Abstract C23: Blocking endogenous TBX2 expression abrogates prostate cancer bone metastasis in a xenograft mouse model
Authors: Nandana Srinivas , Tripathi Manisha , Gururajan Murali , Chu Gina , Matusik Robert J. , Chung Leland W.K. 
Published: August 2014

Identifying factors that mediate the metastatic spread and subsequent remodeling of the bone is highly relevant to successful therapeutic intervention in advanced human Prostate Cancer (PCa). TBX2, a transcription factor that belongs to the T-box family is known to play critical roles in the embryonic development of the organisms. TBX2 acts as a transcriptional repressor by negatively regulating p21 in humans and p19 ARF in mice. Further, TBX2 has been reported to interact with Retinoblastoma (Rb) and mediates its functional specificity. Previously, TBX2 was found to be over-expressed in BRCA1 and BRCA2 associated breast tumors and melanoma and pancreatic cancer cell lines. Interestingly, a recent report found that TBX2 expression is increased 10-fold at the mRNA level in an androgen-independent bone lesion from a human PCa patient when compared to a bone lesion from an androgen-responsive PCa patient.

TBX2 was found to be over-expressed in more aggressive human PCa cell lines and that TBX2 expression correlates with BMP2 expression in these cell lines. Addition of BMP2 to androgen-responsive LAPC4 human prostate cancer cells robustly induces TBX2 expression. Androgen-independent PC3 cells, expressing wild-type TBX2, when infected with the dominant negative TBX2 retroviral construct (PC3-TBX2 DN), display decreased PTHrP, RANKL and SMAD 1,5,8. These results suggest that TBX2, a downstream BMP2 target, csignificantly controls cell signaling network in prostate cancer cells. In vivo, PC3-TBX2 DN cells tagged with firefly luciferase when injected intra-cardiacally in athymic mice lost their ability to metastasize to bone and soft tissues in mice when compared with the control vector infected cells. These results are supported by reduced invasive ability of the PC3-TBX2 DN tumors at the interface between tumor and kidney when grafted under the sub-renal capsule of the male athymic nude mice. Further, utilizing the intra-tibial mouse model, we have found that blocking endogenous TBX2 in PC3 cells significantly reduces their ability to grow in the bone microenvironment. Interestingly, blocking TBX2 does not affect tumor size suggesting that TBX2 is a metastasis suppressor gene. Further, the non-metastatic TrampC2 mouse prostate cancer cells stably over-expressing RANKL show increased Tbx2 expression and develop soft tissue metastases when injected intra-cardiacally in C56/Bl6 immune competent mice. These results suggest that Tbx2 and RANKL pathways cross-talk with each other in prostate cancer metastasis.

In summary, we have found that blocking endogenous TBX2 levels in human PCa cells leads to decreased invasion, metastasis and growth in the bone microenvironment, and that TBX2 potentially mediates its effect through BMP2 and RANKL pathways. TBX2, therefore, is a tumor metastasis suppressor and therapeutic target for PCa metastasis.

Citation Format: Srinivas Nandana, Manisha Tripathi, Murali Gururajan, Gina Chu, Robert J. Matusik, Leland W.K. Chung. Blocking endogenous TBX2 expression abrogates prostate cancer bone metastasis in a xenograft mouse model. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr C23.