Biology of urothelial tumorigenesis: insights from genetically engineered mice

Abstract

Urothelium, one of the slowest cycling epithelia in the body, embodies a unique biological context for cellular transformation. Introduction of oncogenes into or removing tumor suppressor genes from the urothelial cells or a combination of both using the transgenic and/or knockout mouse approaches has provided useful insights into the molecular mechanisms of urothelial transformation and tumorigenesis. It is becoming increasingly clear that over-activation of the receptor tyrosine kinase (RTK) pathway, as exemplified by the constitutively activated Ha-ras oncogene, is both necessary and sufficient to initiate the low-grade, non-invasive urothelial carcinomas. Dosage of the mutated Ha-ras, but not concurrent inactivation of pro-senescence molecules p16Ink4a and p19Arf, dictates whether and when the low-grade urothelial carcinomas arise. Inactivation of both p53 and pRb, a prevailing paradigm previously proposed for muscle-invasive urothelial tumorigenesis, is found to be necessary but insufficient to initiate this urothelial carcinoma variant. Instead, downregulation in p53/pRb co-deficient urothelial cells of p107, a pRb family member, is associated with the genesis of the muscle-invasive bladder cancers. p53 deficiency also seems to be capable of cooperating with that of PTEN in eliciting invasive urothelial carcinomas. The genetically engineered mice have improved the molecular definition of the divergent pathways of urothelial tumorigenesis and progression, helped delineate the intricate crosstalk among different genetic alterations within a urothelium-specific context, identified new prognostic markers and novel therapeutic targets potentially applicable for clinical intervention, and provided in vivo platforms for testing preventive strategies of bladder cancer.

Fig. 1

Dosage-dependence in the activation of ras pathway effectors leading to low-grade, non-invasive urothelial tumorigenesis in the transgenic mice. Low-level urothelial expression of an activated Ha-ras primarily induces MAPK pathway components and urothelial hyperplasia, without provoking urothelial tumors. However, doubling the activated Ha-ras transgene dosage hyper-activates Akt and STAT pathways, resulting in low-grade, non-invasive urothelial tumors. STAT activation may be caused by growth factor (GFs) and cytokines (CTKs) during epithelial/mesenchymal interaction and/or by the functional inactivation of PTEN through C-terminal phosphorylation. Mutations in fibroblast growth factor receptor 3 (FGFR3), which is known to constitutively activate ras GTPase, are likely to transmit signals and induce urothelial tumors in a similar manner. This figure was reprinted with permission from Ref.

Fig. 2

Collaborative effects between pRb family proteins and p53 in invasive urothelial tumorigenesis. a Ablation of pRb specifically in urothelium leads to E2F1 over-expression which in turn up-regulates the p53 pathway and pRb family protein p107 along with transcriptional repressor E2F4. These compensatory urothelial defenses cause urothelial cells to undergo cell-cycle arrest and apoptosis. b Additional ablation of p53 in urothelial cells lacking pRb effectively blunted apoptotic response, resulting in late-onset hyperplasia and nuclear atypia. c Treatment of urothelial cells lacking both pRb and p53 with bladder carcinogen BBN down-regulates p107 and triggers invasive urothelial tumorigenesis. This model emphasizes the collaborative effects among pRb family proteins and p53 in invasive urothelial tumor initiation. This figure was reprinted with permission from Ref.