Mahnaz Janghorban

Targeting c-MYC by antagonizing PP2A inhibitors in breast cancer

Significance Increased kinase activity and suppressed phosphatase activity are hallmarks of oncogenic signaling. The transcription factor c-MYC, a master driver of human cancer, is stabilized and activated by persistent serine 62 phosphorylation. The tumor suppressor protein phosphatase 2A (PP2A) targets this site and negatively regulates c-MYC. Here, we show that two cellular inhibitors of PP2A, the SET oncoprotein and cancerous inhibitor of PP2A (CIP2A), are overexpressed in breast cancer, and depletion or inhibition of SET or CIP2A reduces c-MYC expression and activity and decreases the tumorigenic potential of breast cancer cells. These findings strongly suggest that inhibiting SET or CIP2A to reactivate PP2A may be an effective therapeutic strategy for targeting c-MYC in breast cancer. 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.

Mechanistic insight into Myc stabilization in breast cancer involving aberrant Axin1 expression

High expression of the oncoprotein Myc has been linked to poor outcome in human tumors. Although MYC gene amplification and translocations have been observed, this can explain Myc overexpression in only a subset of human tumors. Myc expression is in part controlled by its protein stability, which can be regulated by phosphorylation at threonine 58 (T58) and serine 62 (S62). We now report that Myc protein stability is increased in a number of breast cancer cell lines and this correlates with increased phosphorylation at S62 and decreased phosphorylation at T58. Moreover, we find this same shift in phosphorylation in primary breast cancers. The signaling cascade that controls phosphorylation at T58 and S62 is coordinated by the scaffold protein Axin1. We therefore examined Axin1 in breast cancer and report decreased AXIN1 expression and a shift in the ratio of expression of two naturally occurring AXIN1 splice variants. We demonstrate that this contributes to increased Myc protein stability, altered phosphorylation at S62 and T58, and increased oncogenic activity of Myc in breast cancer. Thus, our results reveal an important mode of Myc activation in human breast cancer and a mechanism contributing to Myc deregulation involving unique insight into inactivation of the Axin1 tumor suppressor in breast cancer.

Pin1 Regulates the Dynamics of c-Myc DNA Binding To Facilitate Target Gene Regulation and Oncogenesis

ABSTRACT The 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 Mycs 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 Mycs release associated with its degradation. This facilitates Mycs 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.

Serine 62-Phosphorylated MYC Associates with Nuclear Lamins and Its Regulation by CIP2A Is Essential for Regenerative Proliferation

An understanding of the mechanisms determining MYCs transcriptional and proliferation-promoting activities in vivo could facilitate approaches for MYC targeting. However, post-translational mechanisms that control MYC function in vivo are poorly understood. Here, we demonstrate that MYC phosphorylation at serine 62 enhances MYC accumulation on Lamin A/C-associated nuclear structures and that the protein phosphatase 2A (PP2A) inhibitor protein CIP2A is required for this process. CIP2A is also critical for serum-induced MYC phosphorylation and for MYC-elicited proliferation induction in vitro. Complementary transgenic approaches and an intestinal regeneration model further demonstrated the in vivo importance of CIP2A and serine 62 phosphorylation for MYC activity upon DNA damage. However, targeting of CIP2A did not influence the normal function of intestinal crypt cells. These data underline the importance of nuclear organization in the regulation of MYC phosphorylation, leading to an in vivo demonstration of a strategy for inhibiting MYC activity without detrimental physiological effects.

The tumor suppressor phosphatase PP2A-B56α regulates stemness and promotes the initiation of malignancies in a novel murine model

Protein phosphatase 2A (PP2A) is a ubiquitously expressed Serine-Threonine phosphatase mediating 30–50% of protein phosphatase activity. PP2A functions as a heterotrimeric complex, with the B subunits directing target specificity to regulate the activity of many key pathways that control cellular phenotypes. PP2A-B56α has been shown to play a tumor suppressor role and to negatively control c-MYC stability and activity. Loss of B56α promotes cellular transformation, likely at least in part through its regulation of c-MYC. Here we report generation of a B56α hypomorph mouse with very low B56α expression that we used to study the physiologic activity of the PP2A-B56α phosphatase. The predominant phenotype we observed in mice with B56α deficiency in the whole body was spontaneous skin lesion formation with hyperproliferation of the epidermis, hair follicles and sebaceous glands. Increased levels of c-MYC phosphorylation on Serine62 and c-MYC activity were observed in the skin lesions of the B56αhm/hm mice. B56α deficiency was found to increase the number of skin stem cells, and consistent with this, papilloma initiation was accelerated in a carcinogenesis model. Further analysis of additional tissues revealed increased inflammation in spleen, liver, lung, and intestinal lymph nodes as well as in the skin lesions, resembling elevated extramedullary hematopoiesis phenotypes in the B56αhm/hm mice. We also observed an increase in the clonogenicity of bone marrow stem cells in B56αhm/hm mice. Overall, this model suggests that B56α is important for stem cells to maintain homeostasis and that B56α loss leading to increased activity of important oncogenes, including c-MYC, can result in aberrant cell growth and increased stem cells that can contribute to the initiation of malignancy.

Abstract 3857: Pharmacological reactivation of the tumor suppressor protein phosphatase 2A as a novel approach for the treatment of breast cancer

PP2A is a phosphatase that is functionally dysregulated and inactivated in over 50% of all breast cancers independent of histological type. While much attention has focused on oncogenic kinases as targets for cancer treatment, therapeutic targeting of phosphatases, the key negative regulators of these same signaling pathways, has remained largely unexplored. Starting with the observation that tricyclic neuroleptic drugs exert anticancer effects, our laboratory reverse engineered these drugs to generate a novel series of compounds that retain the anti-proliferative effects and favorable pharmacokinetic properties but are devoid of the undesirable central nervous system pharmacology of the parent drugs. It was subsequently demonstrated that these first generation novel derivatives exert anticancer effects in cell culture and xenograft models of cancer (without overt toxicity) by modulation of critical oncogenic signaling pathways. Recently, it was determined that the molecular basis of the actions of these novel derivatives is the activation of PP2A. Comprehensive profiling for activity in a panel of 240 cancer cell lines has shown that these first-in-class small molecule activators of PP2A (SMAPs) have significant anti-proliferative activity in breast cancer cell lines, and studies have been extended to disease relevant in vivo models (patient derived xenograft, GEMM, and traditional xenograft models). PP2A9s ability to negatively regulate a diverse set of oncogenic drivers in breast cancer means SMAPs are able to exert their effects through multiple biological mechanisms, swiftly disrupting cellular energetics and metabolism, inducing apoptosis and inhibiting proliferation. This will be exhibited through a variety of biological assays evaluating cell growth, apoptosis and changes in cellular metabolism. Target engagement assessed via western blotting demonstrates inhibition of PP2A targets critical for mediating SMAP activity, notably c-MYC and AKT. As a result, this research demonstrates SMAPs have potent activity in a wide variety of breast cancer contexts where response to current therapies is modest and limited by the development of treatment resistance. Thus, SMAPs may be a viable treatment option for high-risk populations of patients who do not respond to the current standard of care. This research strives to demonstrate that therapeutic reactivation of PP2A may represent a novel approach for breast cancer treatment and that these molecules may favorably impact the lives of women suffering from breast cancer. Citation Format: Caroline C. Farrington, Xiaoyan Wang, Mahnaz Janghorban, Juan Liang, Analisa Difeo, Rosalie Sears, Goutham Narla. Pharmacological reactivation of the tumor suppressor protein phosphatase 2A as a novel approach for the treatment of breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3857.

Abstract PR03: Serine 62 phosphorylated MYC associates with nuclear lamins and its regulation by CIP2A is essential for proliferation induction in vivo

Targeting MYC function would be highly desirable for hyperproliferative diseases. As MYC itself is refractory to direct chemical inhibition, understanding of the mechanisms determining MYC9s transcriptional and proliferation promoting activities in vivo would be critical for generation of alternative targeting strategies. However, despite 30 years of research, it is very poorly documented what post-translational mechanisms control MYC function in vivo. Here we demonstrate for the first time that Lamin A/C association is critical for MYC phosphorylation regulation. MYC phosphorylation at serine 62 enhances MYC recruitment to Lamin A/C-associated nuclear structures and the Protein Phosphatase 2A (PP2A) inhibitor protein CIP2A is required for retaining this phosphorylation and localization. CIP2A is also critical for serum induced MYC phosphorylation, and for MYC-elicited proliferation induction in vitro. Critically, using complementary transgenic approaches, and intestinal regeneration model, we demonstrate the in vivo importance of this mechanism for MYC´s transcriptional and proliferation promoting activities. However, targeting of this mechanism does not influence basal proliferation, or differentiation of intestinal crypt cells or mouse well-being. Together we discover unprecedented importance of nuclear organization of MYC for its phosphorylation regulation; leading to in vivo demonstration of a strategy for targeting of MYC activity without detrimental physiological effects. Citation Format: Kevin Myant, Xi Qiao, Tuuli Halonen, Christophe Come, Anni Laine, Johanna I. Partanen, Erinn-Lee Ogg, Tiina Laitera, Mahnaz Janghorban, Juha Okkeri, Juha Klefstrom, Rosalie C. Sears, Owen J. Sansom, Jukka Westermarck. Serine 62 phosphorylated MYC associates with nuclear lamins and its regulation by CIP2A is essential for proliferation induction in vivo. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr PR03.

Abstract IA10: Post-translational regulation of MYC's oncogenic activity

In response to growth stimulatory signals in normal cells or in transformed cells with constitutive signaling, the c-Myc oncoprotein is post-translationally stabilized through enhanced phosphorylation at the conserved Serine 62 residue. In addition to increasing c-Myc stability, phosphorylation at Serine 62 (pS62-Myc), in conjunction with Proline 63 isomerization mediated by the Pin1 Proline Isomerase, also increases the rate of recruitment of newly synthesized c-Myc to its target genes involved in pro-proliferative phenotypes, enhancing their expression. In normal cells, phosphorylation of S62 primes phosphorylation at a second conserved residue, Threonine 58 (pT58), which then enhance c-Myc degradation by facilitating a second Pin1-mediated isomerization event, which promotes Protein Phosphatase 2A (PP2A-B56α)-mediated dephosphorylation of Serine 62 and recruits the E3 ubiquitin ligase, SCFFbw7. In cancer cell lines as well as patient tumor samples pS62-Myc is elevated relative to non-transformed cell lines or adjacent normal tissue. In order to study the role of pS62-Myc in vivo, we created c-Myc knock-in mice that express either Myc wild-type (WT) or the MycT58A or MycS62A phosphorylation mutant from the ROSA26 locus in response to Cre-recombinase. Analysis of these mice revealed increased tumorigenic potential of MycT58A, which has constitutive S62 phosphorylation as it is resistant to PP2A-mediated S62 dephosphorylation. Together, this research has revealed an important role for S62 phosphorylation and Pin1-mediated isomerization in the tumorigenic activity of c-Myc, which presents new strategies to target c-Myc by inhibiting these post-translational activation steps. Thus, we are currently testing the therapeutic efficacy of PP2A activation and Pin1 inhibition in mouse models of c-Myc-driven tumorigenesis. To create physiologically relevant c-Myc-driven tumor models that engage post-translational activation of c-Myc, we have crossed our ROSA-LSL-MycWT mice with mice carrying organ-relevant oncogenic signaling transgenes. Thus far, we have generated mouse models of Her2+ and Triple Negative breast cancer, and pancreatic cancer. We have demonstrated dramatic tumor growth inhibition with a novel, orally available small molecule PP2A activator drug in these mouse models. Citation Format: Rosalie Sears, Amy Farrell, Xiaoyan Wang, Juan Liang, Mahnaz Janghorban, Yulong Su, Brittany Allen-Petersen, Michael Ohlemeyer, Narla Goutham. Post-translational regulation of MYC9s oncogenic activity. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr IA10.

Abstract 3180: Regulation of stem cells by the tumor suppressor PP2A-B56α

Protein phosphatase 2 A (PP2A) is one of the major cellular Serine-Threonine phosphatases and regulates numerous signaling pathways, including stem cell self-renewal, proliferation, differentiation, and cell survival. The PP2A holoenzyme has 3 subunits: a catalytic (C) subunit, a structural (A) subunit, and a variable regulatory (B) subunit, which directs PP2A to specific targets. PP2A inhibition is required to completely transform human cells and siRNA screens have shown that loss of B56α, B56γ, or PR72/PR130 B subunits results in cell transformation, which are involved in negatively regulating oncogenic pathways including Wnt, Akt, and Myc. Moreover, PP2A plays an important role in regulating human embryonic stem cells, where both the A and C subunit levels and PP2A activity gradually increase during the course of differentiation. Our preliminary data also demonstrates in breast cancer cell lines that B56α overexpression decreases the number of mammospheres whereas inhibiting B56α increases them. Further research is needed to address the function of specific PP2A complexes in regulating stem cells. Despite the fact that B56α depletion is important for human cell transformation, no one has reported on its role in vivo and no mouse model has ever been generated to our knowledge. Therefore, to study the role of B56α in stem cells and cell transformation in vivo, we have generated B56α knockout mice. We generated B56α knockout mice using commercially available gene trap ES cells. The gene trap sits in the first intron of the B56α gene and, through a splice acceptor site, splices out the first exon to the gene trap and therefore stops whole gene transcription in the whole body. The primary phenotypic effect we observed with B56α loss was spontaneous skin lesion formation. Thus far 40% of mice have developed skin lesions between 11 to 21 months and showed hyper-proliferation of the epidermis, hair follicles, and dermis. Increased number of hair follicles prompted us to perform stem cell assays using BrDU to label long term label-retaining cells and the colony assays for the fast adhering keratinocyte stem cells. We found an increase in skin stem cell numbers. In addition to skin lesions, we found increased inflammation both in skin lesions, liver, and spleen and some enlarged lymph nodes. Next we asked whether loss of B56α also increased the number of hematopoietic stem cell. Although the number of hematopoietic stem cells in the periphery was unchanged under normal condition, treating mice with granulocyte colony-stimulating factor increased the number of circulating stem cells. Our model suggests that loss of B56α increases the number of stem cells and that B56α is important for cells to maintain homeostasis; B56α loss can lead to increased activity of important oncogenes, such as cMyc, leading to aberrant cell growth and defects in stem cell maintenance. More experiments need to be done to understand the primary effect of B56α loss in skin and the interplay between skin lesions and inflammation. Citation Format: Mahnaz Janghorban, Derek Zachman, Xiaoyan Wang, Anupriya Agarwal, William Fleming, Rosalie Sears. Regulation of stem cells by the tumor suppressor PP2A-B56α. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3180. doi:10.1158/1538-7445.AM2015-3180

Abstract 771: Pin1 regulates the dynamics of c-Myc DNA binding to facilitate target gene regulation and oncogenesis.

The c-Myc oncoprotein is generally considered to be a master regulator of gene transcription by virtue of its ability to activate or repress the expression of a large percentage of all genes. However, mechanisms that direct c-Myc9s 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 regulates c-Myc transcriptional activity by altering c-Myc9s association with target gene promoters. Specifically, we show that Pin1 enhances the recruitment of Serine62-phosphorylated c-Myc along with its co-activators to selected target gene promoters during gene activation, followed by promoting c-Myc9s release associated with its degradation, resulting in cyclic c-Myc DNA binding. This facilitates c-Myc9s activation of target genes associated with an increase in elongating RNA Polymerase II in the gene body. Genes impacted by this mechanism are involved in cell growth and metabolism, resulting in enhanced pro-proliferative activity, even while controlling c-Myc expression. In cancer cells with impaired c-Myc degradation, Pin1 still regulates cyclic c-Myc DNA binding, even though it no longer enhances c-Myc degradation. This allows for simultaneous high expression of Pin1 and c-Myc in cancer, which can drive a gene expression pattern that we show is enriched in poor outcome subtypes of breast cancer. This study provides new insight into mechanisms regulating c-Myc DNA binding and oncogenic activity, it reveals a novel role for Pin1 in transcription factor regulation, and it elucidates a mechanism that can contribute to oncogenic cooperation between Pin1 and c-Myc. Citation Format: Amy Farrell, Carl Pelz, Xiaoyan Wang, Colin Daniel, Yulong Su, Mahnaz Janghorban, Soren Impey, Rosalie C. Sears. Pin1 regulates the dynamics of c-Myc DNA binding to facilitate target gene regulation and oncogenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 771. doi:10.1158/1538-7445.AM2013-771