Targeting c-MYC by antagonizing PP2A inhibitors in breast cancer

Abstract

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, ...

Co-expression of FOXL1 and PP2A inhibits proliferation inducing apoptosis in pancreatic cancer cells via promoting TRAIL and reducing phosphorylated MYC

“…On the contrary, the oncogenic MYC (also namely C-MYC) has been found to be deregulated in pancreatic cancers and has been confirmed to promote pancreatic cancers (34,51). The targeted inhibition of MYC has been indicated to inhibit the growth of breast cancers (35). In mammalian cells, Ser-62 phosphorylation of MYC is associated with the MYC stabilization, and the dephosphorylation of the site by PP2A promotes its polyubiquitination and degradation (52).…”

“…TRAIL is a member of the TNF superfamily and triggers apoptosis by recruiting the initiator caspase-8 and by directly activating downstream effector caspases (33), and FOXL1 has been indicated to inhibit the tumor aggressiveness in human pancreatic cancer via promoting TRAIL (22). The oncogenic MYC (also c-MYC) has also been deregulated in pancreatic cancers and has been confirmed to promote pancreatic cancers (34), and the targeted inhibition of MYC by antagonizing PP2A inhibitor has been indicated to inhibit the growth of breast cancers (35). To explore the possible mechanism in apoptosis induction by the co-expression of FOXL1 and PP2A, we then determined the expression of TRAIL and the phosphorylation of MYC, in Panc-1 cells post-infection at 3 MOI Ad (con), Ad (FOXL1), Ad (PP2A) or Ad (FOXL1 + PP2A) virus.…”

Targeting of COX-2 Expression by Recombinant Adenovirus shRNA Attenuates the Malignant Biological Behavior of Breast Cancer Cells

“…It plays an important role in the malignant biological behaviors (Janghorban et al, 2014). Studies have focused on c-myc's effect on regulatory proteins of the G1-S phase transition of the cell cycle.…”

Potenciais mecanismos de regulação da fosfatase PTEN pelas proteínas SET e PP2A e seu envolvimento na predisposição ao carcinoma bucal

“…A proteína SET (TAF-I beta ou I2PP2A), um potente e altamente seletivo inibidor de PP2A (Li et al, 1996), foi identificada em altos níveis no câncer de cabeça e pescoço (Leopoldino, Squarize, Garcia, Almeida, Pestana, Sobral, et al, 2012), câncer de próstata (Hu et al, 2015), de mama (Janghorban et al, 2014), pancreático (Farrell et al, 2014) e em tumor de Wilm´s (Carlson et al, 1998).…”

“…Recentemente, alguns trabalhos apresentaram a regulação negativa de SET como alternativa terapêutica para diversos tipos de cânceres através do aumento de atividade de PP2A e indução de degradação de c-MYC (Mukhopadhyay et al, 2013;Farrell et al, 2014;Janghorban et al, 2014).…”