Utility of DNA postreplication repair protein Rad6B in neoadjuvant chemotherapy response

Nano-delivery of RAD6/Translesion Synthesis Inhibitor SMI#9 for Triple-negative Breast Cancer Therapy

The triple-negative breast cancer (TNBC) subtype, regardless of their BRCA1 status, has the poorest outcome compared with other breast cancer subtypes, and currently there are no approved targeted therapies for TNBC. We have previously demonstrated the importance of RAD6-mediated translesion synthesis pathway in TNBC development/progression and chemoresistance, and the potential therapeutic benefit of targeting RAD6 with a RAD6-selective small-molecule inhibitor, SMI#9. To overcome SMI#9 solubility limitations, we recently developed a gold nanoparticle (GNP)-based platform for conjugation and intracellular release of SMI#9, and demonstrated its in vitro cytotoxic activity toward TNBC cells. Here, we characterized the in vivo pharmacokinetic and therapeutic properties of PEGylated GNP-conjugated SMI#9 in BRCA1 wild-type and BRCA1-mutant TNBC xenograft models, and investigated the impact of RAD6 inhibition on TNBC metabolism by ¹H-NMR spectroscopy. GNP conjugation allowed the released SMI#9 to achieve higher systemic exposure and longer retention as compared with the unconjugated drug. Systemically administered SMI#9-GNP inhibited the TNBC growth as effectively as intratumorally injected unconjugated SMI#9. Inductively coupled mass spectrometry analysis showed highest GNP concentrations in tumors and liver of SMI#9-GNP and blank-GNP-treated mice; however, tumor growth inhibition occurred only in the SMI#9-GNP-treated group. SMI#9-GNP was tolerated without overt signs of toxicity. SMI#9-induced sensitization was associated with perturbation of a common set of glycolytic pathways in BRCA1 wild-type and BRCA1-mutant TNBC cells. These data reveal novel SMI#9 sensitive markers of metabolic vulnerability for TNBC management and suggest that nanotherapy-mediated RAD6 inhibition offers a promising strategy for TNBC treatment.

Gold nanoparticle conjugated Rad6 inhibitor induces cell death in triple negative breast cancer cells by inducing mitochondrial dysfunction and PARP-1 hyperactivation: Synthesis and characterization

We recently developed a small molecule inhibitor SMI#9 for Rad6, a protein overexpressed in aggressive breast cancers and involved in DNA damage tolerance. SMI#9 induces cytotoxicity in cancerous cells but spares normal breast cells; however, its therapeutic efficacy is limited by poor solubility. Here we chemically modified SMI#9 to enable its conjugation and hydrolysis from gold nanoparticle (GNP). SMI#9-GNP and parent SMI#9 activities were compared in mesenchymal and basal triple negative breast cancer (TNBC) subtype cells. Whereas SMI#9 is cytotoxic to all TNBC cells, SMI#9-GNP is endocytosed and cytotoxic only in mesenchymal TNBC cells. SMI#9-GNP endocytosis in basal TNBCs is compromised by aggregation. However, when combined with cisplatin, SMI#9-GNP is imported and synergistically increases cisplatin sensitivity. Like SMI#9, SMI#9-GNP spares normal breast cells. The released SMI#9 is active and induces cell death via mitochondrial dysfunction and PARP-1 stabilization/hyperactivation. This work signifies the development of a nanotechnology-based Rad6-targeting therapy for TNBCs.

FROM THE CLINICAL EDITOR

Protein Rad6 is overexpressed in breast cancer cells and its blockade may provide a new treatment against 3N breast cancer. The authors conjugated a small molecule inhibitor SMI#9 for Rad6 to gold nanoparticles in this study and showed that this new formulation specifically targeted chemo-resistant breast cancer cells and highlighted the importance of nanotechnology in drug carrier development.

Crosstalk between translesion synthesis, Fanconi anemia network, and homologous recombination repair pathways in interstrand DNA crosslink repair and development of chemoresistance

Bifunctional alkylating and platinum based drugs are chemotherapeutic agents used to treat cancer. These agents induce DNA adducts via formation of intrastrand or interstrand (ICL) DNA crosslinks, and DNA lesions of the ICL type are particularly toxic as they block DNA replication and/or DNA transcription. However, the therapeutic efficacies of these drugs are frequently limited due to the cancer cell's enhanced ability to repair and tolerate these toxic DNA lesions. This ability to tolerate and survive the DNA damage is accomplished by a set of specialized low fidelity DNA polymerases called translesion synthesis (TLS) polymerases since high fidelity DNA polymerases are unable to replicate the damaged DNA template. TLS is a crucial initial step in ICL repair as it synthesizes DNA across the lesion thus preparing the damaged DNA template for repair by the homologous recombination (HR) pathway and Fanconi anemia (FA) network, processes critical for ICL repair. Here we review the molecular features and functional roles of TLS polymerases, discuss the collaborative interactions and cross-regulation of the TLS DNA damage tolerance pathway, the FA network and the BRCA-dependent HRR pathway, and the impact of TLS hyperactivation on development of chemoresistance. Finally, since TLS hyperactivation results from overexpression of Rad6/Rad18 ubiquitinating enzymes (fundamental components of the TLS pathway), increased PCNA ubiquitination, and/or increased recruitment of TLS polymerases, the potential benefits of selectively targeting critical components of the TLS pathway for enhancing anti-cancer therapeutic efficacy and curtailing chemotherapy-induced mutagenesis are also discussed.

Melanoma Development and Progression Are Associated with Rad6 Upregulation and β -Catenin Relocation to the Cell Membrane

We have previously demonstrated that Rad6 and β -catenin enhance each other's expression through a positive feedback loop to promote breast cancer development/progression. While β -catenin has been implicated in melanoma pathogenesis, Rad6 function has not been investigated. Here, we examined the relationship between Rad6 and β -catenin in melanoma development and progression. Eighty-eight cutaneous tumors, 30 nevi, 29 primary melanoma, and 29 metastatic melanomas, were immunostained with anti- β -catenin and anti-Rad6 antibodies. Strong expression of Rad6 was observed in only 27% of nevi as compared to 100% of primary and 96% of metastatic melanomas. β -Catenin was strongly expressed in 97% of primary and 93% of metastatic melanomas, and unlike Rad6, in 93% of nevi. None of the tumors expressed nuclear β -catenin. β -Catenin was exclusively localized on the cell membrane of 55% of primary, 62% of metastatic melanomas, and only 10% of nevi. Cytoplasmic β -catenin was detected in 90% of nevi, 17% of primary, and 8% of metastatic melanoma, whereas 28% of primary and 30% of metastatic melanomas exhibited β -catenin at both locations. These data suggest that melanoma development and progression are associated with Rad6 upregulation and membranous redistribution of β -catenin and that β -catenin and Rad6 play independent roles in melanoma development.

Ubiquitin- and ubiquitin-like proteins-conjugating enzymes (E2s) in breast cancer

Breast cancer is the most common malignancy in women and a significant cause of morbidity and mortality. Sub-types of breast cancer defined by the expression of steroid hormones and Her2/Neu oncogene have distinct prognosis and undergo different therapies. Besides differing in their phenotype, sub-types of breast cancer display various molecular lesions that participate in their pathogenesis. BRCA1 is one of the common hereditary cancer predisposition genes and encodes for an ubiquitin ligase. Ubiquitin ligases or E3 enzymes participate together with ubiquitin activating enzyme and ubiquitin conjugating enzymes in the attachment of ubiquitin (ubiquitination) in target proteins. Ubiquitination is a post-translational modification regulating multiple cell functions. It also plays important roles in carcinogenesis in general and in breast carcinogenesis in particular. Ubiquitin conjugating enzymes are a central component of the ubiquitination machinery and are often perturbed in breast cancer. This paper will discuss ubiquitin and ubiquitin-like proteins conjugating enzymes participating in breast cancer pathogenesis, their relationships with other proteins of the ubiquitination machinery and their role in phenotype of breast cancer sub-types.

Value of TP53 status for predicting response to neoadjuvant chemotherapy in breast cancer: a meta-analysis

Background

Numerous studies have yielded inconclusive results regarding the relationship between tumor suppressor protein TP53 overexpression and/or TP53 gene mutations and the response to neoadjuvant chemotherapy in patients with breast cancer. The purpose of the current study was therefore to evaluate the relationship between TP53 status and response to chemotherapy in breast cancer.

Methods and Findings

A total of 26 previously published eligible studies including 3,476 cases were identified and included in this meta-analysis. TP53 status (over expression of TP53 protein and/or TP53 gene mutations) was associated with good response in breast cancer patients who received neoadjuvant chemotherapy (total objective response: risk ratio [RR]  = 1.20, 95% confidence interval [CI]  = 1.09–1.33, p<0.001; pathological objective response: RR = 1.37, 95% CI = 1.20–1.57, p<0.01; total complete response: RR = 1.33, 95% CI = 1.15–1.53, p<0.001; pathological complete response: RR = 1.45, 95% CI = 1.25–1.68, p<0.001). In further stratified analyses, this association also existed among the studies using anthracycline-based neoadjuvant chemotherapy, and the association between response and the presence of gene alterations was stronger than that between response and immunohistochemistry positivity.

Conclusion

The results of the present meta-analysis suggest that TP53 status is a predictive factor for response in breast cancer patients undergoing neoadjuvant chemotherapy. Further larger and well-designed prospective studies are required to evaluate the predictive role of TP53 status in clinical practice.

Impact of mutant β-catenin on ABCB1 expression and therapy response in colon cancer cells

Background:

Colorectal cancers are often chemoresistant toward antitumour drugs that are substrates for ABCB1-mediated multidrug resistance (MDR). Activation of the Wnt/β-catenin pathway is frequently observed in colorectal cancers. This study investigates the impact of activated, gain-of-function β-catenin on the chemoresistant phenotype.

Methods:

The effect of mutant (mut) β-catenin on ABCB1 expression and promoter activity was examined using HCT116 human colon cancer cells and isogenic sublines harbouring gain-of-function or wild-type β-catenin, and patients’ tumours. Chemosensitivity towards 24 anticancer drugs was determined by high throughput screening.

Results:

Cell lines with mut β-catenin showed high ABCB1 promoter activity and expression. Transfection and siRNA studies demonstrated a dominant role for the mutant allele in activating ABCB1 expression. Patients’ primary colon cancer tumours shown to express the same mut β-catenin allele also expressed high ABCB1 levels. However, cell line chemosensitivities towards 24 MDR-related and non-related antitumour drugs did not differ despite different β-catenin genotypes.

Conclusion:

Although ABCB1 is dominantly regulated by mut β-catenin, this did not lead to drug resistance in the isogenic cell line model studied. In patient samples, the same β-catenin mutation was detected. The functional significance of the mutation for predicting patients’ therapy response or for individualisation of chemotherapy regimens remains to be established.