Bimodal expression of Sprouty2 during the cell cycle is mediated by phase-specific Ras/MAPK and c-Cbl activities

The role of CBL family ubiquitin ligases in cancer progression and therapeutic strategies

The CBL (Casitas B-lineage lymphoma) family, as a class of ubiquitin ligases, can regulate signal transduction and activate receptor tyrosine kinases through various tyrosine kinase-dependent pathways. There are three members of the family: c-CBL, CBL-b, and CBL-c. Numerous studies have demonstrated the important role of CBL in various cellular pathways, particularly those involved in the occurrence and progression of cancer, hematopoietic development, and regulation of T cell receptors. Therefore, the purpose of this review is to comprehensively summarize the function and regulatory role of CBL family proteins in different human tumors, as well as the progress of drug research targeting CBL family, so as to provide a broader clinical measurement strategy for the treatment of tumors.

Sprouty3, but Not Sprouty1, Expression Is Beneficial for the Malignant Potential of Osteosarcoma Cells

Sprouty proteins are widely accepted modulators of receptor tyrosine kinase-associated pathways and fulfill diversified roles in cancerogenesis dependent on the originating cells. In this study we detected a high expression of Sprouty3 in osteosarcoma-derived cells and addressed the question of whether Sprouty3 and Sprouty1 influence the malignant phenotype of this bone tumor entity. By using adenoviruses, the Sprouty proteins were expressed in two different cell lines and their influence on cellular behavior was assessed. Growth curve analyses and Scratch assays revealed that Sprouty3 accelerates cell proliferation and migration. Additionally, more colonies were grown in Soft agar if the cells express Sprouty3. In parallel, Sprouty1 had no significant effect on the measured endpoints of the study in osteosarcoma-derived cells. The promotion of the tumorigenic capacities in the presence of Sprouty3 coincided with an increased activation of signaling as measured by evaluating the phosphorylation of extracellular signal-regulated kinases (ERKs). Ectopic expression of a mutated Sprouty3 protein, in which the tyrosine necessary for its activation was substituted, resulted in inhibited migration of the treated cells. Our findings identify Sprouty3 as a candidate for a tumor promoter in osteosarcoma.

A Sprouty4 Mutation Identified in Kallmann Syndrome Increases the Inhibitory Potency of the Protein towards FGF and Connected Processes

Kallmann syndrome is the result of innate genetic defects in the fibroblast growth factor (FGF) regulated signaling network causing diminished signal transduction. One of the rare mutations associated with the syndrome alters the Sprouty (Spry)4 protein by converting the serine at position 241 into a tyrosine. In this study, we characterize the tyrosine Spry4 mutant protein in the primary human embryonic lung fibroblasts WI-38 and osteosarcoma-derived cell line U2OS. As demonstrated in a cell signaling assay, Spry4 gains the capability of inhibiting FGF, but not epithelial growth factor (EGF)-induced signaling as a consequence of the tyrosine substitution. Additionally, migration of normal embryonic lung fibroblasts and osteosarcoma-derived cells is potently inhibited by the tyrosine Spry4 variant, while an effect of the wildtype Spry4 protein is hardly measureable. Concerning cell proliferation, the unaltered Spry4 protein is ineffective to influence the WI-38 cells, while the mutated Spry4 protein decelerates the cell doubling. In summary, these data emphasize that like the other mutations associated with Kallmann syndrome the described Spry4 mutation creates a hyperactive version of a selective inhibitory molecule and can thereby contribute to a weakened FGF signaling. Additionally, the study pinpoints a Spry4 variation expanding the applicability of Spry4 in a potential cancer therapy.

Sprouty3 and Sprouty4, Two Members of a Family Known to Inhibit FGF-Mediated Signaling, Exert Opposing Roles on Proliferation and Migration of Glioblastoma-Derived Cells

Dysregulation of receptor tyrosine kinase-induced pathways is a critical step driving the oncogenic potential of brain cancer. In this study, we investigated the role of two members of the Sprouty (Spry) family in brain cancer-derived cell lines. Using immunoblot analyses we found essential differences in the pattern of endogenous Spry3 and Spry4 expression. While Spry4 expression was mitogen-dependent and repressed in a number of cells from higher malignant brain cancers, Spry3 levels neither fluctuated in response to serum withdrawal nor were repressed in glioblastoma (GBM)-derived cell lines. In accordance to the well-known inhibitory role of Spry proteins in fibroblast growth factor (FGF)-mediated signaling, both Spry proteins were able to interfere with FGF-induced activation of the MAPK pathway although to a different extent. In response to serum solely, Spry4 exerts its role as a negative regulator of MAPK activation. Ectopic expression of Spry4 inhibited proliferation and migration of GBM-originated cells, positioning it as a tumor suppressor in brain cancer. In contrast, elevated Spry3 levels accelerated both proliferation and migration of these cell lines, while repression of Spry3 levels using shRNA caused a significant diminished growth and migration velocity rate of a GBM-derived cell line. This argues for a tumor-promoting function of Spry3 in GBMs. Based on these data we conclude that Spry3 and Spry4 fulfill different if not opposing roles within the cancerogenesis of brain malignancies.

Spatial signal repression as an additional role of Sprouty2 protein variants

Sprouty2 (Spry2) is a prominent member of a protein family with crucial functions in the modulation of signal transduction. One of its main actions is the repression of mitogen-activated protein kinase (MAPK) pathway in response to growth factor-induced signalling. A common single nucleotide polymorphism within the Spry2 gene creates two protein variants where a proline adjacent to the serine rich domain is converted to an additional serine. Both protein variants perform similar functions although their efficiency in fulfilling these tasks varies. In this report, we used biochemical fractionation methods as well as confocal microscopy to analyse quantitative and qualitative differences in the distribution of Spry2 variants. We found that Spry2 proteins localize not solely to the plasma membrane, but also to other membrane engulfed compartments like for example the Golgi apparatus. In these less dense organelles, predominantly slower migrating forms reside indicating that posttranslational modification contributes to the distribution profile of Spry2. However there is no significant difference in the distribution of the two variants. Additionally, we found that Spry2 could be found exclusively in membrane fractions irrespective of the mitogen availability and the phosphorylation status. Considering the interference of extracellular signal-regulated kinase (ERK) activation in the cytoplasm, both Spry2 variants inhibited the levels of phosphorylated ERK (pERK) significantly to a similar extent. In contrast, the induction profiles of pERK levels were completely different in the nuclei. Again, both Spry2 variants diminished the levels of pERK. While the proline variant lowered the activation throughout the observation period, the serine variant failed to interfere with immediate accumulation of nuclear pERK levels, but the signal duration was shortened. Since the extent of the pERK inhibition in the nuclei was drastically more pronounced than in the cytoplasm, we conclude that Spry2 - in addition to its known functions as a repressor of general ERK phosphorylation - functions as a spatial repressor of nucleic ERK activation. Accordingly, a dominant negative version of Spry2 was only able to enhance the pERK levels of serum-deprived cells in the cytosol, while in the nucleus the intensity of the pERK signal in response to serum addition was significantly increased.

Altered regulation of the Spry2/Dyrk1A/PP2A triad by homocysteine impairs neural progenitor cell proliferation

Hyperhomocysteinemia reduces neurogenesis in the adult mouse brain. Homocysteine (Hcy) inhibits postnatal neural progenitor cell (NPC) proliferation by specifically impairing the fibroblast growth factor receptor (FGFR)-Erk1/2-cyclin E signaling pathway. We demonstrate herein that the inhibition of FGFR-dependent NPC proliferation induced by Hcy is mediated by its capacity to alter the cellular methylation potential. Our results show that this alteration modified the expression pattern and activity of Sprouty2 (Spry2), a negative regulator of the above mentioned pathway. Both elevated concentrations of Hcy and methyltransferase activity inhibition induced Spry2 promoter demethylation in NPC cultures leading to a sustained upregulation of the expression of Spry2 mRNA and protein. In addition, protein levels of two kinases responsible for Spry2 activation/deactivation were altered by Hcy: Spry2 kinase Dyrk1A levels diminished while Spry2 phosphatase PP2A increased, leading to changes in the phosphorylation pattern, activity and stability of Spry2. In conclusion, Hcy inhibits NPC proliferation by indirect mechanisms involving alterations in DNA methylation, gene expression, and Spry2 function, causing FGFR signaling impairment.

Sprouty4 mRNA variants and protein expressions in breast and lung-derived cells

Sprouty proteins are modulators of mitogen-induced signalling processes and are therefore hypothesized to affect malignant diseases. As a member of the Sprouty family, Sprouty4 has been previously shown to function as a tumour suppressor in lung and breast cancer. The present study analysed the expression of two known Sprouty4 splice variants in cells established from malignant and normal lung and breast tissues using semi-quantitative reverse transcription-polymerase chain reaction and immunoblotting. The results indicated that the expression of the two messenger RNA (mRNA) variants was reduced in the cells derived from malignant tissue in comparison to the normal counterparts. Although the expression of the two splice variants were associated in both tissue types, on average, the relative expression of the longer variant was slightly increased in malignant cells compared with normal tissues. Notably, the protein levels reflected the expression observed at the mRNA level only in breast-derived cells. Contrarily, with regards to the measured mRNA levels, Sprouty4 protein was disproportional augmented in lung cells known to harbour the mutated K-Ras gene.

MicroRNA-21 Increases Proliferation and Cisplatin Sensitivity of Osteosarcoma-Derived Cells

Osteosarcoma is the most common primary bone tumor and poor prognosis for osteosarcoma patients is mainly due to chemotherapy resistance. MicroRNAs are important to maintain pathophysiological mechanisms of cancer and influence cell sensitivity to chemotherapy. In this study, we tested the functions of microRNA-21 for malignant features as well as for drug resistance of osteosarcoma. We used Northern blot to measure microRNA-21 levels in osteosarcoma-derived cell lines. MicroRNA-21 activity was modulated by either expressing a sponge to decrease its activity in an osteosarcoma-derived cell line expressing high levels of microRNA-21 or by introducing pri-microRNA-21 in a cell line with low endogenous levels. Cell migration was determined in a scratch assay and cell proliferation was measured by performing growth curve analysis. Sensitivity of the cells towards chemotherapeutics was investigated by performing cell viability assays and calculating the IC50 values. While cell migration was unaffected by modulated microRNA-21 levels, microRNA-21 inhibition slowed proliferation and exogenously expressed microRNA-21 promoted this process. Modulated microRNA-21 activity failed to effect sensitivity of osteosarcoma-derived cell lines to doxorubicin or methotrexate. Contrarily, reduction of microRNA-21 activity resulted in enhanced resistance towards cisplatin while ectopic expression of microRNA-21 showed the opposite effect. Increased microRNA-21 levels repressed the expression of Sprouty2 and ectopic expression of Sprouty2 was able to largely rescue the observed effects of microRNA-21 in osteosarcoma. In summary, our data indicate that in osteosarcoma microRNA-21 expression is an important component for regulation of cell proliferation and for determining sensitivity to cisplatin.

The increased Sprouty4 expression in response to serum is transcriptionally controlled by Specific protein 1

Sprouty proteins control length and intensity of the intracellular signal transduction cascade activated by mitogens in the cellular environment. As part of a negative feedback loop, their expression is supposed to be elevated by the same factors. In this report, Sprouty4 expression in response to serum and the underlying regulatory mechanisms were investigated. We verified that Sprouty4 expression is activated by serum addition in all tested cells independent of their origin. Strict correlation between Sprouty4 protein levels and promoter activity indicates mainly transcriptional regulation of Sprouty4 serum-responsiveness. Induction of the mitogen-activated protein kinase pathway is required for Sprouty4 promoter activation in the presence of serum. Nonetheless, signal transduction via this pathway is not sufficient to fully induce the Sprouty4 promoter. Instead, deletion and mutation analysis identified two annotated Specific protein 1 binding sites as the critical cis-elements responsible for conferring the serum induction of the promoter. Corroborating, repressed Specific protein 1 activity or levels result in constitutive lowered transcriptional activity of the Sprouty4 promoter. These data demonstrate that Specific protein 1 plays a crucial role in the regulation of Sprouty4 in response to serum.

The developing story of Sprouty and cancer

Sprouty proteins are evolutionarily conserved modulators of MAPK/ERK pathway. Through interacting with an increasing number of effectors, mediators, and regulators with ultimate influence on multiple targets within or beyond ERK, Sprouty orchestrates a complex, multilayered regulatory system and mediates a crosstalk among different signaling pathways for a coordinated cellular response. As such, Sprouty has been implicated in various developmental and physiological processes. Evidence shows that ERK is aberrantly activated in malignant conditions. Accordingly, Sprouty deregulation has been reported in different cancer types and shown to impact cancer development, progression, and metastasis. In this article, we have tried to provide an overview of the current knowledge about the Sprouty physiology and its regulatory functions in health, as well as an updated review of the Sprouty status in cancer. Putative implications of Sprouty in cancer biology, their clinical relevance, and their proposed applications are also revisited. As a developing story, however, role of Sprouty in cancer remains to be further elucidated.

Sprouty4 interferes with cell proliferation and migration of breast cancer-derived cell lines

Sprouty proteins are modulators of mitogen-induced signal transduction processes and therefore can influence the process of cancerogenesis. In particular, Sprouty2 has been shown to have an important role in cancer development of many tumor entities including breast cancer. In this report, we investigated the role of Sprouty4 in breast cancer-derived cell lines. We have found that ectopic Sprouty4 expression inhibits cell proliferation of breast cancer cell lines independently of their endogenous expression levels. Corroborating Sprouty4 downregulation causes accelerated growth. Furthermore, we demonstrate that an increase in Sprouty4 content interferes with serum-induced activation of mitogen-activated protein kinase pathway. Additionally, Sprouty4 expression negatively influences cell migration. These data suggest that Sprouty4 is a possible candidate for a tumor suppressor in breast cancer.

Sprouty2 but not Sprouty4 is a potent inhibitor of cell proliferation and migration of osteosarcoma cells

As negative regulators of receptor tyrosine kinase-mediated signalling, Sprouty proteins fulfil important roles during carcinogenesis. In this report, we demonstrate that Sprouty2 protein expression inhibits cell proliferation and migration in osteosarcoma-derived cells. Although earlier reports describe a tumour-promoting function, these results indicate that Sprouty proteins also have the potential to function as tumour suppressors in sarcoma. In contrast to Sprouty2, Sprouty4 expression failed to interfere with proliferation and migration of the osteosarcoma-derived cells, possibly due to a less pronounced interference with mitogen-activated protein kinase activity. Sequences within the NH2-terminus are responsible for the specific inhibitory function of Sprouty2 protein.

In non-small cell lung cancer mitogenic signaling leaves Sprouty1 protein levels unaffected

Sprouty1 protein belongs to a family of receptor tyrosine kinase-mediated signaling inhibitors, whose members are usually regulated by growth factors to form a negative feedback loop. Correspondingly fluctuations of Sprouty1 mRNA in response to single growth factors have been observed. In this report, we investigate Sprouty1 protein levels and show that in non-small cell lung carcinoma-derived cells, the expression levels are unaffected by the serum content in the cellular environment. Although cells harboring K-Ras mutations express insignificant higher Sprouty1 levels, ectopic expression of constitutive active Ras in normal human lung fibroblasts fails to augment Sprouty1 protein content. Furthermore, serum starvation for three days has no influence on Sprouty1 expression and addition of serum or of singular growth factors leaves Sprouty protein levels unchanged. Cell cycle analysis reveals that Sprouty1 levels remain constant throughout the whole cell cycle. These data demonstrate that Sprouty1 expression is not connected with mitogenic signaling and cell proliferation.

Overexpression of Aurora-A in primary cells interferes with S-phase entry by diminishing Cyclin D1 dependent activities

Background

Aurora-A is a bona-fide oncogene whose expression is associated with genomic instability and malignant transformation. In several types of cancer, gene amplification and/or increased protein levels of Aurora-A are a common feature.

Results

In this report, we describe that inhibition of cell proliferation is the main effect observed after transient overexpression of Aurora-A in primary human cells. In addition to the known cell cycle block at the G2/M transition, Aurora-A overexpressing cells fail to overcome the restriction point at the G1/S transition due to diminished RB phosphorylation caused by reduced Cyclin D1 expression. Consequently, overexpression of Cyclin D1 protein is able to override the Aurora-A mediated G1 block. The Aurora-A mediated cell cycle arrest in G2 is not influenced by Cyclin D1 and as a consequence cells accumulate in G2. Upon deactivation of p53 part of the cells evade this premitotic arrest to become aneuploid.

Conclusion

Our studies describe that an increase of Aurora-A expression levels on its own has a tumor suppressing function, but in combination with the appropriate altered intracellular setting it might exert its oncogenic potential. The presented data indicate that deactivation of the tumor suppressor RB is one of the requirements for overriding a cell cycle checkpoint triggered by increased Aurora-A levels.

Targeting fibroblast-growth-factor-receptor-dependent signaling for cancer therapy

INTRODUCTION

Fibroblast growth factors (FGF) exert a combination of biological effects that contribute to four of the six essential hallmarks of cancer. It is no surprise that FGF-dependent signaling has increasingly moved to the center of cancer therapy research during the past decade. This is illustrated by the large number of publications focusing on various aspects of this theme that have been published in the past 5 years.

AREAS COVERED

Information from these sources as well as ongoing work from the authors' groups is used to outline the physiological functions of FGF signaling and to highlight how the high oncogenic effects of deregulated FGFs and FGFRs derive from their physiological functions. The biological effect of deregulated FGFR signaling in malignant diseases is described and the current state of therapeutic targeting of FGFR is summarized.

EXPERT OPINION

Strategies for targeting FGFR-signaling for cancer therapy are very promising, but need to be carefully developed based on the physiological roles of FGF signaling. Preventive measures may be necessary for protection from FGF-related side effects. Combined targeting of several receptor tyrosine kinases or combination with other therapies may be a useful way of avoiding or ameliorating side effects. FGF-related markers of prognosis and therapy response still need to be investigated.