Artur Padzik

JNK1 phosphorylation of SCG10 determines microtubule dynamics and axodendritic length

c-Jun NH2-terminal kinases (JNKs) are essential during brain development, when they regulate morphogenic changes involving cell movement and migration. In the adult, JNK determines neuronal cytoarchitecture. To help uncover the molecular effectors for JNKs in these events, we affinity purified JNK-interacting proteins from brain. This revealed that the stathmin family microtubule-destabilizing proteins SCG10, SCLIP, RB3, and RB3′ interact tightly with JNK. Furthermore, SCG10 is also phosphorylated by JNK in vivo on sites that regulate its microtubule depolymerizing activity, serines 62 and 73. SCG10-S73 phosphorylation is significantly decreased in JNK1−/− cortex, indicating that JNK1 phosphorylates SCG10 in developing forebrain. JNK phosphorylation of SCG10 determines axodendritic length in cerebrocortical cultures, and JNK site–phosphorylated SCG10 colocalizes with active JNK in embryonic brain regions undergoing neurite elongation and migration. We demonstrate that inhibition of cytoplasmic JNK and expression of SCG10-62A/73A both inhibited fluorescent tubulin recovery after photobleaching. These data suggest that JNK1 is responsible for regulation of SCG10 depolymerizing activity and neurite elongation during brain development.

Phosphorylation of SCG10/stathmin-2 determines multipolar stage exit and neuronal migration rate

Cell migration is the consequence of the sum of positive and negative regulatory mechanisms. Although appropriate migration of neurons is a principal feature of brain development, the negative regulatory mechanisms remain obscure. We found that JNK1 was highly active in developing cortex and that selective inhibition of JNK in the cytoplasm markedly increased both the frequency of exit from the multipolar stage and radial migration rate and ultimately led to an ill-defined cellular organization. Moreover, regulation of multipolar-stage exit and radial migration in Jnk1−/− (also known as Mapk8) mice, resulted from consequential changes in phosphorylation of the microtubule regulator SCG10 (also called stathmin-2). Expression of an SCG10 mutant that mimics the JNK1-phosphorylated form restored normal migration in the brains of Jnk1−/− mouse embryos. These findings indicate that the phosphorylation of SCG10 by JNK1 is a fundamental mechanism that governs the transition from the multipolar stage and the rate of neuronal cell movement during cortical development.

Competitive binding of Rab21 and p120RasGAP to integrins regulates receptor traffic and migration

P120RasGAP competes with Rab21 for binding to the cytoplasmic domain of integrin α-subunits, thereby promoting receptor escape from early endosomes and recycling to the plasma membrane.

c-Jun N-Terminal Kinase Phosphorylation of MARCKSL1 Determines Actin Stability and Migration in Neurons and in Cancer Cells

ABSTRACT Cell migration is a fundamental biological function, critical during development and regeneration, whereas deregulated migration underlies neurological birth defects and cancer metastasis. MARCKS-like protein 1 (MARCKSL1) is widely expressed in nervous tissue, where, like Jun N-terminal protein kinase (JNK), it is required for neural tube formation, though the mechanism is unknown. Here we show that MARCKSL1 is directly phosphorylated by JNK on C-terminal residues (S120, T148, and T183). This phosphorylation enables MARCKSL1 to bundle and stabilize F-actin, increase filopodium numbers and dynamics, and retard migration in neurons. Conversely, when MARCKSL1 phosphorylation is inhibited, actin mobility increases and filopodium formation is compromised whereas lamellipodium formation is enhanced, as is cell migration. We find that MARCKSL1 mRNA is upregulated in a broad range of cancer types and that MARCKSL1 protein is strongly induced in primary prostate carcinomas. Gene knockdown in prostate cancer cells or in neurons reveals a critical role for MARCKSL1 in migration that is dependent on the phosphorylation state; phosphomimetic MARCKSL1 (MARCKSL1S120D,T148D,T183D) inhibits whereas dephospho-MARCKSL1S120A,T148A,T183A induces migration. In summary, these data show that JNK phosphorylation of MARCKSL1 regulates actin homeostasis, filopodium and lamellipodium formation, and neuronal migration under physiological conditions and that, when ectopically expressed in prostate cancer cells, MARCKSL1 again determines cell movement.

FiloQuant reveals increased filopodia density during breast cancer progression

Defective filopodia formation is linked to pathologies such as cancer, wherein actively protruding filopodia, at the invasive front, accompany cancer cell dissemination. Despite wide biological significance, delineating filopodia function in complex systems remains challenging and is particularly hindered by lack of compatible methods to quantify filopodia properties. Here, we present FiloQuant, a freely available ImageJ plugin, to detect filopodia-like protrusions in both fixed- and live-cell microscopy data. We demonstrate that FiloQuant can extract quantifiable information, including protrusion dynamics, density, and length, from multiple cell types and in a range of microenvironments. In cellular models of breast ductal carcinoma in situ, we reveal a link between filopodia formation at the cell–matrix interface, in collectively invading cells and 3D tumor spheroids, and the in vitro invasive capacity of the carcinoma. Finally, using intravital microscopy, we observe that tumor spheroids display filopodia in vivo, supporting a potential role for these protrusions during tumorigenesis.

PP2A inhibition is a druggable MEK inhibitor resistance mechanism in KRAS-mutant lung cancer cells

Pharmacological PP2A activation is a druggable approach to overcome MEK inhibitor resistance. An oncogene’s enemy is our friend Inhibitors of oncogenic kinases such as MEK are becoming increasingly common as an approach to treating cancer, but these drugs’ effectiveness is often short-lived, as tumors develop resistance. Phosphatases, a class of proteins whose activity counteracts that of kinases, are not routinely targeted by cancer therapies but may offer an alternative approach to treatment in some cases. In particular, Kauko et al. determined that the loss of a phosphatase called PP2A can play a major role in resistance to MEK inhibition in lung cancer. The authors also selected a compound that activates PP2A and demonstrated that it can effectively combine with a MEK inhibitor and overcome drug resistance in multiple mouse models of human lung cancer. Kinase inhibitor resistance constitutes a major unresolved clinical challenge in cancer. Furthermore, the role of serine/threonine phosphatase deregulation as a potential cause for resistance to kinase inhibitors has not been thoroughly addressed. We characterize protein phosphatase 2A (PP2A) activity as a global determinant of KRAS-mutant lung cancer cell resistance across a library of >200 kinase inhibitors. The results show that PP2A activity modulation alters cancer cell sensitivities to a large number of kinase inhibitors. Specifically, PP2A inhibition ablated mitogen-activated protein kinase kinase (MEK) inhibitor response through the collateral activation of AKT/mammalian target of rapamycin (mTOR) signaling. Combination of mTOR and MEK inhibitors induced cytotoxicity in PP2A-inhibited cells, but even this drug combination could not abrogate MYC up-regulation in PP2A-inhibited cells. Treatment with an orally bioavailable small-molecule activator of PP2A DT-061, in combination with the MEK inhibitor AZD6244, resulted in suppression of both p-AKT and MYC, as well as tumor regression in two KRAS-driven lung cancer mouse models. DT-061 therapy also abrogated MYC-driven tumorigenesis. These data demonstrate that PP2A deregulation drives MEK inhibitor resistance in KRAS-mutant cells. These results emphasize the need for better understanding of phosphatases as key modulators of cancer therapy responses.

KIF5C S176 Phosphorylation Regulates Microtubule Binding and Transport Efficiency in Mammalian Neurons

Increased phosphorylation of the KIF5 anterograde motor is associated with impaired axonal transport and neurodegeneration, but paradoxically also with normal transport, though the details are not fully defined. JNK phosphorylates KIF5C on S176 in the motor domain; a site that we show is phosphorylated in brain. Microtubule pelleting assays demonstrate that phosphomimetic KIF5C(1-560)S176D associates weakly with microtubules compared to KIF5C(1-560)WT. Consistent with this, 50% of KIF5C(1-560)S176D shows diffuse movement in neurons. However, the remaining 50% remains microtubule bound and displays decreased pausing and increased bidirectional movement. The same directionality switching is observed with KIF5C(1-560)WT in the presence of an active JNK chimera, MKK7-JNK. Yet, in cargo trafficking assays where peroxisome cargo is bound, KIF5C(1-560)S176D-GFP-FRB transports normally to microtubule plus ends. We also find that JNK increases the ATP hydrolysis of KIF5C in vitro. These data suggest that phosphorylation of KIF5C-S176 primes the motor to either disengage entirely from microtubule tracks as previously observed in response to stress, or to display improved efficiency. The final outcome may depend on cargo load and motor ensembles.

FiloQuant reveals increased filopodia density during DCIS progression

Filopodia are commonly observed cellular protrusions in vitro and in vivo. Defective filopodia formation is linked to several pathologies including cancer, wherein actively protruding filopodia, at the invasive front, and filopodia-mediated probing of the microenvironment accompanies cancer cell dissemination. Despite wide biological significance, delineating the function of these finger-like protrusions in more complex systems remains technically challenging, particularly hindered by lack of compatible methods to quantify filopodia properties. Here, we present FiloQuant, a freely available ImageJ plugin, to detect filopodia and filopodia-like protrusions in both fixed and live-cell microscopy data. We demonstrate that FiloQuant can extract quantifiable information including protrusion dynamics, density and length from multiple cell types and in a range of microenvironments, such as during collective or single cancer cell migration in 2D and 3D, in fixed neuronal cultures, in activated natural killer cells and in sprouting endothelial cells in vivo. In cellular models of breast ductal carcinoma in situ (DCIS) we reveal a link between filopodia formation at the cell-matrix interface, during collective invasion and in 3D tumour spheroids, with the previously reported local invasive potential of these breast cancer models in vivo. Finally, using intravital microscopy, we observed that tumour spheroids display prominent filopodia in vivo, supporting a potential role for these protrusions during tumorigenesis.

SORLA-driven endosomal trafficking regulates the oncogenic fitness of HER2

Human epidermal growth factor receptor 2 (HER2) is an oncogene targeted by several kinase inhibitors and therapeutic antibodies. Endosomal trafficking of many other receptor tyrosine kinases regulates their oncogenic signaling, but the prevailing view is that HER2 is retained on the cell surface. Here we reveal that in cancer cells Sortilin related receptor 1 (SORLA; SORL1) forms a complex with HER2 and regulates its subcellular distribution by promoting recycling of endosomal HER2 back to plasma membrane. Expression of SORLA in cancer cell lines and bladder cancers correlates with HER2 levels. Depletion of SORLA targets HER2 to late endosomal/lysosomal compartments, impairs HER2-driven signaling and in vivo tumor growth. SORLA silencing also disrupts normal lysosome function and sensitizes anti-HER2 therapy sensitive and resistant cancer cells to lysosome-targeting cationic amphiphilic drugs. These findings reveal potentially important SORLA-dependent endosomal trafficking-linked vulnerabilities in HER2-driven cancers.

Abstract 5560: Systemic map of protein phosphatase 2A (PP2A)-regulated phosphotargets and drug responses in cancer cells

Despite the pivotal role of phosphatases in cancer cell signalling, systemic understanding of phosphatase targets is still at infancy. Protein phosphatase 2A (PP2A) is a human tumor suppressor complex. PP2A inhibition is a requirement for human cell transformation and PP2A regulates many cancer critical signalling pathways. Importantly, emerging data indicates that reactivation of PP2A tumor suppressor activity could provide entirely novel approach for cancer therapy. Here, we present first systemic analysis of phosphoprotein targets (dephosphorylome) regulated by PP2A in cancer cells. Based on data, PP2A regulates cancer critical signalling pathways, including entire EGFR-RAS-RAF-MEK-ERK cascade, and functions as a master regulator of MYC function. At network level, PP2A targets critical cellular processes such as chromosome organization, RNA splicing, and nuclear envelope assembly. Surprisingly, soft clustering of PP2A dephosphorylome revealed that most phospho-target residues are subject to only unidirectional regulation in cancer cells. Moreover, targets show intracellular gradient where phosphatase inhibition dominates nuclear phosphorylation balance. Since phosphoregulation is critical for cancer drug responses, dephosphorylome was correlated with cancer cell responses to over 300 drugs. Importantly, cancer therapies could be broadly classified based on their dephosphorylome, both at quantitative and qualitative manner. Finally, we demonstrate the utility of this large dataset by validating the role of PP2A in MEK inhibitor resistance in KRAS mutant cancer cells via regulation of RAF, mTor and MYC. The study presents first systemic resource to understand potential of PP2A manipulation in cancer cell signalling and drug responses. The study also uncovers generally important insights to phosphoregulation. Citation Format: Otto Kauko, Susumu Imanishi, Evgeny Kulesskiy, Teemu D. Laajala, Laxmana Yetukuri, Artur Padzik, Mikael Jumppanen, Pekka Haapaniemi, Bhagwan Yadaw, Veronika Suni, Taru Varila, Garry Corthals, Wennerberg Krister, Tero Aittokallio, Jukka Westermarck. Systemic map of protein phosphatase 2A (PP2A)-regulated phosphotargets and drug responses in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5560. doi:10.1158/1538-7445.AM2017-5560