The HDL receptor SR-BI is associated with human prostate cancer progression and plays a possible role in establishing androgen independence

Malignant Phenotypes in Metastatic Melanoma are Governed by SR-BI and its Association with Glycosylation and STAT5 Activation

Metastatic melanoma is hallmarked by elevated glycolytic flux and alterations in cholesterol homeostasis. The contribution of cholesterol transporting receptors for the maintenance of a migratory and invasive phenotype is not well defined. Here, the scavenger receptor class B type I (SCARB1/SR-BI), a high-density lipoprotein (HDL) receptor, was identified as an estimator of melanoma progression in patients. We further aimed to identify the SR-BI-controlled gene expression signature and its related cellular phenotypes. On the basis of whole transcriptome analysis, it was found that SR-BI knockdown, but not functional inhibition of its cholesterol-transporting capacity, perturbed the metastasis-associated epithelial-to-mesenchymal transition (EMT) phenotype. Furthermore, SR-BI knockdown was accompanied by decreased migration and invasion of melanoma cells and reduced xenograft tumor growth. STAT5 is an important mediator of the EMT process and loss of SR-BI resulted in decreased glycosylation, reduced DNA binding, and target gene expression of STAT5. When human metastatic melanoma clinical specimens were analyzed for the abundance of SR-BI and STAT5 protein, a positive correlation was found. Finally, a novel SR-BI-regulated gene profile was determined, which discriminates metastatic from nonmetastatic melanoma specimens indicating that SR-BI drives gene expression contributing to growth at metastatic sites. Overall, these results demonstrate that SR-BI is a highly expressed receptor in human metastatic melanoma and is crucial for the maintenance of the metastatic phenotype.Implications: High SR-BI expression in melanoma is linked with increased cellular glycosylation and hence is essential for a metastasis-specific expression signature. Mol Cancer Res; 16(1); 135-46. ©2017 AACR.

Synthetic high-density lipoproteins as targeted monotherapy for chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) remains incurable despite the introduction of new drugs. Therapies targeting receptors and pathways active specifically in malignant B cells might provide better treatment options. For instance, in B cell lymphoma, our group has previously shown that scavenger receptor type B-1 (SR-B1), the high-affinity receptor for cholesterol-rich high-density lipoproteins (HDL), is a therapeutic target. As evidence suggests that targeting cholesterol metabolism in CLL cells may have therapeutic benefit, we examined SR-B1 expression in primary CLL cells from patients. Unlike normal B cells that do not express SR-B1, CLL cells express the receptor. As a result, we evaluated cholesterol-poor synthetic HDL nanoparticles (HDL NP), known for targeting SR-B1, as a therapy for CLL. HDL NPs potently and selectively induce apoptotic cell death in primary CLL cells. HDL NPs had no effect on normal peripheral blood mononuclear cells from healthy individuals or patients with CLL. These data implicate SR-B1 as a target in CLL and HDL NPs as targeted monotherapy for CLL.

Bioorthogonal Labeling of Human Prostate Cancer Tissue Slice Cultures for Glycoproteomics

Sialylated glycans are found at elevated levels in many types of cancer and have been implicated in disease progression. However, the specific glycoproteins that contribute to the cancer cell-surface sialylation are not well characterized, specifically in bona fide human disease tissue. Metabolic and bioorthogonal labeling methods have previously enabled the enrichment and identification of sialoglycoproteins from cultured cells and model organisms. Herein, we report the first application of this glycoproteomic platform to human tissues cultured ex vivo. Both normal and cancerous prostate tissues were sliced and cultured in the presence of the azide-functionalized sialic acid biosynthetic precursor Ac₄ ManNAz. The compound was metabolized to the azidosialic acid and incorporated into cell surface and secreted sialoglycoproteins. Chemical biotinylation followed by enrichment and mass spectrometry led to the identification of glycoproteins that were found at elevated levels or uniquely in cancerous prostate tissue. This work therefore extends the use of bioorthogonal labeling strategies to problems of clinical relevance.

Exploiting scavenger receptors in cancer immunotherapy: Lessons from CD5 and SR-B1

Scavenger receptors (SRs) are structurally heterogeneous cell surface receptors characterized by their capacity to remove extraneous or modified self-macromolecules from circulation, thus avoiding the accumulation of noxious agents in the extracellular space. This scavenging activity makes SRs important molecules for host defense and homeostasis. In turn, SRs keep the activation of the steady-state immune response in check, and participate as co-receptors in the priming of the effector immune responses when the macromolecules are associated with a threat that might compromise host homeostasis. Therefore, SRs built up sophisticated sensor mechanisms controlling the immune system, which may be exploited to develop novel drugs for cancer immunotherapy. In this review, we focus on the regulation of the anti-tumor immune response by two paradigmatic SRs: the lymphocyte receptor CD5 and the more broadly distributed scavenger receptor class B type 1 (SR-B1). Cancer immunity can be boosted by blockade of SRs working as immune checkpoint inhibitors (CD5) and/or by proper engagement of SRs working as innate danger receptor (SR-B1). Thus, these receptors illustrate both the complexity of targeting SRs in cancer immunotherapy and also the opportunities offered by such an approach.

Diagnostic and prognostic value of scavenger receptor class B type 1 in clear cell renal cell carcinoma

Aberrant expression of scavenger receptor class B type 1 has been reported in several human cancers. Nevertheless, the roles of scavenger receptor class B type 1 in clear cell renal cell carcinoma remain unclear. The aim of this study was to evaluate the diagnostic and prognostic value of scavenger receptor class B type 1 in clear cell renal cell carcinoma. The messenger RNA level of scavenger receptor class B type 1 in clear cell renal cell carcinoma tissues was detected by quantitative reverse transcription polymerase chain reaction, while protein level was determined by western blot and immunohistochemistry. The lipid content between clear cell renal cell carcinoma tissues and normal kidney tissues was differentiated by Oil Red O and hematoxylin-eosin staining. The diagnostic value of scavenger receptor class B type 1 was determined by receiver operating characteristic curve. The prognostic significance of scavenger receptor class B type 1 was assessed by Kaplan-Meier analysis and Cox regression analysis. Our results showed that the expression of scavenger receptor class B type 1 in clear cell renal cell carcinoma tissues at both messenger RNA and protein level was much higher than that in normal kidney tissues. Receiver operating characteristic curve analysis exhibited a significant value of area under the curve (0.8486, 95% confidence interval: 0.7926-0.9045) with strong sensitivity (0.75, 95% confidence interval: 0.6535-0.8312) and specificity (0.90, 95% confidence interval: 0.8238-0.9510). Kaplan-Meier analysis revealed that patients with higher scavenger receptor class B type 1 expression had shorter progression-free survival time. Cox analysis indicated that scavenger receptor class B type 1 was an independent prognostic biomarker. In conclusion, our findings implied that scavenger receptor class B type 1 might serve as a diagnostic and independent prognostic biomarker in clear cell renal cell carcinoma.

Rediscovering scavenger receptor type BI: surprising new roles for the HDL receptor

PURPOSE OF REVIEW

Scavenger receptor BI (SR-BI) is classically known for its role in antiatherogenic reverse cholesterol transport as it selectively takes up cholesterol esters from HDL. Here, we have highlighted recent literature that describes novel functions for SR-BI in physiology and disease.

RECENT FINDINGS

A large population-based study has revealed that patients heterozygous for the P376L mutant form of SR-BI showed significantly increased levels of plasma HDL-cholesterol and had increased risk of cardiovascular disease, demonstrating that SR-BI in humans is a significant determinant of cardiovascular disease. Furthermore, SR-BI has been shown to modulate the susceptibility to LPS-induced tissue injury and the ability of sphingosine 1 phosphate to interact with its receptor, linking SR-BI to the regulation of inflammation. In addition, important domains within the molecule (Trp-415) as well as novel regulators (procollagen C-endopeptidase enhancer protein 2) of SR-BI's selective uptake function have recently been identified. Moreover, relatively high expression levels of the SR-BI protein have been observed in a variety of cancer tissues, which is associated with a reduced overall survival rate.

SUMMARY

The HDL receptor SR-BI is a potential therapeutic target not only in the cardiovascular disease setting, but also in inflammatory conditions as well as in cancer.

Targeting the SR-B1 Receptor as a Gateway for Cancer Therapy and Imaging

Malignant tumors display remarkable heterogeneity to the extent that even at the same tissue site different types of cells with varying genetic background may be found. In contrast, a relatively consistent marker the scavenger receptor type B1 (SR-B1) has been found to be consistently overexpressed by most tumor cells. Scavenger Receptor Class B Type I (SR-BI) is a high density lipoprotein (HDL) receptor that facilitates the uptake of cholesterol esters from circulating lipoproteins. Additional findings suggest a critical role for SR-BI in cholesterol metabolism, signaling, motility, and proliferation of cancer cells and thus a potential major impact in carcinogenesis and metastasis. Recent findings indicate that the level of SR-BI expression correlate with aggressiveness and poor survival in breast and prostate cancer. Moreover, genomic data show that depending on the type of cancer, high or low SR-BI expression may promote poor survival. This review discusses the importance of SR-BI as a diagnostic as well as prognostic indicator of cancer to help elucidate the contributions of this protein to cancer development, progression, and survival. In addition, the SR-B1 receptor has been shown to serve as a potential gateway for the delivery of therapeutic agents when reconstituted high density lipoprotein nanoparticles are used for their transport to cancer cells and tumors. Opportunities for the development of new technologies, particularly in the areas of cancer therapy and tumor imaging are discussed.

Properties of Native High-Density Lipoproteins Inspire Synthesis of Actively Targeted In Vivo siRNA Delivery Vehicles

Efficient systemic administration of therapeutic short interfering RNA (siRNA) is challenging. High-density lipoproteins (HDL) are natural in vivo RNA delivery vehicles. Specifically, native HDLs: 1) Load single-stranded RNA; 2) Are anionic, which requires charge reconciliation between the RNA and HDL, and 3) Actively target scavenger receptor type B-1 (SR-B1) to deliver RNA. Emphasizing these particular parameters, we employed templated lipoprotein particles (TLP), mimics of spherical HDLs, and self-assembled them with single-stranded complements of, presumably, any highly unmodified siRNA duplex pair after formulation with a cationic lipid. Resulting siRNA templated lipoprotein particles (siRNA-TLP) are anionic and tunable with regard to RNA assembly and function. Data demonstrate that the siRNA-TLPs actively target SR-B1 to potently reduce androgen receptor (AR) and enhancer of zeste homolog 2 (EZH2) proteins in multiple cancer cell lines. Systemic administration of siRNA-TLPs demonstrated no off-target toxicity and significantly reduced the growth of prostate cancer xenografts. Thus, native HDLs inspired the synthesis of a hybrid siRNA delivery vehicle that can modularly load single-stranded RNA complements after charge reconciliation with a cationic lipid, and that function due to active targeting of SR-B1.

SR-BI: Linking Cholesterol and Lipoprotein Metabolism with Breast and Prostate Cancer

Studies have demonstrated the significant role of cholesterol and lipoprotein metabolism in the progression of cancer. The SCARB1 gene encodes the scavenger receptor class B type I (SR-BI), which is an 82-kDa glycoprotein with two transmembrane domains separated by a large extracellular loop. SR-BI plays an important role in the regulation of cholesterol exchange between cells and high-density lipoproteins. Accordingly, hepatic SR-BI has been shown to play an essential role in the regulation of the reverse cholesterol transport pathway, which promotes the removal and excretion of excess body cholesterol. In the context of atherosclerosis, SR-BI has been implicated in the regulation of intracellular signaling, lipid accumulation, foam cell formation, and cellular apoptosis. Furthermore, since lipid metabolism is a relevant target for cancer treatment, recent studies have focused on examining the role of SR-BI in this pathology. While signaling pathways have initially been explored in non-tumoral cells, studies with cancer cells have now demonstrated SR-BI's function in tumor progression. In this review, we will discuss the role of SR-BI during tumor development and malignant progression. In addition, we will provide insights into the transcriptional and post-transcriptional regulation of the SCARB1 gene. Overall, studying the role of SR-BI in tumor development and progression should allow us to gain useful information for the development of new therapeutic strategies.

Targeting SR-BI for Cancer Diagnostics, Imaging and Therapy

Scavenger receptor class B type I (SR-BI) plays an important role in trafficking cholesteryl esters between the core of high density lipoprotein and the liver. Interestingly, this integral membrane protein receptor is also implicated in the metabolism of cholesterol by cancer cells, whereby overexpression of SR-BI has been observed in a number of tumors and cancer cell lines, including breast and prostate cancers. Consequently, SR-BI has recently gained attention as a cancer biomarker and exciting target for the direct cytosolic delivery of therapeutic agents. This brief review highlights these key developments in SR-BI-targeted cancer therapies and imaging probes. Special attention is given to the exploration of high density lipoprotein nanomimetic platforms that take advantage of upregulated SR-BI expression to facilitate targeted drug-delivery and cancer diagnostics, and promising future directions in the development of these agents.

Challenges and opportunities for siRNA-based cancer treatment

As one of the life-threatening diseases involving multi-step genetic and epigenetic disorders, cancer has long been a dynamic research area for siRNA-based therapy as half of the current siRNA-based clinical trials are involved in oncology. However, despite consistent enthusiasm in the academic world, siRNA-based cancer treatment still faces obstacles and difficulties in clinical development. In this article, we discuss key challenges facing siRNA-based cancer treatment revealed from recent clinical and preclinical studies, including chemical modification, tumour penetration, endosomal escape, target selection and off-target effects. In addition, opportunities and avenues for translating siRNA technology from bench to oncologic clinics are explored.

Up-regulated expression of scavenger receptor class B type 1 (SR-B1) is associated with malignant behaviors and poor prognosis of breast cancer

Scavenger receptor class B type 1 (SR-B1) is an integral membrane protein that is expressed in numerous cells and tissue types. The primary role of SR-B1 is to facilitate uptake of cholesteryl esters from high-density lipoproteins (HDL) in the liver. Altered SR-B1 expression contributes to human diseases. This study assessed association of SR-B1 expression in breast tissue specimens with breast cancer development and prognosis. Tissue specimens from 30 cases of adjacent normal breast tissues, ductal carcinoma in situ (DCIS) and invasive ductal breast cancer (IDCA) were subjected to Western blot analysis, and 135 cases of DCIS and IDCA were used for quantitative immunohistochemical analysis of SR-B1 expression. The data showed that SR-B1 was significantly overexpressed in IDCA tissues compared to normal breast and DCIS tissues. SR-B1 expression was associated with pre-menopausal status, tumor size, and worse overall survival of patients. The data from this ex vivo study suggests that up-regulated SR-B1 protein expression is associated with malignant behaviors of breast cancer and that SR-B1 is an independent predictor for poor survival in breast cancer patients.