Too much cleavage of cyclin E promotes breast tumorigenesis

Targeting eukaryotic proteases for natural products-based drug development

Covering: up to April 2020 Proteases are involved in the regulation of many physiological processes. Their overexpression and dysregulated activity are linked to diseases such as hypertension, diabetes, viral infections, blood clotting disorders, respiratory diseases, and cancer. Therefore, they represent an important class of therapeutic targets. Several protease inhibitors have reached the market and >60% of them are directly related to natural products, even when excluding synthetic natural product mimics. Historically, natural products have been a valuable and validated source of therapeutic agents, as over half of the marketed drugs across targets and diseases are inspired by natural product structures. In the past two decades the number of new protease inhibitors discovered from nature has sharply increased. Additionally, the availability of 3D structural information for proteases has permitted structure-based design and accelerated the synthesis of optimized lead structures with improved potency and selectivity profiles, resulting in some of the most-potent-in-class inhibitors. These discoveries were oftentimes maximized by in-depth biological assessments of lead inhibitors, linking them to a relevant disease state. This review will discuss some of the current and emerging drug targets and their involvement in various disease processes, highlighting selected success stories behind several FDA-approved protease inhibitors that have natural products scaffolds as well as recent selected pharmacologically well-characterized inhibitors derived from marine or terrestrial sources.

Low-Molecular-Weight Cyclin E in Human Cancer: Cellular Consequences and Opportunities for Targeted Therapies

Cyclin E, a regulatory subunit of cyclin-dependent kinase 2 (CDK2), is central to the initiation of DNA replication at the G1/S checkpoint. Tight temporal control of cyclin E is essential to the coordination of cell-cycle processes and the maintenance of genome integrity. Overexpression of cyclin E in human tumors was first observed in the 1990s and led to the identification of oncogenic roles for deregulated cyclin E in experimental models. A decade later, low-molecular-weight cyclin E (LMW-E) isoforms were observed in aggressive tumor subtypes. Compared with full-length cyclin E, LMW-E hyperactivates CDK2 through increased complex stability and resistance to the endogenous inhibitors p21CIP1 and p27KIP1 LMW-E is predominantly generated by neutrophil elastase-mediated proteolytic cleavage, which eliminates the N-terminal cyclin E nuclear localization signal and promotes cyclin E's accumulation in the cytoplasm. Compared with full-length cyclin E, the aberrant localization and unique stereochemistry of LMW-E dramatically alters the substrate specificity and selectivity of CDK2, increasing tumorigenicity in experimental models. Cytoplasmic LMW-E, which can be assessed by IHC, is prognostic of poor survival and predicts resistance to standard therapies in patients with cancer. These patients may benefit from therapeutic modalities targeting the altered biochemistry of LMW-E or its associated vulnerabilities. Cancer Res; 78(19); 5481-91. ©2018 AACR.

Pathogenesis and heterogeneity of ovarian cancer

PURPOSE OF REVIEW

The most common type of ovarian cancer, high-grade serous ovarian carcinoma (HGSOC), was originally thought to develop from the ovarian surface epithelium. However, recent data suggest that the cells that undergo neoplastic transformation and give rise to the majority of HGSOC are from the fallopian tube. This development has impacted both translational research and clinical practice, revealing new opportunities for early detection, prevention, and treatment of ovarian cancer.

RECENT FINDINGS

Genomic studies indicate that approximately 50% of HGSOC are characterized by mutations in genes involved in the homologous recombination pathway of DNA repair, especially BRCA1 and BRCA2. Clinical trials have demonstrated successful treatment of homologous recombination-defective cancers with poly-ribose polymerase inhibitors through synthetic lethality. Recently, amplification of CCNE1 was found to be another major factor in HGSOC tumorigenesis, accounting for approximately 20% of all cases. Interestingly, amplification of CCNE1 and mutation of homologous recombination repair genes are mutually exclusive in HGSOC.

SUMMARY

The fallopian tube secretory cell is the cell of origin for the majority of ovarian cancers. Although it remains unclear what triggers neoplastic transformation of these cells, certain tumors exhibit loss of BRCA function or amplification of CCNE1. These alterations represent unique therapeutic opportunities in ovarian cancer.

Cytoplasmic Cyclin E Predicts Recurrence in Patients with Breast Cancer

Purpose: Low molecular weight cyclin E (LMW-E) detected by Western blot analysis predicts for reduced breast cancer survival; however, it is impractical for clinical use. LMW-E lacks a nuclear localization signal that leads to accumulation in the cytoplasm that can be detected by IHC. We tested the hypothesis that cytoplasmic staining of cyclin E can be used as a predictor of poor outcome in different subtypes of breast cancer using patient cohorts with distinct clinical and pathologic features.Experimental Design: We evaluated the subcellular localization of cyclin E in breast cancer specimens from 2,494 patients from 4 different cohorts: 303 from a prospective study and 2,191 from retrospective cohorts [NCI, MD Anderson Cancer Center (MDA), and the United Kingdom (UK)]. Median follow-up times were 8.0, 10.1, 13.5, and 5.7 years, respectively.Results: Subcellular localization of cyclin E on IHC was associated with full-length (nuclear) and low molecular weight isoforms (cytoplasmic) of cyclin E on Western blot analysis. In multivariable analysis, cytoplasmic cyclin E staining was associated with the greatest risk of recurrence compared with other prognostic factors across all subtypes in three (NCI, MDA, and UK) of the cohorts. In the MDA cohort, cytoplasmic cyclin E staining outperformed Ki67 and all other variables as prognostic factors.Conclusions: Cytoplasmic cyclin E identifies patients with the highest likelihood of recurrence consistently across different patient cohorts and subtypes. These patients may benefit from alternative therapies targeting the oncogenic isoforms of cyclin E. Clin Cancer Res; 23(12); 2991-3002. ©2016 AACR.

Cytoplasmic Cyclin E and Phospho-Cyclin-Dependent Kinase 2 Are Biomarkers of Aggressive Breast Cancer

Cyclin E and its co-activator, phospho-cyclin-dependent kinase 2 (p-CDK2), regulate G1 to S phase transition and their deregulation induces oncogenesis. Immunohistochemical assessments of these proteins in cancer have been reported but were based only on their nuclear expression. However, the oncogenic forms of cyclin E (low molecular weight cyclin E or LMW-E) in complex with CDK2 are preferentially mislocalized to the cytoplasm. Here, we used separate nuclear and cytoplasmic scoring systems for both cyclin E and p-CDK2 expression to demonstrate altered cellular accumulation of these proteins using immunohistochemical analysis. We examined the specificity of different cyclin E antibodies and evaluated their concordance between immunohistochemical and Western blot analyses in a panel of 14 breast cell lines. Nuclear versus cytoplasmic staining of cyclin E readily differentiated full-length from LMW-E, respectively. We also evaluated the expression of cyclin E and p-CDK2 in 1676 breast carcinoma patients by immunohistochemistry. Cytoplasmic cyclin E correlated strongly with cytoplasmic p-CDK2 (P < 0.0001), high tumor grade, negative estrogen/progesterone receptor status, and human epidermal growth factor receptor 2 positivity (all P < 0.0001). In multivariable analysis, cytoplasmic cyclin E plus phosphorylated CDK2 (as one variable) predicted breast cancer recurrence-free and overall survival. These results suggest that cytoplasmic cyclin E and p-CDK2 can be readily detected with immunohistochemistry and used as clinical biomarkers for aggressive breast cancer.

Cyclin E Associates with the Lipogenic Enzyme ATP-Citrate Lyase to Enable Malignant Growth of Breast Cancer Cells

Cyclin E is altered in nearly a third of invasive breast cancers where it is a powerful independent predictor of survival in women with stage I-III disease. Full-length cyclin E is posttranslationally cleaved into low molecular weight (LMW-E) isoforms, which are tumor-specific and accumulate in the cytoplasm because they lack a nuclear localization sequence. We hypothesized that aberrant localization of cytosolic LMW-E isoforms alters target binding and activation ultimately contributing to LMW-E-induced tumorigenicity. To address this hypothesis, we used a retrovirus-based protein complementation assay to find LMW-E binding proteins in breast cancer, identifying ATP-citrate lyase (ACLY), an enzyme in the de novo lipogenesis pathway, as a novel LMW-E-interacting protein in the cytoplasm. LMW-E upregulated ACLY enzymatic activity, subsequently increasing lipid droplet formation, thereby providing cells with essential building blocks to support growth. ACLY was also required for LMW-E-mediated transformation, migration, and invasion of breast cancer cells in vitro along with tumor growth in vivo In clinical specimens of breast cancer, the absence of LMW-E and low expression of adipophilin (PLIN2), a marker of lipid droplet formation, associated with favorable prognosis, whereas overexpression of both proteins correlated with a markedly worse prognosis. Taken together, our findings establish a novel relationship between LMW-E isoforms of cyclin E and aberrant lipid metabolism pathways in breast cancer tumorigenesis, warranting further investigation in additional malignancies exhibiting their expression. Cancer Res; 76(8); 2406-18. ©2016 AACR.