The kinase activity of kinase suppressor of Ras1 (KSR1) is independent of bound MEK

The dual function of KSR1: a pseudokinase and beyond

Protein kinases play a pivotal role in regulating many aspects of biological processes, including development, differentiation and cell death. Within the kinome, 48 kinases (~10%) are classified as pseudokinases owing to the fact that they lack at least one conserved catalytic residue in their kinase domain. However, emerging evidence suggest that some pseudokinases, even without the ability to phosphorylate substrates, are regulators of multiple cellular signalling pathways. Among these is KSR1 (kinase suppressor of Ras 1), which was initially identified as a novel kinase in the Ras/Raf pathway. Subsequent studies showed that KSR1 mainly functions as a platform to assemble different cellular components thereby facilitating signal transduction. In the present article, we discuss recent findings regarding KSR1, indicating that it has dual activity as an active kinase as well as a pseudokinase/scaffolding protein. Moreover, the biological functions of KSR1 in human disorders, notably in malignancies, are also reviewed.

KSR1 is overexpressed in endometrial carcinoma and regulates proliferation and TRAIL-induced apoptosis by modulating FLIP levels

The Raf/MEK/extracellular signal-regulated kinase (ERK) pathway participates in many processes altered in development and progression of cancer in human beings such as proliferation, transformation, differentiation, and apoptosis. Kinase suppressor of Ras 1 (KSR1) can interact with various kinases of the Raf/MEK/extracellular signal-regulated kinase pathway to enhance its activation. The role of KSR1 in endometrial carcinogenesis was investigated. cDNA and tissue microarrays demonstrated that expression of KSR1 was up-regulated in endometrial carcinoma. Furthermore, inhibition of KSR1 expression by specific small hairpin RNA resulted in reduction of both proliferation and anchorage-independent cell growth properties of endometrial cancer cells. Because inhibition of apoptosis has a pivotal role in endometrial carcinogenesis, the effects of KSR1 in regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis were investigated. KSR1 knock-down sensitized resistant endometrial cell lines to both TRAIL- and Fas-induced apoptosis. Sensitization to TRAIL and agonistic anti-Fas antibody was caused by down-regulation of FLIP (FLICE-inhibitory protein). Also investigated was the molecular mechanism by which KSR1 regulates FLIP protein levels. It was demonstrated that KSR1 small hairpin RNA did not affect FLIP transcription or degradation. Rather, FLIP down-regulation was caused by Fas-associated death domain protein-dependent inhibition of FLIP translation triggered after TRAIL stimulation in KSR1-silenced cells. Re-expression of heterologous KSR1 in cells with down-regulated endogenous KSR1 restored FLIP protein levels and TRAIL resistance. In conclusion, KSR1 regulates endometrial sensitivity to TRAIL by regulating FLIP levels.

KSR1 is a functional protein kinase capable of serine autophosphorylation and direct phosphorylation of MEK1

The extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway is a highly conserved signaling pathway that regulates diverse cellular processes including differentiation, proliferation, and survival. Kinase suppressor of Ras-1 (KSR1) binds each of the three ERK cascade components to facilitate pathway activation. Even though KSR1 contains a C-terminal kinase domain, evidence supporting the catalytic function of KSR1 remains controversial. In this study, we produced recombinant wild-type or kinase-inactive (D683A/D700A) KSR1 proteins in Escherichia coli to test the hypothesis that KSR1 is a functional protein kinase. Recombinant wild-type KSR1, but not recombinant kinase-inactive KSR1, underwent autophosphorylation on serine residue(s), phosphorylated myelin basic protein (MBP) as a generic substrate, and phosphorylated recombinant kinase-inactive MAPK/ERK kinase-1 (MEK1). Furthermore, FLAG immunoprecipitates from KSR1(-/-) colon epithelial cells stably expressing FLAG-tagged wild-type KSR1 (+KSR1), but not vector (+vector) or FLAG-tagged kinase-inactive KSR1 (+D683A/D700A), were able to phosphorylate kinase-inactive MEK1. Since TNF activates the ERK pathway in colon epithelial cells, we tested the biological effects of KSR1 in the survival response downstream of TNF. We found that +vector and +D683A/D700A cells underwent apoptosis when treated with TNF, whereas +KSR1 cells were resistant. However, +KSR1 cells were sensitized to TNF-induced cell loss in the absence of MEK kinase activity. These data provide clear evidence that KSR1 is a functional protein kinase, MEK1 is an in vitro substrate of KSR1, and the catalytic activities of both proteins are required for eliciting cell survival responses downstream of TNF.

Modulation of the oncogene-dependent tissue factor expression by kinase suppressor of ras 1

INTRODUCTION

Tissue factor (TF) is the key trigger of the coagulation cascade and the membrane signalling receptor for coagulation protease, factor VIIa. In cancer, TF has been implicated in tumor cell survival, growth, and angiogenesis, and is upregulated as a result of oncogenic transformation.

MATERIALS AND METHODS

We assayed TF expression and tumourigenicity in mice in the case of human cancer cell lines expressing oncogenic receptor tyrosine kinases. These cells were also subjected to genetic modulation of the kinase suppressor of ras 1 (KSR1), and treated with oncoprotein inhibitors in vitro and in vivo.

RESULTS

Here we show that herceptin, AG1478 and CI-1033, inhibitors of two different members of the ErbB family of oncogenes (HER-2 and EGFR), reduce TF levels in epithelial cancer cells. In EGFR-driven A431 cells, TF upregulation is diminished upon genetic targeting of KSR1, the scaffolding protein involved in EGFR signalling. Conversely, upregulation of KSR1 in A431 cells increases their TF expression and tumourigenicity in mice. The latter property remains dependent on EGFR, as pan-Erb (EGFR) inhibitor, CI-1033, blocks TF promoter activity and inhibits tumour formation by the parental and KSR1 overexpressing A431 cells.

CONCLUSIONS

KSR1 emerges, as an important modulator of TF expression in EGFR-driven cancer cells, which also impacts their aggressiveness in vivo.

Proteomic characterization of the dynamic KSR-2 interactome, a signaling scaffold complex in MAPK pathway

KSR-1 is a scaffold protein that is essential for Ras-induced activation of the highly conserved RAF-MEK-ERK kinase module. Previously, we identified a close homolog of KSR-1, called KSR-2, through structural homology-based data mining. In order to further understand the role of KSR-2 in MAPK signaling, we undertook a functional proteomics approach to elucidate the dynamic composition of the KSR-2 functional complex in HEK-293 cells under conditions with and without TNF-alpha stimulation. We found nearly 100 proteins that were potentially associated with KSR-2 complex and 43 proteins that were likely recruited to the super molecular complex after TNF-alpha treatment. Our results indicate that KSR-2 may act as a scaffold protein similar as KSR-1 to mediate the MAPK core (RAF-MEK-ERK) signaling but with a distinct RAF isoform specificity, namely KSR-2 may only mediate the A-RAF signaling while KSR-1 is responsible for transducing signals only from c-RAF. In addition, KSR-2 may be involved in the activation of many MAPK downstream signaling molecules such as p38 MAPK, IKAP, AIF, and proteins involved in ubiquitin-proteasome, apoptosis, cell cycle control, and DNA synthesis and repair pathways, as well as mediating crosstalks between MAPK and several other signaling pathways, including PI3K and insulin signaling. While interactions with these molecules are not known for KSR-1, it's reasonable to hypothesize that KSR-1 may also play a similar role in mediating these downstream signaling pathways.

hKSR-2, a vitamin D-regulated gene, inhibits apoptosis in arabinocytosine-treated HL60 leukemia cells

Ras signaling can be modulated by the scaffolding activity of kinase suppressor of Ras-1 (KSR-1) and by the hKSR-2 protein, resulting in diverse phenotypic outcomes. The mitogen-activated protein kinase cascade downstream from Ras and KSRs includes Raf-1 and extracellular signal-regulated kinase 1/2 kinases, known to enhance survival potential of a range of cell types. Because the molecular events that increase survival of HL60 cells induced to differentiate toward monocytic phenotype by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are not known, we investigated if KSR proteins provide a survival function in these cells. We found that whereas kinase suppressor of Ras-1 had no detectable effect on cell survival in the system studied here, 1,25-(OH)2D3-induced up-regulation of hKSR-2 enhanced the resistance of HL60 cells to arabinocytosine. Knockdown of hKSR-2 by either small interfering RNA or antisense oligonucleotides increased arabinocytosine-induced apoptosis, which was accompanied by reduced Bcl-2/Bax and Bcl-2/Bad ratios, and increased caspase-3 activating cleavage. In contrast, up-regulation of Mcl-1 was not abrogated by anti-sense (AS) AS-hKSR-2, pointing to a specific role of Bcl-2 in control of 1,25-(OH)2D3-induced increased cell survival. These findings are consistent with the previously shown lack of fully differentiated monocytic cells in HL60 cultures exposed to 1,25-(OH)2D3 in which hKSR-2 was knocked down, suggesting that optimal differentiation of these cells requires enhanced antiapoptotic mechanisms provided, at least in part, by hKSR-2. Collectively, these results suggest that hKSR-2 may offer a new target for novel therapies of acute myelogenous leukemia.

Expression of human kinase suppressor of Ras 2 (hKSR-2) gene in HL60 leukemia cells is directly upregulated by 1,25-dihydroxyvitamin D(3) and is required for optimal cell differentiation

Induction of terminal differentiation of neoplastic cells offers potential for a novel approach to cancer therapy. One of the agents being investigated for this purpose in preclinical studies is 1,25-dihydroxyvitamin D(3) (1,25D), which can convert myeloid leukemia cells into normal monocyte-like cells, but the molecular mechanisms underlying this process are not fully understood. Here, we report that 1,25D upregulates the expression of hKSR-2, a new member of a small family of proteins that exhibit evolutionarily conserved function of potentiating ras signaling. The upregulation of hKSR-2 is direct, as it occurs in the presence of cycloheximide, and occurs primarily at the transcriptional level, via activation of vitamin D receptor, which acts as a ligand-activated transcription factor. Two VDRE-type motifs identified in the hKSR-2 gene bind VDR-RXR alpha heterodimers present in nuclear extracts of 1,25D-treated HL60 cells, and chromatin immunoprecipitation assays show that these VDRE motifs bind VDR in 1,25D-dependent manner in intact cells, coincident with the recruitment of RNA polymerase II to these motifs. Treatment of the cells with siRNA to hKSR-2 reduced the proportion of the most highly differentiated cells in 1,25D-treated cultures. These results demonstrate that hKSR-2 is a direct target of 1,25D in HL60 cells, and is required for optimal monocytic differentiation.

Kinase suppressor of Ras1 compartmentalizes hippocampal signal transduction and subserves synaptic plasticity and memory formation

The ERK/MAP kinase cascade is important for long-term memory formation and synaptic plasticity, with a myriad of upstream signals converging upon ERK activation. Despite this convergence of signaling, neurons routinely activate appropriate biological responses to different stimuli. Scaffolding proteins represent a mechanism to achieve compartmentalization of signaling and the appropriate targeting of ERK-dependent processes. We report that kinase suppressor of Ras (KSR1) functions biochemically in the hippocampus to scaffold the components of the ERK cascade, specifically regulating the cascade when a membrane fraction of ERK is activated via a PKC-dependent pathway but not via a cAMP/PKA-dependent pathway. Specificity of KSR1-dependent signaling also extends to specific downstream targets of ERK. Behaviorally and physiologically, we found that the absence of KSR1 leads to deficits in associative learning and theta burst stimulation-induced LTP. Our report provides novel insight into the endogenous scaffolding role of KSR1 in controlling kinase activation within the nervous system.

Protein-protein interactions in the regulation of the extracellular signal-regulated kinase

The extracellular signal-regulated kinase (ERK) cascade is a central intracellular signaling pathway that is activated by a variety of extracellular stimuli, and thereby regulates cellular processes such as proliferation, differentiation, and oncogenic transformation. To execute these functions, the signals of those stimuli are transmitted to the cytosolic and nuclear targets in a rapid and specific manner. In the last few years it has become clear that the specificity and the rapid function of the ERK cascade is largely determined by protein-protein interactions with various signaling components and substrates. This review describes interactions of ERK with its immediate regulators, scaffold proteins, substrates, and localizing proteins, and shows their involvement in the functioning of the ERK cascade. Understanding the full scope of ERK-interactions is important for the development of new drugs for the treatment of cancer and other diseases.

hKSR-2 inhibits MEKK3-activated MAP kinase and NF-kappaB pathways in inflammation

Kinase suppressor of ras (KSR) and MEKK3 (MAP kinase kinase kinase) are integral members of the MAP kinase pathway. We have recently identified a new isoform of the KSR family named human kinase suppressor of ras-2 (hKSR-2), and demonstrated that hKSR-2 negatively regulates Cot, a MAP3K family member which is important in inflammation and oncogenesis [P.L. Channavajhala, L. Wu, J.W. Cuozzo, J.P. Hall, W. Liu, L.L. Lin, Y. Zhang, J. Biol. Chem. 278 (2003) 47089-47097]. In this report, we provide evidence that hKSR-2 also regulates the activity of MEKK3 (another MAP3K family member) in HEK-293T cells. We demonstrate that hKSR-2 is a negative regulator of MEKK3-mediated activation of MAP kinase (specifically ERK and JNK) and NF-kappaB pathways, and concurrently inhibits MEKK3-mediated interleukin-8 production. We find that while hKSR-2 blocks MEKK3 activation, it has little to no effect on other members of the MAP3K family, including MEKK4, TAK1, and Ras-Raf, suggesting that its effects are selective.

Inflammatory bowel disease reveals the kinase activity of KSR1

Kinase suppressor of Ras-1 (KSR1) is a recently identified member of the EGFR-Ras-Raf-1-MAPK signaling pathway. A new study demonstrates that KSR1 protects intestinal epithelium from TNF-alpha-induced apoptosis, abrogating inflammatory bowel disease (IBD). Since its discovery, there has been disagreement as to whether KSR1 possesses intrinsic kinase activity. Using transgenic mouse models and genetically modified mouse colon epithelial cells, Polk and coworkers show that the kinase activity of KSR1 is off in normal colon epithelial cells, becoming activated only at the onset of IBD. They also provide strong evidence that KSR1 kinase activity is essential for anti-apoptotic protection of the intestinal epithelium. These new data in support of KSR1 as a kinase highlight an ongoing debate as to whether KSR1 does indeed serve as a specific kinase in transphosphorylating and transactivating c-Raf-1 toward MEK1.

Phosphorylation regulates KSR1 stability, ERK activation, and cell proliferation

Kinase suppressor of Ras (KSR) is a molecular scaffold that interacts with the components of the Raf/MEK/ERK kinase cascade and positively regulates ERK signaling. Phosphorylation of KSR1, particularly at Ser(392), is a critical regulator of KSR1 subcellular localization and ERK activation. We examined the role of phosphorylation of both Ser(392) and Thr(274) in regulating ERK activation and cell proliferation. We hypothesized that KSR1 phosphorylation is involved in generating signaling specificity through the Raf/MEK/ERK kinase cascade in response to stimulation by different growth factors. In fibroblasts, platelet-derived growth factor stimulation induces sustained ERK activation and promotes S-phase entry. Treatment with epidermal growth factor induces transient ERK activation but fails to drive cells into S phase. Mutation of Ser(392) and Thr(274) (KSR1.TVSA) promotes sustained ERK activation and cell cycle progression with either platelet-derived growth factor or epidermal growth factor treatment. KSR1(-/-) mouse embryo fibroblasts expressing KSR1.TVSA proliferate two times faster and grow to a higher density than cells expressing the same level of wild-type KSR1. In addition, KSR1.TVSA is more stable than wild-type KSR1. These data demonstrate that phosphorylation and stability of the molecular scaffold KSR1 are critical regulators of growth factor-specific responses that promote cell proliferation.