Author: Jai Meyers
Meyers, Jai, 2025 The Role of UGT Enzymes as Novel Modulators of Lipid Biosynthesis in Breast Cancer, Flinders University, College of Medicine and Public Health
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The human UDP-glycosyltransferase (UGT) superfamily is comprised of 22 enzymes across four families (UGT1, UGT2, UGT3 and UGT8) that catalyse the covalent addition of sugars to a broad range of small lipophilic molecules. This process not only plays a critical role in the inactivation and elimination of exogenous chemicals but also controls the level and distribution of numerous endogenous signalling molecules, such as steroid hormones, which modulate both breast and prostate cancer progression. Several UGTs are also transcriptionally regulated by such steroids, thus creating a feedback loop to control steroid signalling. Unlike other UGTs, the biological activities of UGT2B11 and UGT2B28 are not well characterised. UGT2B11 and UGT2B28 are frequently overexpressed in cancer, particularly those arising in the breast and prostate. Furthermore, recent data shows that both UGT2B11 and UGT2B28 are dramatically induced by androgens in breast and prostate cancer cells. Although their expression has been linked to pathogenic features of breast and prostate cancers, their precise biological functions remain poorly understood. These enzymes lack well-defined high activity substrates and are often referred to as ‘orphan’ enzymes. This thesis sought to mechanistically explore the functional role of UGT2B11 and UGT2B28 in breast cancer models.
Analysis of RNAseq and microarray data from The Cancer Genome Atlas breast cancer (TCGA-BRCA) dataset and the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) dataset revealed a clear association between the expression of both UGT2B11 and UGT2B28 isoforms. Increased expression of these UGTs was observed in breast cancer patients with tumours that were estrogen receptor alpha negative (ER-), human epidermal growth factor receptor 2 (HER2) enriched and had high expression of the androgen receptor (AR). Interestingly, breast cancers with this expression profile are broadly consistent with molecular apocrine tumours and are similar to prostate cancers in many ways. Within the ER- subsets of breast cancer, high expression of either UGT2B11 or UGT2B28 was associated with significantly worse survival outcomes. In these patients, functional pathway analysis identified enrichment of gene signatures associated with lipid biosynthesis, particularly those involved in Sterol Regulatory Element Binding Protein (SREBP) mediated lipid biosynthetic pathways.
SREBPs are the master regulators of lipid metabolism. When cellular lipid levels are low, an endoplasmic reticulum (ER)-based lipid-sensing mechanism induces trafficking of SREBP from ER to Golgi, where proteolytic processing generates the mature transcription factor form that enters the nucleus to drive lipogenic gene expression. The roles of UGT2B11 and UGT2B28 in lipid metabolism were assessed in the molecular apocrine MDA-MB-453 breast cancer cell line. Stable overexpression was associated with increased proliferation and increased expression of SREBP-target genes. Moreover, transient UGT2B11 and UGT2B28 expression triggered ER processing events that lead to increased nuclear accumulation of nSREBPs. These affects were also recapitulated with a series of naturally occurring truncated variants of UGT2B11 and UGT2B28 lacking critical domains for catalytic activity, suggesting a non-catalytic mechanism. HEK-293T co-expression models demonstrated physical interactions between UGT2B11 and all components of the ER-based lipid sensing complex; SCAP (SREBP cleavage activating protein), INSIG (Insulin induced gene product) and the SREBP precursor. Together these data support a model whereby, through novel functional interactions, UGT2B11 and UGT2B28 can increase SREBP activation and lead to increased SREBP target gene expression.
Finally, as excessive lipogenesis can result in lipotoxicity and cellular death, mechanisms by which cancerous cells can prevent this whilst still maintaining a proliferative advantage were examined. Intriguingly, UGT2B11 and UGT2B28 were able to promote the turnover of transcriptionally active nSREBPs in MDA-MB-453 breast cancer cells. This appeared to be independent of the canonical glycogen synthase kinase-3β (GSK-3β) phosphorylation, which is generally required for nSREBP turnover. As an alternative mechanism for preventing lipotoxicity, the capacity for nSREBPs to reduce expression of UGTs through reducing AR transcriptional activity was examined. Whilst UGT promoter constructs showed reduced transactivation following transient overexpression of nSREBPs, this did not translate to a significant change in UGT mRNA levels in doxycycline-inducible nSREBP overexpression cell lines.
Overall, the findings of this thesis demonstrate that UGT2B11 and UGT2B28 can modulate the activation of SREBPs, leading to enhanced proliferative capacity of breast cancer cells and to worse survival outcomes for patients with increased UGT expression. Additionally, UGT2B11 and UGT2B28 may also modulate nSREBP transcriptional activity by inducing nSREBP turnover, suggesting an important role for these enzymes in preventing toxic hyperactivation of lipogenesis. This research highlights the need for further studies on the enigmatic UGT2B11 and UGT2B28 ‘orphan’ enzymes.
Keywords: UDP-glycosyltransferase, UGT, UGT2B11, UGT2B28, Lipid Biosynthesis, breast cancer, Sterol Regulatory Element Binding Protein, SREBP
Subject: Medicine thesis
Thesis type: Doctor of Philosophy
Completed: 2025
School: College of Medicine and Public Health
Supervisor: Robyn Meech