The RAC1 activator Tiam1 regulates centriole duplication through controlling PLK4 levels

Centriole duplication is tightly controlled to maintain correct centriole number through the cell cycle. Key to this is the regulated degradation of PLK4, the master regulator of centriole duplication. Here, we show that the Rac1 guanine nucleotide exchange factor (GEF) Tiam1 localises to centrosomes during S-phase, where it is required for the maintenance of normal centriole number. Depletion of Tiam1 leads to an increase in centrosomal PLK4 and centriole overduplication, whereas overexpression of Tiam1 can restrict centriole overduplication. Ultimately, Tiam1 depletion leads to lagging chromosomes at anaphase and aneuploidy, which are potential drivers of malignant progression. The effects of Tiam1 depletion on centrosomal PLK4 levels and centriole overduplication can be rescued by re-expression of both wild-type Tiam1 and catalytically inactive (GEF*) Tiam1, but not by Tiam1 mutants unable to bind to the F-box protein βTRCP (also known as F-box/WD repeat-containing protein 1A) implying that Tiam1 regulates PLK4 levels through promoting βTRCP-mediated degradation independently of Rac1 activation.

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 MYC's 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 Laiterä, Mahnaz Janghorban, Juha Okkeri, Juha Klefström, 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.

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

An understanding of the mechanisms determining MYC's 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.

ROS production and NF-κB activation triggered by RAC1 facilitate WNT-driven intestinal stem cell proliferation and colorectal cancer initiation

The Adenomatous Polyposis Coli (APC) gene is mutated in the majority of colorectal cancers (CRCs). Loss of APC leads to constitutively active WNT signaling, hyperproliferation, and tumorigenesis. Identification of pathways that facilitate tumorigenesis after APC loss is important for therapeutic development. Here, we show that RAC1 is a critical mediator of tumorigenesis after APC loss. We find that RAC1 is required for expansion of the LGR5 intestinal stem cell (ISC) signature, progenitor hyperproliferation, and transformation. Mechanistically, RAC1-driven ROS and NF-κB signaling mediate these processes. Together, these data highlight that ROS production and NF-κB activation triggered by RAC1 are critical events in CRC initiation.

The miR-15a-miR-16-1 locus is homozygously deleted in a subset of prostate cancers

MicroRNAs (miRNAs) are small, non-coding RNAs that negatively regulate the expression of protein coding genes. In this study, we screened highly informative prostate cancer cell lines and xenografts (n = 42) for miRNA gene copy number and expression changes. The expression profiling showed distinction between cell lines and xenografts as well as between androgen sensitive and independent models. Only a few copy number alterations that were associated with expression changes were identified. Most importantly, the miR-15a-miR-16-1 locus was found to be homozygously deleted in two samples leading to the abolishment of miR-15a, but not miR-16, expression. miR-16 is also expressed from another genomic locus. Mutation screening of the miR-15a-miR-16-1 gene in the model systems as well as clinical samples (n = 50) revealed no additional mutations. In conclusion, our data indicate that putative tumor suppressors, miR-15a and miR-16-1, are homozygously deleted in a subset of prostate cancers, further suggesting that these miRNAs could be important in the development of prostate cancer.