SUMO and estrogen receptors in breast cancer

Small ubiquitin-like modifier (SUMO) is a family of proteins structurally similar to ubiquitin that have been found to be covalently attached to certain lysine residues of specific target proteins. By contrast to ubiquitination, however, SUMO proteins do not promote protein degradation but, instead, modulate important functional properties, depending on the protein substrate. These properties include—albeit not limited to—subcellular localization, protein dimerization, DNA binding and/or transactivation of transcription factors, among them estrogen receptors. Moreover, it has been suggested that SUMO proteins might affect transcriptional co-factor complexes of the estrogen receptor signalling cascade. Tissue and/or state specificity seems to be one of their intriguing features. In this regard, elucidation of their contribution to estrogen receptor-mediated transcriptional activity during breast carcinogenesis will offer new insights into the molecular mechanisms governing sensitivity/resistance in currently applied endocrine treatment and/or chemoprevention, and provide novel routes to breast carcinoma therapeutics.