Normal and neoplastic urothelial stem cells: getting to the root of the problem

Abstract

Most epithelial tissues contain self-renewing stem cells that mature into downstream progenies with increasingly limited differentiation potential. It is not surprising that cancers arising from such hierarchically organized epithelial tissues retain features of cellular differentiation. Accumulating evidence suggests that the urothelium of the urinary bladder is a hierarchically organized tissue, containing tissue-specific stem cells that are important for both normal homeostasis and injury response. The phenotypic and functional properties of cancer stem cells (CSCs; also known as tumour-initiating cells) from bladder cancer tissue have been studied in detail. Urothelial CSCs are not isolated by a 'one-marker-fits-all' approach; instead, various cell surface marker combinations (possibly reflecting the cell-of-origin) are used to isolate CSCs from distinct differentiation subtypes of urothelial carcinomas. Additional CSC markers, including cytokeratin 14 (CK14), aldehyde dehydrogenase 1 family, member A1 (ALDH1A1), and tumour protein 63 (p63), have revealed prognostic value for urothelial carcinomas. Signalling pathways involved in normal stem cell self-renewal and differentiation are implicated in the malignant transformation of different subsets of urothelial carcinomas. Early expansion of primitive CK14+ cells--driven by genetic pathways such as STAT3--can lead to the development of carcinoma in situ, and CSC-enriched urothelial carcinomas are associated with poor clinical outcomes. Given that bladder CSCs are the proposed root of malignancy and drivers of cancer initiation and progression for urothelial carcinomas, these cells are ideal targets for anticancer therapies.

Figure 1

Markers and signalling pathways associated with urothelial cellular differentiation. a | Composition of the normal adult bladder urothelium. Spiky bright-green symbols represent infiltrating immune cells, stromal cells are shown in grey, basal cells are shown in light brown, a single stem cell is shown in dark brown, intermediate cells are shown in blue, and umbrella cells are shown in purple. b | The association of markers with normal urothelial cellular differentiation. Thin black arrows depict transitions from one cell type to another. Dotted thin black lines show an alternative (hypothetical) pathway for cellular differentiation. Thick black arrows depict increases in levels of expression. c | The role of signalling proteins and pathways in injury-induced cell regeneration. Thin black arrows depict transitions from one cell type to another. Dotted thin black lines show alternative (hypothetical) signalling pathways for cellular regeneration. Abbreviations: 67LR, 67L-kDa laminin receptor; CD, cluster of differentiation; CK, cytokeratin; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; GLI1, zinc finger protein GLI1; LRC, label-retaining cell; MAPK, mitogen-activated protein kinase; PPARγ, peroxisome proliferator-activated receptor γ; SHH, sonic hedgehog protein; SOX9, transcription factor SOX-9; Wnt, protein Wnt.

Figure 2

Clinical relevance of urothelial CSCs. a | Process of cellular differentiation in the normal urothelium. During the process of differentiation, loss of expression marker CD90 occurs early on, followed by changes to expression status of CD44, and, in later stages, CD49. At least three subtypes of UC exist on the basis of differentiation status. Differentiated and intermediate subtypes are associated with better outcomes, whereas basal subtypes are associated with poor clinical outcomes. b | Early-stage UCs (pTa or pT1) commonly contain activating mutations in FGFR3, HRAS, or PIK3CA. About 15% of the high-risk subgroup will progress to invasive UCs. Advanced-stage UCs (>pT2) are commonly linked to mutations in the tumour suppressor genes P53, RB, and PTEN. These advanced-stage UCs commonly arise from high-grade dysplastic lesions or CIS, usually without an intermediary stage of noninvasive papillary disease. STAT3-driven expansion of CK14 primitive stem cells directs urothelial cells towards the CIS-invasive pathway. Some CSC markers (CK5, CK14, and 34BE12) have been linked to histological UC variants, such as squamous cell carcinomas and micropapillary UCs, which are associated with poor prognosis and aggressive phenotype. Abbreviations: CIS, carcinoma in situ; CK, cytokeratin; CSCs, cancer stem cells; FGFR3, fibroblast growth factor receptor 3; HRAS, v-Ha-ras Harvey rat sarcoma viral oncogene homolog; PIK3CA, phosphoinositide-3-kinase, catalytic, alpha polypeptide; PTEN, phosphatase and tensin homolog; p53, tumour protein 53; RB, retinoblastoma protein; SCC, squamous cell carcinoma; STAT3, signal transducer and activator of transcription 3 (acute-phase response factor).