Repression of endometrial tumor growth by targeting SREBP1 and lipogenesis

Inhibition of mTOR complex 2 induces GSK3/FBXW7-dependent degradation of sterol regulatory element-binding protein 1 (SREBP1) and suppresses lipogenesis in cancer cells

Cancer cells feature increased de novo lipogenesis. Sterol regulatory element-binding protein 1 (SREBP1), when presented in its mature form (mSREBP1), enhances lipogenesis through increasing transcription of several of its target genes. Mammalian target of rapamycin (mTOR) complexes, mTORC1 and mTORC2, are master regulators of cellular survival, growth and metabolism. A role for mTORC1 in the regulation of SREBP1 activity has been suggested; however the connection between mTORC2 and SREBP1 has not been clearly established and hence is the focus of this study. mTOR kinase inhibitors (e.g., INK128), which inhibit both mTORC1 and mTORC2, decreased mSREBP1 levels in various cancer cell lines. Knockdown of rictor, but not raptor, also decreased mSREBP1. Consistently, reduced mSREBP1 levels were detected in cells deficient in rictor or Sin1 compared to parent or rictor-deficient cells with re-expression of ectopic rictor. Hence it is mTORC2 inhibition that causes mSREBP1 reduction. As a result, expression of the mSREBP1 target genes acetyl-CoA carboxylase and fatty acid synthase was suppressed, accompanied with suppressed lipogenesis in cells exposed to INK128. Moreover, mSREBP1 stability was reduced in cells treated with INK128 or rictor knockdown. Inhibition of proteasome, GSK3 or the E3 ubiquitin ligase, FBXW7, prevented mSREBP1 reduction induced by mTORC2 inhibition. Thus mTORC2 inhibition clearly facilitates GSK3-dependent, FBXW7-mediated mSREBP1 degradation, leading to mSREBP1 reduction. Accordingly, we conclude that mTORC2 positively regulates mSREBP1 stability and lipogenesis. Our findings reveal a novel biological function of mTORC2 in the regulation of lipogenesis and warrant further study in this direction.

SREBP1 regulates tumorigenesis and prognosis of pancreatic cancer through targeting lipid metabolism

Sterol regulatory element-binding protein 1 (SREBP1) is a known transcription factor of lipogenic genes, which plays important roles in regulating de novo lipogenesis. Accumulating evidences indicate SREBP1 is involved in tumorigenesis, yet its role in pancreatic cancer remains unclear. Here, we explored the expression characteristic and function of SREBP1 in pancreatic cancer. Analysis of 60 patients with pancreatic ducat cancer showed that SREBP1 level was significantly higher in pancreatic cancer than that in adjacent normal tissues. High expression of SREBP1 predicted poor prognosis in patients with pancreatic cancer. Multivariate analysis revealed that SREBP1 was an independent factor affecting overall survival. SREBP1 silencing resulted in proliferation inhibition and induction of apoptosis in pancreatic cancer cells. Mechanistically, lipogenic genes (acetyl-CoA carboxylase (ACC), fatty acid synthase (FASN), and stearoyl-CoA desaturase-1 (SCD1)) and de novo lipogenesis were promoted by SREBP1. Inhibition of lipogenic genes through specific inhibitors ablated SREBP1-mediated growth regulation. Furthermore, depletion of SREBP1 could suppress lipid metabolism and tumor growth in vivo. Our results indicate that SREBP1 had important role in tumor progression and appears to be a novel prognostic marker for pancreatic cancer.

ErbB2 activation upregulates glutaminase 1 expression which promotes breast cancer cell proliferation

Active glutamine utilization is critical for tumor cell proliferation. Glutaminolysis represents the first and rate-limiting step of glutamine utilization and is catalyzed by glutaminase (GLS). Activation of ErbB2 is one of the major causes of breast cancers, the second most common cause of death for women in many countries. However, it remains unclear whether ErbB2 signaling affects glutaminase expression in breast cancer cells. In this study, we show that MCF10A-NeuT cell line has higher GLS1 expression at both mRNA and protein levels than its parental line MCF10A, and knockdown of ErbB2 decreases GLS1 expression in MCF10A-NeuT cells. We further show that in these cells, ErbB2-mediated upregulation of GLS1 is not correlated to c-Myc expression. Moreover, activation of neither PI3K-Akt nor MAPK pathway is sufficient to upregulate GLS1 expression. Interestingly, inhibition of NF-κB blocks ErbB2-stimulated GLS1 expression, whereas stimulation of NF-κB is sufficient to enhance GLS1 levels in MCF10A cells, suggesting a PI3K-Akt-independent activation of NF-κB upregulates GLS1 in ErbB2-positive breast cancer cells. Finally, knockdown or inhibition of GLS1 significantly decreased the proliferation of breast cancer cells with high GLS1 levels. Taken together, our data indicate that ErbB2 activation promotes GLS1 expression via a PI3K-Akt-independent NF-κB pathway in breast cancer cells, identifying another oncogenic signaling pathway which stimulates GLS1 expression, and thus promoting glutamine utilization in cancer cells. These findings, if validated by in vivo model, may facilitate the identification of novel biochemical targets for cancer prevention and therapy.

SIRT1 promotes endometrial tumor growth by targeting SREBP1 and lipogenesis

Silent information regulator 1 (SIRT1) is involved in a number of cellular regulatory mechanisms affecting cellular life span, stress resistance, apoptosis and cellular metabolism. Recent studies have revealed that SIRT1 plays a dual role as a tumor suppressor and a tumor promoter in multiple stages of carcinogenesis. Increased lipogenesis has been found in cancer cells, sterol regulatory element binding protein 1 (SREBP1) are nuclear lipogenic transcription factors, which mainly regulate lipogenic processes by activating genes involved in fatty acid and triglyceride biosynthesis. In the present study, we detected expression of SIRT1 in endometrial cancer (EC) and illustrated the relationship between SIRT1 and SREBP1, which indicated that SIRT1 could stimulate endometrial tumor growth through the lipogenic pathway. Gene expression levels of SIRT1 were assayed using quantitative real-time PCR and protein expression levels were detected by western blotting. RNA interference was conducted in order to explore the subsequent effect on tumor cells and on the expression of SREBP1. Expression levels of SIRT1 in EC were found to be significantly higher than in normal endometrium. Knockdown of SIRT1 could downregulate expression of SREBP1 and suppress cell proliferation. These results demonstrated that SIRT1 may play a role as a tumor promoter in EC and can promote endometrial tumor growth by promoting lipogenesis. Our findings suggest that targeting SIRT1 may provide a theoretical basis for the management of EC.

Gradually elevated expression of Gankyrin during human hepatocarcinogenesis and its clinicopathological significance

Gankyrin is an important oncoprotein that is overexpressed in human hepatocellular carcinoma (HCC). However, the gradual alteration of Gankyrin in successive stages during human HCC development and the mechanism of Gankyrin-mediated hepatocarcinogenesis remain largely unknown. In this study, we evaluated the pattern and level of Gankyrin protein expression using immunohistochemistry in various liver tissues, including normal liver, chronic hepatitis, cirrhosis, adenomatous hyperplasia (AH), and HCC tissues, to analyze its clinicopathological significance. Furthermore, we stably transfected the shRNA-Gan vector, which targets human Gankyrin, into HepG2 cells to assess the role of Gankyrin in cell proliferation and tumorigenicity. The expression level of Gankyrin in the cytoplasm, nucleus, and whole cell was gradually elevated during consecutive stages of hepatocarcinogenesis. The nuclear Gankyrin level in AH was significantly higher than that in normal liver, chronic hepatitis, and cirrhotic tissues. The cytoplasmic, nuclear, and total cellular Gankyrin expression levels in HCC were significantly correlated with capsular invasion and intrahepatic metastasis. Silencing Gankyrin expression using shRNA-Gan repressed tumor cell proliferation, tumorigenicity, migration, and invasion in vitro. Our findings demonstrate that Gankyrin is aberrantly expressed beginning at the initiation stage and plays an important role in the initiation, promotion, and progression of hepatocarcinogenesis.

Sterol regulatory element-binding protein-1/fatty acid synthase involvement in proliferation inhibition and apoptosis promotion induced by progesterone in endometrial cancer

BACKGROUND

The number of endometrial cancer (EC) cases is escalating rapidly, with no evident improvements in survival rates. The downregulation of progesterone receptor, resulting in progestin resistance, is presently a major problem regarding the therapeutic aspect. On the basis of this, we can focus more on the downstream signaling pathways that are controlled by progesterone. Lipid biosynthesis mediated by sterol regulatory element-binding protein-1/fatty acid synthase (SREBP-1/FASN) is of utmost importance to the growth and the proliferation of EC cells, so we hypothesize that SREBP-1/FASN might be involved in suppressing the proliferation and promoting apoptosis in EC cells through the effects induced by progesterone.

MATERIAL AND METHODS

The Cell Counting Kit-8 was used to analyze the growth inhibition ratio of Ishikawa cells upon treatment with megestrol acetate (MA; MA is a progesterone derivative, also known as 17α-acetoxy-6-dehydro-6-methylprogesterone) and to determine the 50% inhibitory concentration. Apoptosis ratio was analyzed by treatment of the cells with MA at 50% inhibitory concentration at different time intervals using Annexin V-FITC/propidium iodide. The protein and messenger RNA levels of SREBP-1 and FASN were compared between the experimental and control groups (MA-treated Ishikawa cells were considered to be the experimental group).

RESULTS

The experimental group showed obvious growth inhibition that was time and concentration dependent. The apoptosis ratio was also significantly higher in the experimental group compared with the control group (P < 0.01). The protein and messenger RNA levels of SREBP-1 and FASN were significantly reduced by MA too.

CONCLUSIONS

Sterol regulatory element-binding protein-1/FASN is involved in the proliferation suppression and apoptosis promotion brought about by MA in Ishikawa cells.

SREBP-1 has a prognostic role and contributes to invasion and metastasis in human hepatocellular carcinoma

Sterol regulatory element-binding protein 1 (SREBP-1) is a well-known nuclear transcription factor involved in lipid synthesis. Recent studies have focused on its functions in tumor cell proliferation and apoptosis, but its role in cell migration and invasion, especially in hepatocellular carcinoma (HCC), is still unclear. In this study, we found that the expression of SREBP-1 in HCC tissues was significantly higher than those in matched tumor-adjacent tissues (p < 0.05). SREBP-1 was expressed at significantly higher levels in patients with large tumor size, high histological grade and advanced tumor-node-metastasis (TNM) stage (p < 0.05). The positive expression of SREBP-1 correlated with a worse 3-year overall and disease-free survival of HCC patients (p < 0.05). Additionally, SREBP-1 was an independent factor for predicting both 3-year overall and disease-free survival of HCC patients (p < 0.05). In vitro studies revealed that downregulation of SREBP-1 inhibited cell proliferation and induced apoptosis in both HepG2 and MHCC97L cells (p < 0.05). Furthermore, wound healing and transwell assays showed that SREBP-1 knockdown prominently inhibited cell migration and invasion in both HepG2 and MHCC97L cells (p < 0.05). These results suggest that SREBP-1 may serve as a prognostic marker in HCC and may promote tumor progression by promoting cell growth and metastasis.

Single nucleotide polymorphism of SREBF-1 gene associated with an increased risk of endometrial cancer in Chinese women

Aim

Elevated levels of sterol regulatory element-binding protein-1 (SREBP-1) have been found in endometrial cancer (EC), suggesting that it is essential to the development of EC. Obesity and diabetes have been established as known risk factors of EC, while SREBF-1 gene polymorphisms have also been found to be associated with obesity and type II diabetes. Therefore, we hypothesize that single nucleotide polymorphism (SNP) in SREBF-1 gene may be associated with increased risk of EC.

Method

We analyzed the sequence of SREBF-1 in tissue samples from 30 EC cases and 6 benign controls using high throughput method. Based on the primary results, we selected one SNP (rs2297508) as a genetic marker to conduct a hospital-based case-control study with 139 EC cases and 129 benign controls. The samples were examined under the microscope to determine their histopathology prior to the SNP analysis using RT-PCR.

Results

Through sequence analysis, we found 10 SNPs of SREBF-1 associated with EC, including 3 new SNPs. Fourteen percent of EC showed the rs2297508 SNP with C allele, while only 7% had the C allele was present in benign controls (p = 0.027, OR = 1.983). Additionally, the C allele was associated with cancer differentiation (p<0.05) and the depth of myometrial invasion (p<0.05).

Conclusion

Our study indicates that SNP (rs2297508) of SREBF-1 may serve as a genetic predisposition factor for the development of EC and screening of such genetic marker may be helpful in its early detection.