Abstract 79: Crosstalk of PIM and LKB1 kinases in driving growth of prostate cancer cells

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.

PIM kinases inhibit AMPK activation and promote tumorigenicity by phosphorylating LKB1

BACKGROUND

The oncogenic PIM kinases and the tumor-suppressive LKB1 kinase have both been implicated in the regulation of cell growth and metabolism, albeit in opposite directions. Here we investigated whether these kinases interact with each other to influence AMPK activation and tumorigenic growth of prostate and breast cancer cells.

METHODS

We first determined how PIM and LKB1 kinases affect AMPK phosphorylation levels. We then used in vitro kinase assays to demonstrate that LKB1 is phosphorylated by PIM kinases, and site-directed mutagenesis to identify the PIM target sites in LKB1. The cellular functions of PIM and LKB1 kinases were evaluated using either pan-PIM inhibitors or CRISPR/Cas9 genomic editing, with which all three PIM family members and/or LKB1 were knocked out from PC3 prostate and MCF7 breast cancer cell lines. In addition to cell proliferation assays, we examined the effects of PIM and/or LKB1 loss on tumor growth using the chick embryo chorioallantoic membrane (CAM) xenograft model.

RESULTS

We provide both genetic and pharmacological evidence to demonstrate that inhibition of PIM expression or activity increases phosphorylation of AMPK at Thr172 in both PC3 and MCF7 cells, but not in their derivatives lacking LKB1. This is explained by our observation that all three PIM family kinases can phosphorylate LKB1 at Ser334. Wild-type LKB1, but not its phosphodeficient derivative, can restore PIM inhibitor-induced AMPK phosphorylation in LKB1 knock-out cells. In the CAM model, loss of LKB1 enhances tumorigenicity of PC3 xenografts, while cells lacking both LKB1 and PIMs exhibit slower proliferation rates and form smaller tumors.

CONCLUSION

PIM kinases are novel negative regulators of LKB1 that affect AMPK activity in an LKB1-dependent fashion. The impairment of cell proliferation and tumor growth in cells lacking both LKB1 and PIMs indicates that these kinases possess a shared signaling role in the context of cancer. These data also suggest that PIM inhibitors may be a rational therapeutic option for LKB1-deficient tumors. Video Abstract.

PIM-induced phosphorylation of Notch3 promotes breast cancer tumorigenicity in a CSL-independent fashion

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.

PIM1 accelerates prostate cancer cell motility by phosphorylating actin capping proteins

Background

The PIM family kinases promote cancer cell survival and motility as well as metastatic growth in various types of cancer. We have previously identified several PIM substrates, which support cancer cell migration and invasiveness. However, none of them are known to regulate cellular movements by directly interacting with the actin cytoskeleton. Here we have studied the phosphorylation-dependent effects of PIM1 on actin capping proteins, which bind as heterodimers to the fast-growing actin filament ends and stabilize them.

Methods

Based on a phosphoproteomics screen for novel PIM substrates, we have used kinase assays and fluorescence-based imaging techniques to validate actin capping proteins as PIM1 substrates and interaction partners. We have analysed the functional consequences of capping protein phosphorylation on cell migration and adhesion by using wound healing and real-time impedance-based assays. We have also investigated phosphorylation-dependent effects on actin polymerization by analysing the protective role of capping protein phosphomutants in actin disassembly assays.

Results

We have identified capping proteins CAPZA1 and CAPZB2 as PIM1 substrates, and shown that phosphorylation of either of them leads to increased adhesion and migration of human prostate cancer cells. Phosphorylation also reduces the ability of the capping proteins to protect polymerized actin from disassembly.

Conclusions

Our data suggest that PIM kinases are able to induce changes in actin dynamics to support cell adhesion and movement. Thus, we have identified a novel mechanism through which PIM kinases enhance motility and metastatic behaviour of cancer cells.

Phosphorylation of NFATC1 at PIM1 target sites is essential for its ability to promote prostate cancer cell migration and invasion

Background

Progression of prostate cancer from benign local tumors to metastatic carcinomas is a multistep process. Here we have investigated the signaling pathways that support migration and invasion of prostate cancer cells, focusing on the role of the NFATC1 transcription factor and its post-translational modifications. We have previously identified NFATC1 as a substrate for the PIM1 kinase and shown that PIM1-dependent phosphorylation increases NFATC1 activity without affecting its subcellular localization. Both PIM kinases and NFATC1 have been reported to promote cancer cell migration, invasion and angiogenesis, but it has remained unclear whether the effects of NFATC1 are phosphorylation-dependent and which downstream targets are involved.

Methods

We used mass spectrometry to identify PIM1 phosphorylation target sites in NFATC1, and analysed their functional roles in three prostate cancer cell lines by comparing phosphodeficient mutants to wild-type NFATC1. We used luciferase assays to determine effects of phosphorylation on NFAT-dependent transcriptional activity, and migration and invasion assays to evaluate effects on cell motility. We also performed a microarray analysis to identify novel PIM1/NFATC1 targets, and validated one of them with both cellular expression analyses and in silico in clinical prostate cancer data sets.

Results

Here we have identified ten PIM1 target sites in NFATC1 and found that prevention of their phosphorylation significantly decreases the transcriptional activity as well as the pro-migratory and pro-invasive effects of NFATC1 in prostate cancer cells. We observed that also PIM2 and PIM3 can phosphorylate NFATC1, and identified several novel putative PIM1/NFATC1 target genes. These include the ITGA5 integrin, which is differentially expressed in the presence of wild-type versus phosphorylation-deficient NFATC1, and which is coexpressed with PIM1 and NFATC1 in clinical prostate cancer specimens.

Conclusions

Based on our data, phosphorylation of PIM1 target sites stimulates NFATC1 activity and enhances its ability to promote prostate cancer cell migration and invasion. Therefore, inhibition of the interplay between PIM kinases and NFATC1 may have therapeutic implications for patients with metastatic forms of cancer.

Graphical abstract

Phosphorylation of Notch1 by Pim kinases promotes oncogenic signaling in breast and prostate cancer cells

Tumorigenesis is a multistep process involving co-operation between several deregulated oncoproteins. In this study, we unravel previously unrecognized interactions and crosstalk between Pim kinases and the Notch signaling pathway, with implications for both breast and prostate cancer. We identify Notch1 and Notch3, but not Notch2, as novel Pim substrates and demonstrate that for Notch1, the serine residue 2152 is phosphorylated by all three Pim family kinases. This target site is located in the second nuclear localization sequence (NLS) of the Notch1 intracellular domain (N1ICD), and is shown to be important for both nuclear localization and transcriptional activity of N1ICD. Phosphorylation-dependent stimulation of Notch1 signaling promotes migration of prostate cancer cells, balances glucose metabolism in breast cancer cells, and supports in vivo growth of both types of cancer cells on chick embryo chorioallantoic membranes. Furthermore, Pim-induced growth of orthotopic prostate xenografts in mice is associated with enhanced nuclear Notch1 activity. Finally, simultaneous inhibition of Pim and Notch abrogates the cellular responses more efficiently than individual treatments, opening up new vistas for combinatorial cancer therapy.

The PIM1 kinase promotes prostate cancer cell migration and adhesion via multiple signalling pathways

The ability of cells to migrate and form metastases is one of the fatal hallmarks of cancer that can be conquered only with better understanding of the molecules and regulatory mechanisms involved. The oncogenic PIM kinases have been shown to support cancer cell survival and motility, but the PIM-regulated pathways stimulating cell migration and invasion are less well characterized than those affecting cell survival. Here we have identified the glycogen synthase kinase 3β (GSK3B) and the forkhead box P3 (FOXP3) transcription factor as direct PIM targets, whose tumour-suppressive effects in prostate cancer cells are inhibited by PIM-induced phosphorylation, resulting in increased cell migration. Targeting GSK3B is also essential for the observed PIM-enhanced expression of the prostaglandin-endoperoxide synthase 2 (PTGS2), which is an important regulator of both cell migration and adhesion. Accordingly, selective inhibition of PIM activity not only reduces cell migration, but also affects integrin-mediated cell adhesion. Taken together, these data provide novel mechanistic insights on how and why patients with metastatic prostate cancer may benefit from therapies targeting PIM kinases, and how such approaches may also be applicable to inflammatory conditions.

Tricyclic Benzo[cd]azulenes selectively inhibit activities of Pim kinases and restrict growth of Epstein-Barr virus-transformed cells

Oncogenic Pim family kinases are often overexpressed in human hematopoietic malignancies as well as in solid tumours. These kinases contribute to tumorigenesis by promoting cell survival and by enhancing resistance against chemotherapy and radiation therapy. Furthermore, we have recently shown that they increase the metastatic potential of adherent cancer cells. Here we describe identification of tricyclic benzo[cd]azulenes and their derivatives as effective and selective inhibitors of Pim kinases. These compounds inhibit Pim autophosphorylation and abrogate the anti-apoptotic effects of Pim kinases. They also reduce cancer cell motility and suppress proliferation of lymphoblastoid cell lines infected and immortalized by the Epstein-Barr virus. Thus, these novel Pim-selective inhibitors provide promising compounds for both research and therapeutic purposes.

Use of copper(I) catalyzed azide alkyne cycloaddition (CuAAC) for the preparation of conjugated pyrrolo[2,3-a]carbazole Pim kinase inhibitors

We have previously demonstrated that pyrrolo[2,3-a]carbazole-3-carbaldehydes are potent Pim kinase inhibitors with in vitro antiproliferative activities. In the present study, we report the synthesis of new pyrrolocarbazoles substituted at the N-10 position. When their ability to inhibit Pim kinase activities were evaluated in in vitro assays, we observed that this nitrogen atom can be substituted without loss of Pim-1 and Pim-3 inhibitory potencies. Moreover, when we added a fluorescent dansyl group (compound 13), we were able to show that 13 penetrates the plasma membrane and enters the cytoplasm.

Pim-selective inhibitor DHPCC-9 reveals Pim kinases as potent stimulators of cancer cell migration and invasion

BACKGROUND

Pim family kinases are small constitutively active serine/threonine-specific kinases, elevated levels of which have been detected in human hematopoietic malignancies as well as in solid tumours. While we and others have previously shown that the oncogenic Pim kinases stimulate survival of hematopoietic cells, we now examined their putative role in regulating motility of adherent cancer cells. For this purpose, we inhibited Pim kinase activity using a small molecule compound, 1,10-dihydropyrrolo[2,3-a]carbazole-3-carbaldehyde (DHPCC-9), which we had recently identified as a potent and selective inhibitor for all Pim family members.

RESULTS

We now demonstrate that the Pim kinase inhibitor DHPCC-9 is very effective also in cell-based assays. DHPCC-9 impairs the anti-apoptotic effects of Pim-1 in cytokine-deprived myeloid cells and inhibits intracellular phosphorylation of Pim substrates such as Bad. Moreover, DHPCC-9 slows down migration and invasion of cancer cells derived from either prostate cancer or squamocellular carcinoma patients. Silencing of Pim expression reduces cell motility, while Pim overexpression enhances it, strongly suggesting that the observed effects of DHPCC-9 are dependent on Pim kinase activity. Interestingly, DHPCC-9 also abrogates NFATc-dependent migration of cancer cells, implying that NFATc factors mediate at least part of the pro-migratory effects of Pim kinases.

CONCLUSIONS

Altogether, our data indicate that DHPCC-9 is not only a powerful tool to investigate physiological effects of the oncogenic Pim family kinases, but also an attractive molecule for drug development to inhibit invasiveness of Pim-overexpressing cancer cells.