Researchers generated mice with urothelium-specific knockout of Pten or Tp53. In the Pten knockout mice, the urothelium exhibited mild hyperplasia and increased the number of urothelial layers. Urothelial morphology was normal in Tp53 knockout mice. When both Tp53 and Pten were depleted, all mice developed MIBC. Tumor cells exhibited squamous features and were positive for the basal cell marker KRT5 and squamous differentiation markers KRT6B and KRT13. These were absent in wild-type mice. Since Pten is only altered in 0-6% of MIBC cases, researchers then examined the effects of PTEN inactivation using patient-derived bladder cancer cell lines representing different grades and stages. In many of these cell lines, PTEN was phosphorylated at the C-terminus, reflecting inactivation. This was confirmed in non-invasive bladder tumors. Mutagenesis of potential phosphorylation sites on PTEN in UMUC3 cells lacking endogenous PTEN abolished phosphorylation and decreased cell survival.
The investigators then generated a tailless PTEN lacking the C-terminus that undergoes phosphorylation. UMUC3 cells transfected with tailless PTEN exhibited significantly reduced cell survival and AKT activation (downstream of PTEN). Tailless PTEN incapable of C-terminus phosphorylation also showed higher binding to p53 than wild-type PTEN. This was also associated with increased expression of p21 and p27, which are targets of p53. These results were then tested in vivo using a xenograft mouse model. Mice injected with UMUC3 cells transfected with full-length PTEN developed tumors similar in size and weight to control mice injected with UMUC3 cells alone. However, mice injected with UMUC3 cells transfected with inducible tailless PTEN had significantly lower tumor weight. These results were confirmed via intravesical injection of Adenovirus-driven tailless PTEN and control full-length PTEN into the bladders of Upk2-HRas*/* transgenic mice that develop early-onset non-muscle invasive bladder tumors.
These findings reveal the potency of tailless PTEN as a tumor suppressor. The local administration of tailless PTEN into patient tumors may lead to tumor growth inhibition. It could potentially be used in combination with other treatments to boost efficacy since it can counteract the effect of oncogenic drivers.
Written by: Bishoy M. Faltas, MD, Director of Bladder Cancer Research, Englander Institute for Precision Medicine, Weill Cornell Medicine
References:
- He F, Zhang F, Liao Y, Tang MS, Wu XR. Structural or functional defects of PTEN in urothelial cells lacking P53 drive basal/squamous-subtype muscle-invasive bladder cancer. Cancer Lett. 2022;550:215924. doi:10.1016/j.canlet.2022.215924