Suppressor of cytokine signaling-2 gene disruption promotes Apc(Min/+) tumorigenesis and activator protein-1 activation

Unveiling the pathological functions of SOCS in colorectal cancer: Current concepts and future perspectives

Colorectal cancer (CRC) remains a significant global health challenge, marked by increasing incidence and mortality rates in recent years. The pathogenesis of CRC is complex, involving chronic inflammation of the intestinal mucosa, heightened immunoinflammatory responses, and resistance to apoptosis. The suppressor of cytokine signaling (SOCS) family, comprised of key negative regulators within cytokine signaling pathways, plays a crucial role in cell proliferation, growth, and metabolic regulation. Deficiencies in various SOCS proteins can trigger the activation of the Janus kinase (JAK) and signal transducers and activators of transcription (STAT) pathways, following the binding of cytokines and growth factors to their receptors. Mounting evidence indicates that SOCS proteins are integral to the development and progression of CRC, positioning them as promising targets for novel anticancer therapies. This review delves into the structure, function, and molecular mechanisms of SOCS family members, examining their roles in cell proliferation, apoptosis, migration, epithelial-mesenchymal transition (EMT), and immune modulation. Additionally, it explores their potential impact on the regulation of CRC immunotherapy, offering new insights and perspectives that may inform the development of innovative therapeutic strategies for CRC.

A novel signature incorporating lipid metabolism- and immune-related genes to predict the prognosis and immune landscape in hepatocellular carcinoma

Background

Liver hepatocellular carcinoma (LIHC) is a highly malignant tumor with high metastasis and recurrence rates. Due to the relation between lipid metabolism and the tumor immune microenvironment is constantly being elucidated, this work is carried out to produce a new prognostic gene signature that incorporates immune profiles and lipid metabolism of LIHC patients.

Methods

We used the "DEseq2" R package and the "Venn" R package to identify differentially expressed genes related to lipid metabolism (LRDGs) in LIHC. Additionally, we performed unsupervised clustering of LIHC patients based on LRDGs to identify their subgroups and immuno-infiltration and Gene Ontology (GO) enrichment analysis on the subgroups. Next, we employed multivariate, LASSO and univariate Cox regression analyses to determine variables and to create a prognostic profile on the basis of immune- and lipid metabolism-related differential genes (IRDGs and LRDGs). We separated patients into low- and high-risk groups in accordance with the best cut-off value of risk score. We conducted Decision Curve Analysis (DCA), Receiver Operating Characteristic curve analysis as a function of time as well as Survival Analysis to evaluate this signature's prognostic value. We incorporated the clinical characteristics of patients into the risk model to obtain a nomogram prognostic model. GEO14520 and ICGC-LIRI JP datasets were employed to externally confirm the accuracy and robustness of signature. The gene set variation analysis (GSVA) and gene set enrichment analysis (GSEA) were applied for investigating the underlying mechanisms. Immune infiltration analysis was implemented to examine the differences in immune between both risk groups. Single-cell RNA sequencing (scRNA-SEQ) was utilized to characterize the genes that were involved in the distribution of signature and expression characteristics of different LIHC cell types. The patients' sensitivity in both risk groups to commonly used chemotherapeutic agents and semi-inhibitory concentrations (IC50) of the drugs was assessed using the GDSC database. On the basis of the differentially expressed genes (DEGs) in the two groups, the CMAP database was adopted for the prediction of potential small-molecule compounds. Small-molecule compounds were molecularly docked with prognostic markers. Lastly, we investigated the prognostic gene expression levels in normal and LIHC tissues with immunohistochemistry (IHC) and quantitative reverse transcription polymerase chain reaction(qRT-PCR).

Results

We built and verified a prognostic signature with seven genes that incorporated immune profiles and lipid metabolism. Patients were classified as low- and high-risk groups depending on their prognostic profiles. The overall survival (OS) was markedly lower in the high-risk group as compared to low-risk group. Time-dependent ROC curves more precisely predicted patients' survival at 1, 3 and 5 years; the area under the ROC curve was 0.81 (1 year), 0.75 (3 years) and 0.77 (5 years). The DCA curves showed the value of the prognostic genes in this signature for clinical applications. We included the patients' clinical characteristics in the risk model for both multivariate and univariate Cox regression analyses, and the findings revealed that the risk model represents an independent factor that influences OS in LIHC patients. With immune analysis, GSVA and GSEA, we identified that there are remarkable differences between the two risk groups in immune pathways, lipid metabolism, tumor development, immune cell infiltration and immune microenvironment, response to immunotherapy, and sensitivity to chemotherapy. Moreover, those with higher risk scores presented greater sensitivity to the chemotherapeutic agents. Experiments in vitro further elucidated the roles of SPP1 and FLT3 in the LIHC immune microenvironment. Furthermore, four small-molecule drugs that could target LIHC were screened. In vitro qRT-PCR , IHC revealed that the SPP1,KIF18A expressions were raised in LIHC in tumor samples, whereas FLT3,SOCS2 showed the opposite trend.

Conclusions

We developed and verified a new signature comprising immune- and lipid metabolism-associated markers and to assess the prognosis and the immune status of LIHC patients. This signature can be applied to survival prediction, individualized chemotherapy, and immunotherapeutic guidance for patients with liver cancer. This study also provides potential targeted therapeutics and novel ideas for the immune evasion and progression of LIHC.

SOCS2 protects against chemical-induced hepatocellular carcinoma progression by modulating inflammation and cell proliferation in the liver

Hepatocellular carcinoma (HCC) is one of the most prevalent and lethal cancers worldwide, but the precise intracellular mechanisms underlying the progression of this inflammation associated cancer are not well established. SOCS2 protein plays an important role in the carcinogenesis of different tumors by regulating cytokine signalling through the JAK/STAT axis. However, its role in HCC is unclear. Here, we investigate the role of SOCS2 in HCC progression and its potential as HCC biomarker. The effects of SOCS2 in HCC progression were evaluated in an experimental model of diethylnitrosamine (DEN)-induced HCC in C57BL/6 and SOCS2 deficient mice, in cultured hepatic cells, and in liver samples from HCC patients. Mice lacking SOCS2 showed higher liver tumor burden with increased malignancy grade, inflammation, fibrosis, and proliferation than their controls. Protein and gene expression analysis reported higher pSTAT5 and pSTAT3 activation, upregulation of different proteins involved in survival and proliferation, and increased levels of proinflammatory and pro-tumoral mediators in the absence of SOCS2. Clinically relevant, downregulated expression of SOCS2 was found in neoplasia from HCC patients compared to healthy liver tissue, correlating with the malignancy grade. In summary, our data show that lack of SOCS2 increases susceptibility to chemical-induced HCC and suggest the tumor suppressor role of this protein by regulating the oncogenic and inflammatory responses mediated by STAT5 and STAT3 in the liver. Hence, SOCS2 emerges as an attractive target molecule and potential biomarker to deepen in the study of HCC treatment.

Comprehensive analysis of suppressor of cytokine signaling proteins in human breast Cancer

Background

Abnormal expression of suppressor of cytokine signaling (SOCS) proteins regulates tumor angiogenesis and development in cancers. In this study, we aimed to perform a comprehensive bioinformatic analysis of SOCS proteins in breast invasive carcinoma (BRCA).

Methods

The gene expression, methylation level, copy number, protein expression and patient survival data related to SOCS family members in BRCA patients were obtained from the following databases: Oncomine, The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), Human Protein Atlas (HPA), Gene Expression Profiling Interactive Analysis (GEPIA), PCViz, cBioPortal and Kaplan-Meier plotter. Correlation analyses, identification of interacting genes and construction of regulatory networks were performed by functional and pathway enrichment analyses, weighted gene coexpression network analysis (WGCNA) and gene set enrichment analysis (GSEA).

Results

Data related to 1109 BRCA tissues and 113 normal breast tissue samples were extracted from the TCGA database. SOCS2 and SOCS3 exhibited significantly lower mRNA expression levels in BRCA tissues than in normal tissues. BRCA patients with high mRNA levels of SOCS3 (p < 0.01) and SOCS4 (p < 0.05) were predicted to have significantly longer overall survival (OS) times. Multivariate analysis showed that SOCS3 was an independent prognostic factor for OS. High mRNA expression levels of SOCS2 (p < 0.001), SOCS3 (p < 0.001), and SOCS4 (p < 0.01), and a low expression level of SOCS5 (p < 0.001) were predicted to be significantly associated with better recurrence-free survival (RFS). Multivariate analysis showed that SOCS2 was an independent prognostic factor for RFS. Lower expression levels of SOCS2 and SOCS3 were observed in patients with tumors of more advanced clinical stage (p < 0.05). Functional and pathway enrichment analyses, together with WGCNA and GSEA, showed that SOCS3 and its interacting genes were significantly involved in the JAK-STAT signaling pathway, suggesting that JAK-STAT signaling might play a critical role in BRCA angiogenesis and development. Western blot results showed that overexpression of SOCS3 inhibited the activity of the JAK-STAT signaling pathway in vitro.

Conclusions

SOCS family proteins play a very important role in BRCA. SOCS3 may be a prognostic factor and SOCS2 may be a potential therapeutic target in breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08434-y.

Development of a macrophages-related 4-gene signature and nomogram for the overall survival prediction of hepatocellular carcinoma based on WGCNA and LASSO algorithm

BACKGROUND

Immune system instability and poor prognosis are the two major clinical performance of hepatocellular carcinoma (HCC). Abnormal expression of MiR-424-5p has been reported to accelerate the progression of liver cancer, but it mediated immune cell infiltration imbalance is still unknown. We aim to mine the immune-related genes (IRGs) targeted by miR-424-5p and construct a multi-gene signature to improve the prognostic prediction of HCC.

METHODS

The HCC-related data of the cancer genome atlas (TCGA) database and the GSE14520 dataset of the gene expression omnibus (GEO) database were downloaded as the discovery dataset and the validation dataset, respectively. Weighted gene co-expression network analysis (WGCNA), the deconvolution algorithm of CIBERSORT and LASSO algorithm participated in the identification of IRGs and the development of prognostic signature and nomogram.

RESULTS

Our study found that the abundance of macrophages M0, M1 and M2 are all drastically changed during the cancerous process. A total of 920 macrophages infiltration-related LRGs were identified and a novel 4-gene signature (CDCA8, CBX2, UCK2 and SOCS2) with superior prognostic independence was established. The prognostic signature based risk score has superior capability to identify high-risk patients and predict overall survival (p < 0.001; AUC = 0.798 for 1 year; AUC = 0.748 for 3 years; AUC = 0.721 for 5 years). And it (C-index = 0.726) has a better prognostic potential than the TNM stage (C-index = 0.619), which is widely adopted in clinical practice. Additionally, the nomogram formed by combining the risk score and TNM stage further improved the accuracy of survival prediction (C-index = 0.733).

CONCLUSION

In summary, the immune landscape with abnormal infiltration of macrophages may be one of the prelude to the cancerous process. The novel macrophages-related 4-gene signature is expected to become a potential prognostic marker in liver cancer.

m6A methyltransferase METTL3 maintains colon cancer tumorigenicity by suppressing SOCS2 to promote cell proliferation

N⁶-methyladenosine (m⁶A) RNA modification maintained by N6-methyltransferases and demethylases is involved in multiple biological functions. Methyltransferase like 3 (METTL3) is a major N⁶-methyltransferase. However, the role of METTL3 and its installed m⁶A modification in colorectal tumorigenesis remains to be fully elucidated. METTL3 is highly expressed as indicated in colorectal cancer samples in the TCGA and Oncomine databases, implying its potential role in colon tumorigenesis. SW480 cell line with stable METTL3 knockout (METTL3-KO) was generated using CRISPR/Cas9 and were confirmed by the loss of METTL3 expression and suppression of m⁶A modification. The proliferation of METTL3-KO cells was significantly inhibited compared with that of control cells. METTL3-KO decreased the decay rate of suppressor of cytokine signaling 2 (SOCS2) RNA, resulting in elevated SOCS2 protein expression. m⁶A-RNA immunoprecipitation-qPCR (MeRIP-qPCR) revealed that SOCS2 mRNA was targeted by METTL3 for m⁶A modification. Similar to METTL3-KO SW480 cells, SW480 cells treated with 3-deazaadenosine, an RNA methylation inhibitor, exhibited elevated SOCS2 protein expression. Increased levels of SOCS2 in METTL3-KO SW480 cells were associated with decreased expression of leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5), contributing to the inhibition of cell proliferation. The underlying associations among METTL3, SOCS2, and LGR5 were further confirmed in SW480 cells transfected with si-METTL3 and in tumor samples from patients with colorectal cancer. Taken together, our data demonstrate that an increased level of METTL3 may maintain the tumorigenicity of colon cancer cells by suppressing SOCS2.

Control of Tyrosine Kinase Signalling by Small Adaptors in Colorectal Cancer

Tyrosine kinases (TKs) phosphorylate proteins on tyrosine residues as an intracellular signalling mechanism to coordinate intestinal epithelial cell communication and fate decision. Deregulation of their activity is ultimately connected with carcinogenesis. In colorectal cancer (CRC), it is still unclear how aberrant TK activities contribute to tumour formation because TK-encoding genes are not frequently mutated in this cancer. In vertebrates, several TKs are under the control of small adaptor proteins with potential important physiopathological roles. For instance, they can exert tumour suppressor functions in human cancer by targeting several components of the oncogenic TK signalling cascades. Here, we review how the Src-like adaptor protein (SLAP) and the suppressor of cytokine signalling (SOCS) adaptor proteins regulate the SRC and the Janus kinase (JAK) oncogenic pathways, respectively, and how their loss of function in the intestinal epithelium may influence tumour formation. We also discuss the potential therapeutic value of these adaptors in CRC.

Development and Validation of a Three-gene Prognostic Signature for Patients with Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the leading cause of cancer-related death worldwide, because recurrence often occurs in most HCC patients undergoing hepatectomy. It is necessary to identify patients with high risk for recurrence and adopt effective therapies. An obstacle to monitor patients at high risk for poor prognosis has been the lack of useful predictive biomarkers. Fortunately, recent progress in system biology allows to screen the biomarkers for HCC prognosis in a high-throughput manner. In this study, we performed systematic Kaplan-Meier survival analysis of the whole mRNA transcriptomics based on the Cancer Genome Atlas project (TCGA) and developed a three-gene prognostic signature composing of three genes UPB1, SOCS2 and RTN3. The model was validated in two independent microarray data sets retrieved from Gene Expression Omnibus (GEO) and the expression pattern of these three predictive genes in HCC was confirmed by western blot and immunohistochemistry with our HCC samples. In conclusion, our results showed that this three-gene signature has prognostic value for HCC patients.

The suppressor of cytokine signaling 2 (SOCS2) inhibits tumor metastasis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common cause of cancer-related death worldwide, and its incidence continues to increase. However, the mechanism underlying the development and progression of HCC remains unknown. The suppressor of cytokine signaling 2 (SOCS2) is a member of the SOCS family and influences the carcinogenesis of multiple types of tumors, but the biological roles of SOCS2 in HCC remain unclear. In this study, we found that SOCS2 expression was reduced in HCC tissues compared with matched noncancerous liver tissues. Moreover, decreased SOCS2 expression was significantly associated with the presence of intrahepatic metastasis and high histological grade in HCC patients. Colony formation assays and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays demonstrated that overexpression of SOCS2 or knockdown of endogenous SOCS2 did not significantly affect cell proliferation and tumorigenicity in HCC cells in vitro and in vivo. However, SOCS2 overexpression significantly inhibited the migration and invasion of HCC cells in vitro and inhibited metastasis in vivo. Consistent with these findings, the knockdown of endogenous SOCS2 enhanced migration and invasion in HCC cells in vitro. Our study demonstrated that SOCS2 inhibited human HCC metastasis, and SOCS2 might provide a new potential therapeutic strategy for treating HCC.

IL-8 induces miR-424-5p expression and modulates SOCS2/STAT5 signaling pathway in oral squamous cell carcinoma

Suppressor of cytokine signaling (SOCS) proteins are negative feedback regulators of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Dysregulation of SOCS protein expression in cancers can be one of the mechanisms that maintain STAT activation, but this mechanism is still poorly understood in oral squamous cell carcinoma (OSCC). Here, we report that SOCS2 protein is significantly downregulated in OSCC patients and its levels are inversely correlated with miR‐424‐5p expression. We identified the SOCS2 protein, which modulates STAT5 activity, as a direct target of miR‐424‐5p. The miR‐424‐5p‐induced STAT5 phosphorylation, matrix metalloproteinases (MMPs) expression, and cell migration and invasion were blocked by SOCS2 restoration, suggesting that miR‐424‐5p exhibits its oncogenic activity through negatively regulating SOCS2 levels. Furthermore, miR‐424‐5p expression could be induced by the cytokine IL‐8 primarily through enhancing STAT5 transcriptional activity rather than NF‐κB signaling. Antagomir‐mediated inactivation of miR‐424‐5p prevented the IL‐8‐induced cell migration and invasion, indicating that miR‐424‐5p is required for IL‐8‐induced cellular invasiveness. Taken together, these data indicate that STAT5‐dependent expression of miR‐424‐5p plays an important role in mediating IL‐8/STAT5/SOCS2 feedback loop, and scavenging miR‐424‐5p function using antagomir may have therapeutic potential for the treatment of OSCC.

miR‐424‐5p is overexpressed in OSCC.

miR‐424‐5p directly targets SOCS2, leading to increased cell migration and invasion.

STAT5 activation is required for IL‐8‐mediated miR‐424‐5p transcription.

miR‐424‐5p plays an important role in mediating IL‐8/STAT5/SOCS2 feedback loop.

Mitochondrial STAT3 contributes to transformation of Barrett's epithelial cells that express oncogenic Ras in a p53-independent fashion

Metaplastic epithelial cells of Barrett's esophagus transformed by the combination of p53-knockdown and oncogenic Ras expression are known to activate signal transducer and activator of transcription 3 (STAT3). When phosphorylated at tyrosine 705 (Tyr705), STAT3 functions as a nuclear transcription factor that can contribute to oncogenesis. STAT3 phosphorylated at serine 727 (Ser727) localizes in mitochondria, but little is known about mitochondrial STAT3's contribution to carcinogenesis in Barrett's esophagus, which is the focus of this study. We introduced a constitutively active variant of human STAT3 (STAT3CA) into the following: 1) non-neoplastic Barrett's (BAR-T) cells; 2) BAR-T cells with p53 knockdown; and 3) BAR-T cells that express oncogenic H-Ras(G12V). STAT3CA transformed only the H-Ras(G12V)-expressing BAR-T cells (evidenced by loss of contact inhibition, formation of colonies in soft agar, and generation of tumors in immunodeficient mice), and did so in a p53-independent fashion. The transformed cells had elevated levels of both mitochondrial (Ser727) and nuclear (Tyr705) phospho-STAT3. Introduction of a STAT3CA construct with a mutated tyrosine phosphorylation site into H-Ras(G12V)-expressing Barrett's cells resulted in high levels of mitochondrial phospho-STAT3 (Ser727) with little or no nuclear phospho-STAT3 (Tyr705), and the cells still formed tumors in immunodeficient mice. Thus tyrosine phosphorylation of STAT3 is not required for tumor formation in Ras-expressing Barrett's cells. We conclude that mitochondrial STAT3 (Ser727) can contribute to oncogenesis in Barrett's cells that express oncogenic Ras. These findings suggest that agents targeting STAT3 might be useful for chemoprevention in patients with Barrett's esophagus.

Identification of SOCS2 and SOCS6 as biomarkers in human colorectal cancer

BACKGROUND

Over the past years, some members of the family of suppressor of cytokine signalling (SOCS) proteins have emerged as potential tumour suppressors. This study aimed at investigating the clinical significance of SOCS proteins in colorectal carcinoma (CRC).

METHODS

We integrated publicly available microarray expression data on CRC in humans, analysed the expression pattern of SOCSs and assessed the predictive power of SOCS2 and SOCS6 for diagnostic purposes by generating receiver operating characteristic curves. Using laser microdissected patient material we assessed SOCS expression on RNA and protein levels as well as their methylation status in an independent CRC patient cohort. Finally, we investigated the prognostic value of SOCS2 and SOCS6.

RESULTS

The meta-analysis as well as the independent patient cohort analysis reveal a stage-independent downregulation of SOCS2 and SOCS6 and identify both molecules as diagnostic biomarkers for CRC. We demonstrate a different methylation pattern within the SOCS2 promoter between tumour tissue and normal control tissue in 25% of CRC patients. Furthermore, early CRC stage patients with low expression of SOCS2 display significantly shorter disease-free survival.

CONCLUSIONS

Our data offers evidence that SOCS2 and SOCS6 levels are reduced in CRC and may serve as diagnostic biomarkers for CRC patients.

Insulin receptor isoform switching in intestinal stem cells, progenitors, differentiated lineages and tumors: evidence that IR-B limits proliferation

Despite evidence for the impact of insulin on intestinal epithelial physiology and pathophysiology, the expression patterns, roles, and regulation of insulin receptor (IR) and IR isoforms in the intestinal epithelium are not well characterized. IR-A is thought to mediate the proliferative effects of insulin or insulin growth factors (IGFs) in fetal or cancer cells. IR-B is considered to be the metabolic receptor for insulin in specialized tissues. This study used a novel Sox9-EGFP reporter mouse that permits isolation of intestinal epithelial stem cells (IESCs), progenitors, enteroendocrine cells and differentiated lineages, the Apc(Min/+) mouse model of precancerous adenoma and normal human intestinal and colorectal cancer (CRC) cell lines. We tested the hypothesis that there is differential expression of IR-A or IR-B in stem and tumor cells versus differentiated intestinal epithelial cells (IECs) and that IR-B impacts cell proliferation. Our findings provide evidence that IR-B expression is significantly lower in highly proliferative IESCs and progenitor cells versus post-mitotic, differentiated IECs and in subconfluent and undifferentiated versus differentiated Caco-2 cells. IR-B is also reduced in Apc(Min/+) tumors and highly tumorigenic CRC cells. These differences in IR-B were accompanied by altered levels of mRNAs encoding muscleblind-like 2 (MBNL2), a known regulator of IR alternative splicing. Forced IR-B expression in subconfluent and undifferentiated Caco-2 cells reduced proliferation and increased biomarkers of differentiation. Our findings indicate that the impact of insulin on different cell types in the intestinal epithelium might differ depending on relative IR-B IR-A expression levels and provide new evidence for the roles of IR-B to limit proliferation of CRC cells.

Insulin-like growth factor 1: common mediator of multiple enterotrophic hormones and growth factors

PURPOSE OF REVIEW

To summarize the recent evidence that insulin-like growth factor 1 (IGF1) mediates growth effects of multiple trophic factors and discuss clinical relevance.

RECENT FINDINGS

Recent reviews and original reports indicate benefits of growth hormone (GH) and long-acting glucagon-like peptide 2 (GLP2) analogs in short bowel syndrome and Crohn's disease. This review highlights the evidence that biomarkers of sustained small intestinal growth or mucosal healing and evaluation of intestinal epithelial stem cell biomarkers may improve clinical measures of intestinal growth or response to trophic hormones. Compelling evidence that IGF1 mediates growth effects of GH and GLP2 on intestine or linear growth in preclinical models of resection or Crohn's disease is presented, along with a concept that these hormones or IGF1 may enhance sustained growth if given early after bowel resection. Evidence that suppressor of cytokine signaling protein induction by GH or GLP2 in normal or inflamed intestine may limit IGF1-induced growth, but protect against risk of dysplasia or fibrosis, is reviewed. Whether IGF1 receptor mediates IGF1 action and potential roles of insulin receptors are addressed.

SUMMARY

IGF1 has a central role in mediating trophic hormone action in small intestine. Better understanding of benefits and risks of IGF1, receptors that mediate IGF1 action, and factors that limit undesirable growth are needed.

STAT5A-mediated SOCS2 expression regulates Jak2 and STAT3 activity following c-Src inhibition in head and neck squamous carcinoma

PURPOSE

The inhibition of c-Src results in a striking reduction in cancer cell invasion, but the effect on cell survival is modest. Defining mechanisms that limit apoptosis following c-Src inhibition could result in an ideal therapeutic approach that both inhibits invasion and leads to apoptosis. In this regard, we discovered a novel feedback loop that results in STAT3 reactivation following sustained c-Src inhibition. Here we define the mechanism underlying this feedback loop and examine the effect of inhibiting it in vivo.

EXPERIMENTAL DESIGN

We measured levels and activity of pathway components using PCR, Western blotting, and kinase assays following their manipulation using both molecular and pharmacologic approaches. We used a heterotransplant animal model in which human oral squamous cancer is maintained exclusively in vivo.

RESULTS

Following c-Src inhibition, STAT5 is durably inhibited. The inhibition of STAT5A, but not STAT5B, subsequently reduces the expression of suppressors of cytokine signaling 2 (SOCS2). SOCS2 inhibits Janus kinase 2 (Jak2) activity and Jak2-STAT3 binding. SOCS2 expression is necessary for STAT3 inhibition by c-Src inhibitors. Overexpression of SOCS2 is adequate to prevent STAT3 reactivation and to enhance the cytotoxic effects of c-Src inhibition. Likewise, the combination of Jak and c-Src inhibitors led to significantly more apoptosis than either agent alone in vivo.

CONCLUSIONS

To our knowledge, ours is the first study that fully defines the mechanism underlying this feedback loop, in which sustained c-Src inhibition leads to diminished SOCS2 expression via sustained inhibition of STAT5A, allowing activation of Jak2 and STAT3, Jak2-STAT3 binding, and survival signals.