Ablation of Goalpha overrides G1 restriction point control through Ras/ERK/cyclin D1-CDK activities

Preclinical Evaluation of the Association of the Cyclin-Dependent Kinase 4/6 Inhibitor, Ribociclib, and Cetuximab in Squamous Cell Carcinoma of the Head and Neck

Simple Summary

We previously showed that ribociclib induces cell cycle arrest in some human papillomavirus (HPV)-negative squamous cell carcinomas of the head and neck (SCCHN) models. However, in vivo, ribociclib has only a cytostatic effect, suggesting that its activity needs to be optimized in combination with other treatments. We investigated the activity of ribociclib in combination with cetuximab in several HPV-negative SCCHN patient-derived tumor xenograft (PDTX) models. We found that the combination of cetuximab and ribociclib was not significantly more active than cetuximab monotherapy. In addition, our observations also suggest that the combination of cetuximab with a cyclin-dependent kinase (CDK) 4/6 inhibitor may reduce the activity of the CDK4/6 inhibitor in some cetuximab-resistant models. Our work has significant clinical implications since combinations of anti-epidermal growth factor receptor (EGFR) therapy and CDK4/6 inhibitors are currently being investigated in clinical trials.

Abstract

Epidermal growth factor receptor (EGFR) overexpression is observed in 90% of human papillomavirus (HPV)-negative squamous cell carcinomas of the head and neck (SCCHN). Cell cycle pathway impairments resulting in cyclin-dependent kinase (CDK) 4 and 6 activation, are frequently observed in SCCHN. We investigated the efficacy of ribociclib, a CDK4/6 inhibitor, in combination with cetuximab, a monoclonal antibody targeting the EGFR, in HPV-negative SCCHN patient-derived tumor xenograft (PDTX) models. The combination of cetuximab and ribociclib was not significantly more active than cetuximab monotherapy in all models investigated. In addition, the combination of cetuximab and ribociclib was less active than ribociclib monotherapy in the cetuximab-resistant PDTX models. In these models, a significant downregulation of the retinoblastoma (Rb) protein was observed in cetuximab-treated mice. We also observed Rb downregulation in the SCCHN cell lines chronically exposed and resistant to cetuximab. In addition, Rb downregulation induced interleukin 6 (Il-6) secretion and the Janus kinase family member/signal transducer and activator of transcription (JAK/STAT) pathway activation that might be implicated in the cetuximab resistance of these cell lines. To conclude, cetuximab is not an appropriate partner for ribociclib in cetuximab-resistant SCCHN models. Our work has significant clinical implications since the combination of anti-EGFR therapy with CDK4/6 inhibitors is currently being investigated in clinical trials.

Phase 1 Study Evaluating the Association of the Cyclin-Dependent Kinase 4/6 Inhibitor Ribociclib and Cetuximab in Recurrent/Metastatic p16-Negative Squamous Cell Carcinoma of the Head and Neck

Background: The majority of human papillomavirus (HPV)-negative squamous cell carcinoma of the head and neck (SCCHN) present upregulation of the epidermal growth factor receptor (EGFR) and frequent alterations in the cyclin D1-cyclin dependent kinase (CDK) 4/6 (CDK 4/6)-retinoblastoma protein (pRb) pathway, resulting in cell cycle progression and tumor proliferation. This study investigated the combination of ribociclib, an orally highly selective inhibitor of CDK 4/6, and cetuximab in recurrent and/or metastatic (R/M) SCCHN. Methods: A phase I trial using a 3 + 3 design was performed to determine the dose limiting toxicity (DLT) and maximum tolerated dose (MTD) of ribociclib with standard dose of weekly cetuximab in HPV-negative patients with R/M SCCHN. Ribociclib was administered orally (3 weeks on/1 week off) at dose level 1 of 400 mg daily and dose level 2 of 600 mg daily. The MTD of ribocilib was then further evaluated in an expansion cohort. Results: 10 patients were enrolled in the escalation trial. No DLTs were observed at dose level 1 (n = 3); at dose level 2, one patient was replaced due to rapid disease progression, and one patient out of six evaluable patients experienced a DLT (grade 4 thrombocytopenia >7 days). Ribociclib 600 mg daily was thus determined to be the MTD. Eleven additional patients were enrolled in the expansion cohort. Diarrhea (52%), rash (52%), fatigue (43%), nausea (33%), and mucositis (28%) were the most frequent grade 1-2 adverse events (AE). Neutropenia was the most frequent grade 3-4 AE (20%). Median progression-free survival (PFS) was 3.5 months (range 0.4-17.3 months) and median overall survival (OS) was 8.3 months (range 0.4-24.1 months). Among the 19 radiologically evaluable patients, two (10.5%) achieved a partial response and 11 (58%) had stable disease. Conclusions: The MTD of ribociclib is 600 mg daily when administered in combination with standard dose cetuximab for 3 weeks on and 1 week off. This combination was safe and showed efficacy. Further clinical trials should be conducted to evaluate the antitumor effects of this combination. Trial Information: ClinicalTrials.gov: NCT02429089; Eudract number 2014-005371-83.

EGFR and RB1 as Dual Biomarkers in HPV-Negative Head and Neck Cancer

Clinical decision making for human papillomavirus (HPV)-negative head and neck squamous cell carcinoma (HNSCC) is predominantly guided by disease stage and anatomic location, with few validated biomarkers. The epidermal growth factor receptor (EGFR) is an important therapeutic target, but its value in guiding therapeutic decision making remains ambiguous. We integrated analysis of clinically annotated tissue microarrays with analysis of data available through the TCGA, to investigate the idea that expression signatures involving EGFR, proteins regulating EGFR function, and core cell-cycle modulators might serve as prognostic or drug response-predictive biomarkers. This work suggests that consideration of the expression of NSDHL and proteins that regulate EGFR recycling in combination with EGFR provides a useful prognostic biomarker set. In addition, inactivation of the tumor suppressor retinoblastoma 1 (RB1), reflected by CCND1/CDK6-inactivating phosphorylation of RB1 at T356, inversely correlated with expression of EGFR in patient HNSCC samples. Moreover, stratification of cases with high EGFR by expression levels of CCND1, CDK6, or the CCND1/CDK6-regulatory protein p16 (CDKN2A) identified groups with significant survival differences. To further explore the relationship between EGFR and RB1-associated cell-cycle activity, we evaluated simultaneous inhibition of RB1 phosphorylation with the CDK4/6 inhibitor palbociclib and of EGFR activity with lapatinib or afatinib. These drug combinations had synergistic inhibitory effects on the proliferation of HNSCC cells and strikingly limited ERK1/2 phosphorylation in contrast to either agent used alone. In summary, combinations of CDK and EGFR inhibitors may be particularly useful in EGFR and p^T356RB1-expressing or CCND1/CDK6-overexpressing HPV-negative HNSCC. Mol Cancer Ther; 15(10); 2486-97. ©2016 AACR.

Age-specific functional epigenetic changes in p21 and p16 in injury-activated satellite cells

The regenerative capacity of muscle dramatically decreases with age because old muscle stem cells fail to proliferate in response to tissue damage. Here, we uncover key age-specific differences underlying this proliferative decline: namely, the genetic loci of cyclin/cyclin-dependent kinase (CDK) inhibitors (CDKIs) p21 and p16 are more epigenetically silenced in young muscle stem cells, as compared to old, both in quiescent cells and those responding to tissue injury. Interestingly, phosphorylated ERK (pERK) induced in these cells by ectopic FGF2 is found in association with p21 and p16 promoters, and moreover, only in the old cells. Importantly, in the old satellite cells, FGF2/pERK silences p21 epigenetically and transcriptionally, which leads to reduced p21 protein levels and enhanced cell proliferation. In agreement with the epigenetic silencing of the loci, young muscle stem cells do not depend as much as old on ectopic FGF/pERK for their myogenic proliferation. In addition, other CDKIs, such asp15(INK4B) and p27(KIP1) , become elevated in satellite cells with age, confirming and explaining the profound regenerative defect of old muscle. This work enhances our understanding of tissue aging, promoting strategies for combating age-imposed tissue degeneration.

Sustained ERK activation underlies reprogramming in regeneration-competent salamander cells and distinguishes them from their mammalian counterparts

In regeneration-competent vertebrates, such as salamanders, regeneration depends on the ability of various differentiated adult cell types to undergo natural reprogramming. This ability is rarely observed in regeneration-incompetent species such as mammals, providing an explanation for their poor regenerative potential. To date, little is known about the molecular mechanisms mediating natural reprogramming during regeneration. Here, we have identified the extent of extracellular signal-regulated kinase (ERK) activation as a key component of such mechanisms. We show that sustained ERK activation following serum induction is required for re-entry into the cell cycle of postmitotic salamander muscle cells, partially by promoting the downregulation of p53 activity. Moreover, ERK activation induces epigenetic modifications and downregulation of muscle-specific genes such as Sox6. Remarkably, while long-term ERK activation is found in salamander myotubes, only transient activation is seen in their mammalian counterparts, suggesting that the extent of ERK activation could underlie differences in regenerative competence between species.

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Sustained ERK activation is required for serum reprogramming of salamander cells

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Only transient ERK activation is observed in their mammalian counterparts

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Constant ERK activation promotes expression of S phase genes in mammalian myotubes

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The extent of ERK activation could underlie differences in regenerative competence

Proliferation potential of Müller glia after retinal damage varies between mouse strains

Retinal Müller glia can serve as a source for regeneration of damaged retinal neurons in fish, birds and mammals. However, the proliferation rate of Müller glia has been reported to be low in the mammalian retina. To overcome this problem, growth factors and morphogens have been studied as potent promoters of Müller glial proliferation, but the molecular mechanisms that limit the proliferation of Müller glia in the mammalian retina remain unknown. In the present study, we found that the degree of damage-induced Müller glia proliferation varies across mouse strains. In mouse line 129×1/SvJ (129), there was a significantly larger proliferative response compared with that observed in C57BL/6 (B6) after photoreceptor cell death. Treatment with a Glycogen synthase kinase 3 (GSK3) inhibitor enhanced the proliferation of Müller glia in 129 but not in B6 mouse retinas. We therefore focused on the different gene expression patterns during retinal degeneration between B6 and 129. Expression levels of Cyclin D1 and Nestin correlated with the degree of Müller glial proliferation. A comparison of genome-wide gene expression between B6 and 129 showed that distinct sets of genes were upregulated in the retinas after damage, including immune response genes and chromatin remodeling factors.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

Zinc induces ERK-dependent cell death through a specific Ras isoform

The effect of Zn on p53-independent cell death was examined in IIC9 embryonic fibroblasts. Despite the fact that these cells are p53-minus, Zn-mediated death occurs via an apoptotic mechanism. Death is facilitated by the presence of the Zn ionophore, pyrithione, indicating that intracellular Zn initiates the death response. Our investigations of the mechanism of Zn action demonstrate that Zn induces the death of IIC9 cells in a manner that is ERK-dependent. Expression of dn-(dominant negative)Ras attenuates ERK1/2 activation by Zn, and correspondingly reduces its cytotoxic effects. Raf-RBD pull-down experiments confirm that Zn treatment activates Ras and identified H-Ras as the specific isoform activated. This contrasts the activation of N-Ras that occurs when IIC9 cells are stimulated with thrombin. Thus, although the prolonged activation of the Ras/ERK pathway by Zn is similar to that seen when induced by mitogen, the distinguishing feature appears to be the isoform specificity of Ras activation.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

Expression of cyclin E renders cyclin D-CDK4 dispensable for inactivation of the retinoblastoma tumor suppressor protein, activation of E2F, and G1-S phase progression

The activation of CDK2-cyclin E in late G1 phase has been shown to play a critical role in retinoblastoma protein (pRb) inactivation and G1-S phase progression of the cell cycle. The phosphatidylinositol 3-OH-kinase inhibitor LY294002 has been shown to block cyclin D1 accumulation, CDK4 activity and, thus, G1 progression in alpha-thrombin-stimulated IIC9 cells (Chinese hamster embryonic fibroblasts). Our previous results show that expression of cyclin E rescues S phase progression in alpha-thrombin-stimulated IIC9 cells treated with LY294002, arguing that cyclin E renders CDK4 activity dispensable for G1 progression. In this work we investigate the ability of alpha-thrombin-induced CDK2-cyclin E activity to inactivate pRb in the absence of prior CDK4-cyclin D1 activity. We report that in the absence of CDK4-cyclin D1 activity, CDK2-cyclin E phosphorylates pRb in vivo on at least one residue and abolishes pRb binding to E2F response elements. We also find that expression of cyclin E rescues E2F activation and cyclin A expression in cyclin D kinase-inhibited, alpha-thrombin-stimulated cells. Furthermore, the rescue of E2F activity, cyclin A expression, and DNA synthesis by expression of E can be blocked by the expression of either CDK2(D145N) or RbDeltaCDK, a constitutively active mutant of pRb. However, restoring four known cyclin E-CDK2 phosphorylation sites to RbDeltaCDK renders it susceptible to inactivation in late G1, as assayed by E2F activation, cyclin A expression, and S phase progression. These data indicate that CDK2-cyclin E, without prior CDK4-cyclin D activity, can phosphorylate and inactivate pRb, activate E2F, and induce DNA synthesis.

The ethanol metabolite, linolenic acid ethyl ester, stimulates mitogen-activated protein kinase and cyclin signaling in hepatic stellate cells

Chronic ethanol consumption can result in hepatic fibrosis and cirrhosis. In addition to oxidative metabolism, ethanol can be metabolized by esterification with fatty acids to form fatty acid ethyl esters (FAEE) such as linolenic acid ethyl ester (LAEE). We have previously demonstrated that LAEE has promitogeinc and activating effects on hepatic stellate cells (HSC), but the mechanisms of these actions are not known. Intracellular signaling through MAP kinase pathways, including extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) can influence the activity of the transcription factor AP-1, while cell-cycle regulatory proteins such as cyclin E and cyclin-dependent kinase (CDK), play an important role in cell proliferation. In this study, we demonstrate that treatment of HSC with LAEE increases cyclin E expression and cyclin E/CDK2 activity, which may underlie the promitogenic effects of this compound. In addition, LAEE increases ERK and JNK activity, and these pathways play an important role in the activation of AP-1-dependent gene expression by LAEE. The stimulation of intracellular signaling pathways in HSC by this well-characterized ethanol metabolite may contribute to ethanol-induced hepatic fibrogenesis.

Cyclin-dependent kinase 2 nucleocytoplasmic translocation is regulated by extracellular regulated kinase

Activation of cyclin-dependent kinase 2 (CDK2)-cyclin E in the late G(1) phase of the cell cycle is important for transit into S phase. In Chinese hamster embryonic fibroblasts (IIC9) phosphatidylinositol 3-kinase and ERK regulate alpha-thrombin-induced G(1) transit by their effects on cyclin D1 protein accumulation (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053). Here, we show that ERK also affects CDK2-cyclin E activation by regulating the subcellular localization of CDK2. Ectopic expression of cyclin E rescues the inhibition of alpha-thrombin-induced activation of CDK2-cyclin E and transit into S phase brought about by treatment of IIC9 cells with LY29004, a selective inhibitor of mitogen stimulation of phosphatidylinositol 3-kinase activity. However, cyclin E expression is ineffectual in rescuing these effects when ERK activation is blocked by treatment with PD98059, a selective inhibitor of MEK activation of ERK. Investigation into the mechanistic reasons for this difference found the following. 1) Although treatment with LY29004 inhibits alpha-thrombin-stimulated nuclear localization, ectopic expression of cyclin E rescues CDK2 translocation. 2) In contrast to treatment with LY29004, ectopic expression of cyclin E fails to restore alpha-thrombin-stimulated nuclear CDK2 translocation in IIC9 cells treated with PD98059. 3) CDK2-cyclin E complexes are not affected by treatment with either inhibitor. These data indicate that, in addition to its effects on cyclin D1 expression, ERK activity is an important controller of the translocation of CDK2 into the nucleus where it is activated.

Phosphatidylinositol 3-kinase activity regulates alpha -thrombin-stimulated G1 progression by its effect on cyclin D1 expression and cyclin-dependent kinase 4 activity

In this study, we present evidence that PI 3-kinase is required for alpha-thrombin-stimulated DNA synthesis in Chinese hamster embryonic fibroblasts (IIC9 cells). Previous results from our laboratory demonstrate that the mitogen-activated protein kinase (extracellular signal-regulated kinase (ERK)) pathway controls transit through G(1) phase of the cell cycle by regulating the induction of cyclin D1 mRNA levels and cyclin dependent kinase 4 (CDK4)-cyclin D1 activity. In IIC9 cells, PI 3-kinase activation also is an important controller of the expression of cyclin D1 protein and CDK4-cyclin D1 activity. Pretreatment of IIC9 cells with the selective PI 3-kinase inhibitor, LY294002 blocks the alpha-thrombin-stimulated increase in cyclin D1 protein and CDK4 activity. However, LY294002 does not affect alpha-thrombin-induced cyclin D1 steady state message levels, indicating that PI 3-kinase acts independent of the ERK pathway. Interestingly, expression of a dominant-negative Ras significantly decreased both alpha-thrombin-stimulated ERK and PI 3-kinase activities. These data clearly demonstrate that the alpha-thrombin-induced Ras activation coordinately regulates ERK and PI 3-kinase activities, both of which are required for expression of cyclin D1 protein and progression through G(1).

Ras-stimulated extracellular signal-related kinase 1 and RhoA activities coordinate platelet-derived growth factor-induced G1 progression through the independent regulation of cyclin D1 and p27

Platelet-derived growth factor (PDGF)-induced Ras activation is required for G1 progression in Chinese hamster embryo fibroblasts (IIC9 cells). Ras stimulates both extracellular signal-related kinase (ERK) activation and RhoA activation in response to PDGF stimulation. Inhibition of either of these Ras-stimulated pathways results in growth arrest. We have shown previously that Ras-stimulated ERK activation is essential for the induction and continued G1 expression of cyclin D1. In this study we examine the role of Ras-induced RhoA activity in G1 progression. Unstimulated IIC9 cells expressed high levels of the G1 cyclin-dependent kinase inhibitor p27(KIP1). Stimulation with PDGF resulted in a dramatic decrease in p27(KIP1) protein expression. This decrease was attributed to increased p27(KIP1) protein degradation. Overexpression of dominant-negative forms of Ras or RhoA completely blocked PDGF-induced p27(KIP1) degradation, but only dominant-negative Ras inhibited cyclin D1 protein expression. C3 transferase also inhibited PDGF-induced p27(KIP1) degradation, thus further implicating RhoA in p27(KIP1) regulation. Overexpression of dominant-negative ERK resulted in inhibition of PDGF-induced cyclin D1 expression but had no effect on PDGF-induced p27(KIP1) degradation. These data suggest that Ras coordinates the independent regulation of cyclin D1 and p27(KIP1) expression by the respective activation of ERK and RhoA and that these pathways converge to determine the activation state of complexes of cyclin D1 and cyclin-dependent kinase in response to mitogen.

Ablation of Go alpha-subunit results in a transformed phenotype and constitutively active phosphatidylcholine-specific phospholipase C

Modulation of the components involved in mitogenic signaling cascades is critical to the regulation of cell growth. GTP-binding proteins and the stimulation of phosphatidylcholine (PC) hydrolysis have been shown to play major roles in these cascades. One of the enzymes involved in PC hydrolysis, a PC-specific phospholipase C (PC-PLC) has received relatively little attention. In this paper we examined the role of a particular heterotrimeric GTP-binding protein, Go, in the regulation of cell growth and PC-PLC-mediated hydrolysis of PC in IIC9 fibroblasts. The Go alpha-subunit was ablated in IIC9 cells by stable expression of antisense RNA. These stably transfected cells acquired a transformed phenotype as indicated by: (a) the formation of multiple foci in monolayer cultures, (b) the acquisition of anchorage-independent growth in soft agar; and (c) an increased level of thymidine incorporation in the absence of added mitogens. These data implicate Goalpha as a novel tumor suppressor. Interestingly, PC-PLC activity was constitutively active in the Goalpha-ablated cells as evidenced by the chronically elevated levels of diacylglycerol and phosphorylcholine in the absence of growth factors. In contrast, basal activities of PC-phospholipase D, phospholipase A2, or phosphoinositol-PLC were not affected. These data demonstrate, for the first time, a role for Go in regulating cell growth and provide definitive evidence for the existence of a PC-PLC in eukaryotic cells. The data further indicate that a subunit of Go, is involved in regulating this enzyme.