Control of cell proliferation by Myc proteins

Dermatopontin inhibits papillary thyroid cancer cell proliferation through MYC repression

Dermatopontin (DPT), a noncollagenous extracellular matrix component, has been illustrated to regulate cellular proliferation and invasiveness in several types of neoplasms. Nevertheless, the biological functions of DPT in cell proliferation, especially papillary thyroid cancer (PTC) cell proliferation, remain unknown, as do the mechanisms underlying its effects. In this study, we detected low DPT expression in PTC, which was related to higher T classifications. Ectopic DPT expression impeded cell proliferation both in vitro and in vivo. Furthermore, we illustrated that DPT down-regulated MYC, which in turn targeted CDK4, CDK6 and p21, through the ERK pathway. These results suggest that DPT regulates CDK4, CDK6 and p21, through MEK-ERK-MYC signaling to repress PTC proliferation.

Global regulation of nucleotide biosynthetic genes by c-Myc

Background

The c-Myc transcription factor is a master regulator and integrates cell proliferation, cell growth and metabolism through activating thousands of target genes. Our identification of direct c-Myc target genes by chromatin immunoprecipitation (ChIP) coupled with pair-end ditag sequencing analysis (ChIP-PET) revealed that nucleotide metabolic genes are enriched among c-Myc targets, but the role of Myc in regulating nucleotide metabolic genes has not been comprehensively delineated.

Methodology/Principal Findings

Here, we report that the majority of genes in human purine and pyrimidine biosynthesis pathway were induced and directly bound by c-Myc in the P493-6 human Burkitt's lymphoma model cell line. The majority of these genes were also responsive to the ligand-activated Myc-estrogen receptor fusion protein, Myc-ER, in a Myc null rat fibroblast cell line, HO.15 MYC-ER. Furthermore, these targets are also responsive to Myc activation in transgenic mouse livers in vivo. To determine the functional significance of c-Myc regulation of nucleotide metabolism, we sought to determine the effect of loss of function of direct Myc targets inosine monophosphate dehydrogenases (IMPDH1 and IMPDH2) on c-Myc-induced cell growth and proliferation. In this regard, we used a specific IMPDH inhibitor mycophenolic acid (MPA) and found that MPA dramatically inhibits c-Myc-induced P493-6 cell proliferation through S-phase arrest and apoptosis.

Conclusions/Significance

Taken together, these results demonstrate the direct induction of nucleotide metabolic genes by c-Myc in multiple systems. Our finding of an S-phase arrest in cells with diminished IMPDH activity suggests that nucleotide pool balance is essential for c-Myc's orchestration of DNA replication, such that uncoupling of these two processes create DNA replication stress and apoptosis.

The interplay between MYC and HIF in the Warburg effect

c-MYC and the hypoxia-inducible factors (HIFs) are critical factors for tumorigenesis in a large number of human cancers. While the normal function of MYC involves the induction of cell proliferation and enhancement of cellular metabolism, the function of HIF, particularly HIF-1, involves adaptation to the hypoxic microenvironment, including activation of anaerobic glycolysis. When MYC-dependent tumors grow, the hypoxic tumor microenvironment elevates the levels of HIF, such that oncogenic MYC and HIF collaborate to enhance the cancer cell's metabolic needs through increased uptake of glucose and its conversion to lactate. HIF is also able to attenuate mitochondrial respiration through the induction of pyruvate dehydrogenase kinase 1 (PDK1), which in part accounts for the Warburg effect that describes the propensity for cancers to avidly take up glucose and convert it to lactate with the concurrent decrease in mitochondrial respiration. Target genes that are common to both HIF and MYC, such as PDK1, LDHA, HK2, and TFRC, are therefore attractive therapeutic targets, because their coordinate induction by HIF and MYC widens the therapeutic window between cancer and normal tissues.

The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation

A large body of evidence has been accumulated that demonstrates dominant effects of Myc proto-oncoproteins on different aspects of cellular growth. Myc is one of the few proteins that is sufficient to drive resting cells into the cell cycle and promote DNA synthesis. In line with this finding is that the constitutive expression of Myc in cells blocks their differentiation. These growth stimulating properties are most likely responsible for Myc's ability to initiate and promote tumor formation. Interestingly Myc can also sensitize cells to apoptosis, suggesting that this protein is part of a life-and-death switch. Molecularly Myc functions as a transcriptional regulator that needs to heterodimerize with Max to exert the biological activities described above and to regulate gene transcription. Myc and Max are just two members of a growing family of proteins referred to as the Myc/Max/Mad network. A hallmark of these proteins is that they possess a C-terminal basic region/helix-loop-helix/leucine zipper domain (bHLHZip). The bHLHZip domain specifies dimerization within the network and determines sequence specific DNA binding. Importantly this domain together with the N-terminal transactivation domain is essential for Myc biology. Here we have summarized the structural, functional, and regulatory aspects of the bHLHZip domain of Myc proteins.

The 400 kDa subunit of the PCAF histone acetylase complex belongs to the ATM superfamily

PCAF histone acetylase is found in a complex with more than 20 associated polypeptides. Here we report cloning and characterization of the 400 kDa PCAF-associated factor referred to as PAF400. PAF400 is almost identical to TRRAP, which binds to c-Myc and E2F, and has significant sequence similarities to the ATM superfamily including FRAP, ATM, ATR, and the catalytic subunit of DNA-PK. Remarkably, PAF400 and FRAP share sequence similarity in broad regions that cover 80% of the entire PAF400 sequence. However, unlike the other members of the ATM superfamily, PAF400 is not a protein kinase as judged from the lack of kinase motif and autophosphorylation activity. We discuss the possibility that PAF400 may play a role in signaling of DNA damage to p53 by stimulation of p53 acetylation.