Abstract 3810: Muscle invasion produced drug-resistant and bone metastatic prostate cancer
Authors: Marr Kendra D. , Knudsen Beatrice S. , Gard Jaime M. , Bird Malia , Nagle Raymond B. , Cress Anne E. 
Published: June 2022

Abstract
Overtreatment of prostate cancer is a significant source of patient morbidity and cost. The human prostate is bounded by a smooth muscle capsule, and aggressive tumors invade through the muscle layer, called extracapsular extension (ECE), to escape organ confinement. The presence of ECE defines pT3a pathologic stage and is associated with increased risk of biochemical recurrence, metastases, and cancer-specific mortality. Although muscle invasion is required for ECE and metastatic spread, both the invasive tumor network phenotypes and muscle responses are understudied. The goal was to probe the molecular events of ECE to understand this critical step in metastasis.
Muscle invasion of prostate cancer cell lines was tested in vivo by injecting cells into the peritoneal cavity of male NSG mice. The cells colonize the inferior surface of the muscular respiratory diaphragm and invade through to the superior surface. To explore transcriptional regulators, we performed whole genome RNAseq on cells from three compartments: (1) “Inferior” non-invading cells on the underside of the diaphragm; (2) “Muscle-resident” cells that have invaded and now reside within the diaphragm muscle; and (3) “Superior” cells that have completely traversed the diaphragm. Tumors cells reaching the superior side of the diaphragm were established ex vivo as polyclonal cell lines termed the “KM” series.
RNAseq reveals 1,482 differentially expressed sequences (DES) between Inferior and Muscle-resident cells, 253 DES between Muscle-resident and Superior cells, and 896 DES between Inferior and Superior cells (padj &lt;0.05, |fold change| ≥ 1.3). Further, we identified 84 DES that overlap between the Superior vs. Muscle-resident and Inferior vs. Muscle-resident groups, highlighting candidate gene changes unique to cancer cells in a muscle microenvironment. Importantly, 4,076 differentially expressed, mouse-specific RNA sequences (padj &lt; 0.05, |fold change| ≥ 2.0) highlight pathways within muscle that respond to the presence of invasive tumors. Strikingly, the KM series have aggressive features including newly acquired bone metastasis and resistance to a taxane.
The gene expression patterns imply a dominant effect of the muscle microenvironment in evoking a new and transient tumor transcriptional response as revealed by the DESs. Independent of this response, the KM sub-populations successfully navigating the muscle were more aggressive in laboratory assays, including bone metastatic potential and chemotherapeutic resistance. These studies support the hypothesis that successful invasion into and through a contractile muscle layer results in aggressive cancer cells. Using the in vivo assay provides the unique ability to parse out unexplored patterns of changes that occur in early, muscle-invasive disease to improve prostate cancer patient selection for definitive treatment versus active surveillance.
Citation Format: Kendra D. Marr, Beatrice S. Knudsen, Jaime M. Gard, Malia Bird, Raymond B. Nagle, Anne E. Cress. Muscle invasion produced drug-resistant and bone metastatic prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3810.