Functional analysis of the N-terminal domain of the Myc oncoprotein.

Myc is a multifunctional nuclear phosphoprotein that can drive cell cycle progression, apoptosis and cellular transformation. Myc orchestrates these activities at the molecular level by functioning as a regulator of gene transcription to activate or repress specific target genes. Previous studies have shown that both the Myc N-terminal domain (NTD) and the C-terminal domain (CTD) are essential for Myc functions. The role of the CTD is relatively well understood as it encodes a basic helix-loop-helix leucine zipper motif important for DNA binding and protein-protein interactions. By contrast, the role of the NTD and the specific domains responsible for different Myc activities are not as well defined. To investigate the regions of the NTD necessary for Myc function and to determine whether these activities are overlapping or independent of one another, we have conducted a detailed structure-function analysis of the Myc NTD. We assessed the ability of a number of deletion and point mutants within the highly conserved regions of the Myc NTD to induce cell cycle progression, apoptosis and transformation as well as repress and activate expression of endogenous target genes. Our analyses highlight the complexity of the Myc NTD and extend previous studies. For example, we show most Myc mutants that were compromised as repressors of gene transcription retained the ability to activate gene transcription, reinforcing the concept that these activities can be uncoupled. Repression of two different target genes could be distinguished by specific mutants, further supporting the notion of at least two different Myc repression mechanisms. Mutants disabled at both inducing and repressing gene transcription could not maximally drive the biological activities of Myc, indicating these functions are tightly linked. Indeed, a close association of Myc repression and apoptosis was also observed.

The myc oncogene: MarvelouslY Complex.

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

Identifying genes regulated in a Myc-dependent manner.

The c-myc proto-oncogene can direct a diverse array of biological activities, including cell cycle progression, apoptosis, and differentiation. It is believed that Myc can affect this wide variety of activities by functioning as a regulator of gene transcription, although few targets have been identified to date. To delineate the molecular program regulated downstream of Myc, we used a cDNA microarray approach and identified 52 putative targets out of >6000 cDNAs analyzed. To further distinguish the subset of genes whose regulation was dependent upon Myc per se from those regulated in response to activation of general mitogenic or apoptotic programs, the putative cDNA targets were then screened by a series of assays. By this approach 37 putative targets were ruled out and 15 Myc target genes were uncovered. Interestingly, comparing our results with other high throughput screens reveals that certain putative Myc targets previously reported are shown not to be regulated downstream of Myc (e.g. ribosomal proteins, HSP90beta), whereas others are further supported by our analyses (e.g. pdgfbetar, nucleolin). The identity of genes specifically regulated downstream of Myc provides the critical tools required to understand the role Myc holds in the transformation process and to delineate how Myc functions as a regulator of gene transcription.