Estrogen receptor α (ERα) is a nuclear hormone receptor that mediates the biological effects of estrogen and is known to be phosphorylated at multiple sites. Despite several reports highlighting serine-167 (S167) of ERα as a common phosphorylation site for various kinases, there have been contradictory data regarding its relevance in the context of ERα-expressing luminal A breast cancer. In this study, we show that S167 is targeted by the oncogenic PIM family kinases in these cells. Loss of PIM expression or catalytic activity reduces S167 phosphorylation and restricts ERα signaling, while PIM overexpression has the opposite effect. Nonetheless, we found no evidence that PIMs protect breast cancer cells from ERα blockade, as both PIM protein levels and ERα S167 phosphorylation levels were substantially lower in populations of cells that had developed tamoxifen resistance as compared to untreated cells. However, this does not rule out the possibility that PIMs provide protection against ERα-targeting therapies in such breast cancer cells in which they are overexpressed, warranting further investigations.

Abstract The oncogenic PIM kinases and the tumor-suppressive LKB1 kinase have been linked to regulation of cell growth and proliferation. Here we have investigated their potential interactions as well as their relative impacts on tumorigenic growth of prostate cancer cells. The cellular functions of PIM and LKB1 kinases were evaluated using either pan-PIM inhibitors or CRISPR/Cas9-based genomic editing, with which all three PIM family members and/or LKB1 were knocked out from PC3 prostate cancer cells. In addition to cell-based 2D proliferation assays, we examined tumor growth using the chick embryo chorioallantoic membrane (CAM) xenograft model. In this report, we show that inhibition of PIM expression or activity results in increased phosphorylation of AMPK at Thr172 in an LKB1-dependent fashion. With in vitro kinase assays, we demonstrate that LKB1 is a novel direct substrate for PIM kinases and that Ser334 is one of the PIM target sites in LKB1. Accordingly, wild-type LKB1, but not the phosphodeficient S334A mutant can restore PIM inhibitor-induced AMPK phosphorylation in LKB1 knock-out cells. Whereas loss of LKB1 exaggerates formation of PC3-based tumors in the CAM xenograft model, co-deletion of PIM kinases attenuates it. The impairment of cell proliferation and tumor growth in cells lacking both PIM and LKB1 kinases not only underscores the potential crosstalk in signaling between these kinases, but also suggest that PIM inhibitors could be used to restrain growth of LKB1-deficient tumors. Citation Format: Kwan Long Mung, William Eccleshall, Niina M. Santio, Adolfo Rivero-Müller, Cecilia Sahlgren, Päivi J. Koskinen. Crosstalk of PIM and LKB1 kinases in driving growth of prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 79.

Dysregulation of the developmentally important Notch signaling pathway is implicated in several types of cancer, including breast cancer. However, the specific roles and regulation of the four different Notch receptors have remained elusive. We have previously reported that the oncogenic PIM kinases phosphorylate Notch1 and Notch3. Phosphorylation of Notch1 within the second nuclear localization sequence of its intracellular domain (ICD) enhances its transcriptional activity and tumorigenicity. In this study, we analyzed Notch3 phosphorylation and its functional impact. Unexpectedly, we observed that the PIM target sites are not conserved between Notch1 and Notch3. Notch3 ICD (N3ICD) is phosphorylated within a domain, which is essential for formation of a transcriptionally active complex with the DNA-binding protein CSL. Through molecular modeling, X-ray crystallography, and isothermal titration calorimetry, we demonstrate that phosphorylation of N3ICD sterically hinders its interaction with CSL and thereby inhibits its CSL-dependent transcriptional activity. Surprisingly however, phosphorylated N3ICD still maintains tumorigenic potential in breast cancer cells under estrogenic conditions, which support PIM expression. Taken together, our data indicate that PIM kinases modulate the signaling output of different Notch paralogs by targeting distinct protein domains and thereby promote breast cancer tumorigenesis via both CSL-dependent and CSL-independent mechanisms.

Kwan Long Mung University of Turku Faculty of Mathematics and Natural Sciences: Turun yliopiston luonnontieteiden ja tekniikan tiedekunta William Eccleshall University of Turku Faculty of Mathematics and Natural Sciences: Turun yliopiston luonnontieteiden ja tekniikan tiedekunta Niina M Santio University of Turku Faculty of Mathematics and Natural Sciences: Turun yliopiston luonnontieteiden ja tekniikan tiedekunta Adolfo Rivero-Müller Medical University of Lublin: Uniwersytet Medyczny w Lublinie Päivi J. Koskinen (  paivi.koskinen@utu. ) Turun Yliopisto https://orcid.org/0000-0002-6864-4457