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Cancer stem cells (CSCs) represent a subset of tumor cells with tumor-initiating potential. We recently demonstrated that inhibition of the sumoylation pathway cleared the CSC population and repressed the outgrowth of basal breast cancer xenografts. Targeting the sumoylation pathway offers a novel treatment strategy for basal breast cancer.
The clinical subtypes of breast cancer are defined by expression of estrogen receptor-alpha (ERα), progesterone receptor (PgR), and amplification and overexpression of ERBB2/HER2. The molecular subtype of breast cancer based on patterns of gene expression is predictive of outcome and response to therapy.1 The luminal breast cancer subtypes (accounting for approximately 75% of breast cancers in postmenopausal women) are characterized by the expression of a set of ERα-associated genes.1 The triple-negative breast cancer subtype is a heterogeneous group that represents 10–20% of breast cancers and includes basal breast cancers, which have an aggressive clinical course and do not respond to directed therapy that is effective for cancers that express ERα or HER2.2, 3 Hence, there is an intense need for research focused on understanding the molecular characterization of this group with the goal of defining novel molecular targets.3 Basal-like breast cancers are further distinguished from luminal cancers by frequent mutations of tumor protein p53 (also known as TP53), gene expression patterns characteristic of epithelial–mesenchymal transition (EMT), and an increase in the percentage of cancer stem cells (CSCs).3 The CSC population is identified by the marker expression CD44+/hi/CD24−/low and defines a population of cells capable of forming tumor xenografts. CSCs are relatively chemoresistant and become enriched after chemotherapy, leading to the theory that CSCs drive cancer recurrence and metastasis.4 Therapeutic strategies that specifically target the CSC population offer the potential of reducing cancer recurrence and metastasis and may augment the therapeutic benefit of conventional chemotherapy.
The process of sumoylation results in the post-translational modification of a target protein through the attachment of small ubiquitin-like modifier (SUMO) proteins to a lysine residue and is mediated by an enzymatic cascade involving E1-activation, E2-conjugation, and E3-ligation.5 The activity of many transcription factors is altered by sumoylation, often through repression of activity at a subset of promoters.6,7 Several interesting findings are associated with the regulation of transcriptional activity by sumoylation. First, SUMO conjugation of transcription factors is often identified in only a small population of the total protein yet can profoundly influence the transcriptional activity of the factor.7 Second, SUMO conjugation of transcriptional regulators can influence the transcriptional activity at a subset of promoters but may leave a subset of target promoters unaffected. For example, Holmstrom et al.8 showed that sumoylation repressed transcriptional activity of glucocorticoid receptor at promoter regions with compound but not single sites, indicating that the effects of sumoylation may be related to the degree of stability of the protein–chromatin complex or to higher order structures formed at complex promoter sites.
Our recent publication9 provides important insight into the role of the SUMO pathway in regulating the phenotypic characteristics of basal breast cancer. The transcription factors TFAP2A/AP-2α and TFAP2C/AP-2γ play key roles in the regulation of genes that establish patterns of gene expression characteristic of the different breast cancer subtypes. TFAP2C has a unique role in maintaining the luminal pattern of expression by inducing ERα-associated genes and repressing basal-associated genes. One of the molecular characteristics of basal cancer is relatively high expression of CD44, a key marker of the CSC population, and TFAP2C has been shown to repress CD44 expression.9 On the other hand, the highly homologous AP-2 family member TFAP2A lacks functional effects on luminal patterning. The molecular basis for functional specificity of the AP-2 family members depends upon sumoylation. SUMO-conjugated TFAP2A is transcriptionally inactive with respect to luminal gene expression; however, inhibition of the SUMO pathway allows TFAP2A to acquire TFAP2C-like transcriptional activity including the ability to repress CD44. Of particular clinical importance, we showed that blocking the SUMO pathway in basal cancers resulted in TFAP2A-dependent repression of CD44 and clearance of the CSC population (Fig. 1). Mice that are gavaged with SUMO inhibitors failed to form basal cancer xenografts, demonstrating that clearing of the CD44+/hi/CD24−/low cell population resulted in a loss of tumor-initiating ability of basal cell lines.
The findings presented in Bogachek et al.9 offer a novel approach to the treatment of basal breast cancer and potentially other carcinomas in which the CD44+/hi/CD24−/low CSC population may be abrogated through inhibition of the SUMO pathway. Whereas conventional chemotherapy may preferentially eradicate the more differentiated tumor bulk, the addition of SUMO inhibitors may specifically target the CSC population, resulting in a more durable therapeutic response. Certain key issues must be addressed in order to advance the pre-clinical role of SUMO inhibitors in cancer therapy. For example, it will be important to examine in more detail the transcriptional effects of SUMO inhibition. Inhibiting the SUMO pathway may induce a number of molecular effects that are critical to cancer progression and metastasis. In the basal cell lines examined, TFAP2A was clearly shown to mediate repression of CD44 in response to SUMO inhibition. However, other transcription factors sensitive to sumoylation may also play important roles in the regulation of gene expression. For example, recent findings indicated that RUNX activity is regulated by sumoylation and that mutations of the SUMO site may be a mechanism whereby breast cancers bypass regulation through the SUMO pathway.10 Hence, understanding the molecular basis for sensitivity to SUMO inhibition will be an important area for further investigation. These exciting findings are encouraging indications that targeting the SUMO pathway provides a novel approach to eliminate the CSC population of tumors and thereby establish a more durable clinical response.
No potential conflicts of interest were disclosed.
This work was supported by the National Institutes of Health grants R01CA109294 (PI: R.J. Weigel), T32CA148062 (PtdIns: R. J. Weigel) and by a generous gift from the Kristen Olewine Milke Breast Cancer Research Fund. JPD was supported by the NIH grant T32CA148062.