tumors. There was a trend for decreased survival in double mutant animals compared to mice developing c-MYC-induced tumors. Thus, loss of RB function does not provide a proliferative advantage to c-MYC-expressing HCC cells but the RB and c-MYC pathways may cooperate to control the polyploidy of mature hepatocytes.     

Novel epigenetic therapeutic strategies and targets in cancer

The critical role of dysregulated epigenetic pathways in cancer genesis, development, and therapy has typically been established as a result of scientific and technical innovations in next generation sequencing. RNA interference, histone modification, DNA methylation and chromatin remodelling are epigenetic processes that control gene expression without causing mutations in the DNA. Although epigenetic abnormalities are thought to be a symptom of cell tumorigenesis and malignant events that impact tumor growth and drug resistance, physicians believe that related processes might be a key therapeutic target for cancer treatment and prevention due to the reversible nature of these processes. A plethora of novel strategies for addressing epigenetics in cancer therapy for immuno-oncological complications are currently available - ranging from basic treatment to epigenetic editing. – and they will be the subject of this comprehensive review. In this review, we cover most of the advancements made in the field of targeting epigenetics with special emphasis on microbiology, plasma science, biophysics, pharmacology, molecular biology, phytochemistry, and nanoscience.     

Interaction Between the Cdk2/Cyclin A Complex and a Small Molecule Derived from the pRb2/p130 Spacer Domain: A Theoretical Model

Retinoblastoma (RB) family proteins pRb, p107 and pRb2/p130 are important cellular factors which play a well-recognized role as tumor and growth suppressors. These proteins are actively involved in the negative control of the cell cycle and their function is modulated via complex homeostatic processes, most of them involving post-translational regulation of their phosphorylation status. Interestingly, the family members p107 and pRb2/p130 share the ability to physically interact and inhibit the kinase activity of the Cdk2/Cyclin A and Cdk2/Cyclin E complexes. Regarding pRb2/p130, its inhibitory effect on the Cdk2/Cyclin A activity has been attributed to the “spacer” region. Recently, a 39 aa-long pRb2/p130 spacer-derived peptide (Spa310, aa 641-679) has been selected as the sequence responsible for Cdk2/Cyclin A inhibition. Following the identification of this active sequence, here we propose a computer-generated three-dimensional model of the interaction between the Cdk2/Cyclin A complex and the N-terminal 9-amino acid sequence of the Spa310 peptide. We believe this model as useful for the rational development of peptide or peptidomimetic kinase inhibitors to be used for the negative modulation of cell cycle in cancer cells.     

The impact of p53 and p73 on aneuploidy and cancer

Initiation, progression and evasion are sequential steps in cancer formation, with autonomous cell proliferation as a final outcome. Genetic or epigenetic alterations of key regulatory genes of the cell cycle are frequently associated with these phenomena. Recently, chromosomal instability, a long-supposed driving force of tumorigenesis, was associated with dysregulation of mitotic genes, providing advantages to tumor cells. Numerous molecules thus provide a key link in the chain of relationships between chromosomal instability and cancer. Here, we discuss emerging evidence revealing that two p53 family members, p53 and p73, might be key regulatory genes at the heart of the relationship between chromosomal instability and cancer. We argue that the role of members of the p53 family as tumor suppressor proteins, their impact on the control of cellular ploidy, and their newly emerging connection with mitotic checkpoint regulatory genes support the suggestion that p73 and p53 could be two of the missing links among chromosomal instability, the mitotic checkpoint and cancer.     

SET8-mediated methylations of histone H4 lysine 20 mark silent heterochromatic domains in apicomplexan genomes

Posttranslational histone modifications modulate chromatin-templated processes in various biological systems. H4K20 methylation is considered to have an evolutionarily ancient role in DNA repair and genome integrity, while its function in heterochromatin function and gene expression is thought to have arisen later during evolution. Here, we identify and characterize H4K20 methylases of the Set8 family in Plasmodium and Toxoplasma, two medically important members of the protozoan phylum Apicomplexa. Remarkably, parasite Set8-related proteins display H4K20 mono-, di-, and trimethylase activities, in striking contrast to the monomethylase-restricted human Set8. Structurally, few residues forming the substrate-specific channel dictate enzyme methylation multiplicity. These enzymes are cell cycle regulated and focally enriched at pericentric and telomeric heterochromatin in both parasites. Collectively, our findings provide new insights into the evolution of Set8-mediated biochemical pathways, suggesting that the heterochromatic function of the marker is not restricted to metazoans. Thus, these lower eukaryotes have developed a diverse panel of biological stages through their high capacity to differentiate, and epigenetics only begins to emerge as a strong determinant of their biology.     

Bridging epigenetics and metabolism

Recent research suggests that chromatin-modifying enzymes are metabolic sensors regulating gene expression. Epigenetics is linked to metabolomics in response to the cellular microenvironment. Specific metabolites involved in this sensing mechanism include S-adenosylmethionine, acetyl-CoA, alphaketoglutarate and NAD⁺. Although the core metabolic pathways involving glucose have been emphasized as the source of these metabolites, the reprogramming of pathways involving non-essential amino acids may also play an important role, especially in cancer. Examples include metabolic pathways for glutamine, serine and glycine. The coupling of these pathways to the intermediates affecting epigenetic regulation occurs by “parametabolic” mechanisms. The metabolism of proline may play a special role in this parametabolic linkage between metabolism and epigenetics. Both proline degradation and biosynthesis are robustly affected by oncogenes or suppressor genes, and they can modulate intermediates involved in epigenetic regulation. A number of mechanisms in a variety of animal species have been described by our laboratory and by others. The challenge we now face is to identify the specific chromatin-modifying enzymes involved in coupling of proline metabolism to altered reprogramming of gene expression.     

Carcinogenesis and the hypothesis of phylogenetic reversion

Chemoprevention must target early molecular events involved in malignant transformation. The sequence of events leading from a normally functioning interphase cell to an uncontrolled tumor cell is only partially understood, impeding systematic design of chemopreventive agents. The respective roles of mutagenic and epigenetic mechanisms have not been definitively established. Also, traditional models do not appear to incorporate cellular response to events leading to carcinogenesis. A perspective on system response offered by complexity science elucidates the roles of feedback and control in maintaining functional stability during carcinogenesis. Carcinogenesis is seen as a process of epigenetic redifferentiation resulting in a cell behaving like an archetypal karyocyte free of growth restraints (phylogenetic reversion). Genes that evolved during the development of multicellular organisms, restraining uncontrolled growth and regulating intercell communication may be systematically silenced during carcinogenesis. The formation of heterochromatin, which results in epigenetic silencing by hypermethylation in CpG-dense islands, finds expression in the nuclear chromatin pattern. Karyometry is an integrating biomarker of chromatin pattern information that accommodates the possibility of multiple, differently ordered pathways and provides exquisite sensitivity, allowing detection of very early transformation events. Its use can monitor the impact of chemopreventive agents on the earliest events in progression to cancer.     

N-terminal RASSF family: RASSF7-RASSF10

Epigenetic inactivation of tumor suppressor genes is a hallmark of cancer development. RASSF1A (Ras Association Domain Family 1 isoform A) tumor suppressor gene is one of the most frequently epigenetically inactivated genes in a wide range of adult and children''s cancers and could be a useful molecular marker for cancer diagnosis and prognosis. RASSF1A has been shown to play a role in several biological pathways, including cell cycle control, apoptosis and microtubule dynamics. RASSF2, RASSF4, RASSF5 and RASSF6 are also epigenetically inactivated in cancer but have not been analyzed in as wide a range of malignancies as RASSF1A. Recently four new members of the RASSF family were identified these are termed N-Terminal RASSF genes (RASSF7–RASSF10). Molecular and biological analysis of these newer members has just begun. This review highlights what we currently know in respects to structural, functional and molecular properties of the N-Terminal RASSFs.Key words: N-terminal RASSF, RAS, cancer, epigenetic, tumor suppressor