Determinants of Muscle Invasion by Prostate Cancer

Abstract

Aggressive prostate tumors invade through a smooth muscle pseudocapsule in a process called extracapsular extension (ECE), escaping organ confinement. The presence of ECE defines the pT3a category in the pathologic staging of prostate adenocarcinoma and is associated with an increased risk of biochemical recurrence, distant metastases, and cancer-specific mortality. Although muscle invasion is required for ECE and distant metastasis, the cellular phenotypes that dictate this behavior are not understood. In the following work, we investigate this early cancer invasion event using a unique in vivo model. Muscle invasion was tested by the ability of prostate cancer cells to colonize the inferior surface of the respiratory diaphragm of male NGS mice, invade into and through to the superior surface. We isolated and compared the biological, cellular, and molecular characteristics of “Inferior” non-invasive tumor, “Muscle-resident” cells that have invaded and now reside within the diaphragm muscle and “Superior” cells that have completely traversed the diaphragm muscle. The results show that the Superior cells have gained the ability to reach a bone metastatic site and have a differential gene expression signature as compared to inferior cells of 264 genes that was positively associated with metastatic prostate cancer in humans (p = 0.003, Rank sum test). The Superior tumor cells had an IC50 of 7.30nM to docetaxel as compared to the Inferior cells with an IC50 of 3.8nm. Taken together, these data indicate that the traverse of the smooth muscle barrier resulted in bone trophic and drug resistant phenotypes in the successful tumor cells and during residency in the muscle and specifically remodeled the muscle transcriptome. In studying our muscle-invasion model it has become clear that the laminin-binding integrins α3β1 and α6pβ1 are critical regulator of this process, and a functional epithelial-mesenchymal cooperation (EMC) is likely to facilitate tumor invasion through collective migration. Of further interest is that the Muscle-resident tumor cells differentially expressed 84 genes compared to both the Inferior and Superior cancer cells, highlighting the unique environment of the active mouse muscle. A bioinformatic analysis of mouse muscle specific genes showed a remarkable reciprocity with expression of 2,213 genes in response to the Muscle-resident tumor cells, correlating with GO terms related to ECM deposition and inflammatory responses. Moving toward clinical samples, the relatively new tool of patient-derived organoids provides a method for investigating human cancer cells in culture in three dimensions and reveals differential phenotypic behavior on extracellular components between organoids derived from prostate tumors after radical prostatectomy versus matched normal tissue from the same patient. It is further revealed using clinical samples from image-guided prostate biopsies that tissue-based aggressive markers of prostate cancer may impact the performance of non-invasive imaging methods to detect clinically significant prostate cancer.