The MYC oncoprotein is an essential transcription factor that regulates the expression of many genes involved in cell growth, proliferation, and metabolic pathways. Thus, it is important to keep MYC activity in check in normal cells in order to avoid unwanted oncogenic changes. Normal cells have adapted several ways to control MYC levels, and these mechanisms can be disrupted in cancer cells. One of the major ways in which MYC levels are controlled in cells is through targeted degradation by the ubiquitin-proteasome system (UPS). Here, we discuss the role of the UPS in the regulation of MYC protein levels and review some of the many proteins that have been shown to regulate MYC protein stability. In addition, we discuss how this relates to MYC transcriptional activity, human cancers, and therapeutic targeting.

The transcription factor c-MYC is stabilized and activated by phosphorylation at serine 62 (S62) in breast cancer. Protein phosphatase 2A (PP2A) is a critical negative regulator of c-MYC through its ability to dephosphorylate S62. By inactivating c-MYC and other key signaling pathways, PP2A plays an important tumor suppressor function. Two endogenous inhibitors of PP2A, I2PP2A, Inhibitor-2 of PP2A (SET oncoprotein) and cancerous inhibitor of PP2A (CIP2A), inactivate PP2A and are overexpressed in several tumor types. Here we show that SET is overexpressed in about 50-60% and CIP2A in about 90% of breast cancers. Knockdown of SET or CIP2A reduces the tumorigenic potential of breast cancer cell lines both in vitro and in vivo. Treatment of breast cancer cells in vitro or in vivo with OP449, a novel SET antagonist, also decreases the tumorigenic potential of breast cancer cells and induces apoptosis. We show that this is, at least in part, due to decreased S62 phosphorylation of c-MYC and reduced c-MYC activity and target gene expression. Because of the ubiquitous expression and tumor suppressor activity of PP2A in cells, as well as the critical role of c-MYC in human cancer, we propose that activation of PP2A (here accomplished through antagonizing endogenous inhibitors) could be a novel antitumor strategy to posttranslationally target c-MYC in breast cancer.breast cancer therapy | phosphatase activator T he c-MYC (MYC) oncoprotein is overexpressed in human breast cancer and this is associated with poor clinical outcome (1, 2). Expression of MYC is regulated at multiple levels, including protein stability, which is increased in several cancer types (1,3,4). MYC stability is regulated in part by sequential and interdependent phosphorylation at two conserved residues, threonine 58 (T58) and serine 62 (S62) (5). MYC is phosphorylated at S62 (pS62) through the mitogen-activated protein kinase (MAPK) pathway or cyclin-dependent kinase (CDK) activation in response to growth signals and this modification increases its stability and oncogenic activity (5-8). When growth signals cease, GSK3, in a manner dependent upon prior phosphorylation at S62, phosphorylates T58 (pT58) (5, 6). T58 phosphorylation facilitates protein phosphatase 2A (PP2A)-mediated dephosphorylation of pS62 and recruitment of the E3 ubiquitin ligase SCF Fbw7 to initiate proteasomal destruction of MYC (9, 10). This process is facilitated by AXIN1, which helps nucleate a destruction complex for MYC at target gene promoters (11, 12). Our previous work has shown that MYC stability is increased in breast cancers and that this correlates with high pS62-and low pT58-MYC (4).PP2A is a ubiquitously expressed, heterotrimeric serinethreonine (S/T) phosphatase that mediates 30-50% of cellular S/T phosphatase activity (13). Target specificity of PP2A is directed by a variable regulatory (B) subunit, and we have shown that B56α is the isoform that directs PP2A to MYC (9, 13). Human cell transformation requires inhibition of PP2A activity and, in an siRNA screen, B56α, ...

cThe Myc oncoprotein is considered a master regulator of gene transcription by virtue of its ability to modulate the expression of a large percentage of all genes. However, mechanisms that direct Myc's recruitment to DNA and target gene selection to elicit specific cellular functions have not been well elucidated. Here, we report that the Pin1 prolyl isomerase enhances recruitment of serine 62-phosphorylated Myc and its coactivators to select promoters during gene activation, followed by promoting Myc's release associated with its degradation. This facilitates Myc's activation of genes involved in cell growth and metabolism, resulting in enhanced proproliferative activity, even while controlling Myc levels. In cancer cells with impaired Myc degradation, Pin1 still enhances Myc DNA binding, although it no longer facilitates Myc degradation. Thus, we find that Pin1 and Myc are cooverexpressed in cancer, and this drives a gene expression pattern that we show is enriched in poor-outcome breast cancer subtypes. This study provides new insight into mechanisms regulating Myc DNA binding and oncogenic activity, it reveals a novel role for Pin1 in the regulation of transcription factors, and it elucidates a mechanism that can contribute to oncogenic cooperation between Pin1 and Myc.

Expression of the c-Myc proto-oncoprotein is tightly regulated in normal cells. Phosphorylation at two conserved residues, threonine58 (T58) and serine62 (S62), regulates c-Myc protein stability. In cancer cells, c-Myc can become aberrantly stabilized associated with altered T58 and S62 phosphorylation. A complex signalling cascade involving GSK3b kinase, the Pin1 prolyl isomerase, and the PP2A-B56a phosphatase controls phosphorylation at these sites. We report here a novel role for the tumour suppressor scaffold protein Axin1 in facilitating the formation of a degradation complex for c-Myc containing GSK3b, Pin1, and PP2A-B56a. Although knockdown of Axin1 decreases the association of c-Myc with these proteins, reduces T58 and enhances S62 phosphorylation, and increases c-Myc stability, acute expression of Axin1 reduces c-Myc levels and suppresses c-Myc transcriptional activity. Moreover, the regulation of c-Myc by Axin1 is impaired in several tested cancer cell lines with known stabilization of c-Myc or loss of Axin1. This study provides critical insight into the regulation of c-Myc expression, how this can be disrupted in three cancer types, and adds to our knowledge of the tumour suppressor activity of Axin1.

No abstract