Abstract B38: Identification of highly metastatic disseminating tumor cells in late-stage liver cancer lung metastasis

As the 6th most frequent cancer worldwide, liver cancer is the 2nd leading cause of cancer death globally due to its often late detection and high rate of metastasis. Although relatively uncommon in the United States, a recent study led by AACR has projected it will surpass breast, prostate, and colorectal cancers to become the third leading cause of cancer death in US by 2030. Therefore, there is an urgent need to understand the biology driving liver cancer metastasis, particularly the late-stage metastasis, in order to develop more efficacious therapies to improve the poor prognosis. We have previously shown that inactivation of the conditional Pten and p53 alleles in a Prom1-expressing progenitor population induced the formation of liver cancer with almost 100% penetrance. 3-5% of these liver tumors metastasized to the lung, the predominant metastatic site of liver cancer. By enriching the tumorigenic cancer stem cells from the primary tumors via in vitro liver stem cell culture, in combination with a novel ultrasound-guided intrahepatic implantation method we developed, we established a series of highly metastatic orthotopic liver cancer allograft mouse models with >60% lung metastasis rate. These mice also developed extensive intrahepatic metastases as well as invasions and metastases into the abdominal organs, predominantly in the diaphragm, peritoneum, lymph nodes in the omentum and mesentery. In more severe cases, metastases are also found in the stomach, intestine, kidney, ovary and uterus. Cross-species comparison has demonstrated the high resemblance of our model to human liver cancer at both histological and genomic levels. All tumor cells used in this model carry a strong GFP reporter that allows their easy identification during metastasis. We primarily focused on the lung metastasis for its clinical significance. Very interestingly, in addition to identifying GFP+ tumor cells in the bulk of the metastases (metastasis tumor cells, or MTCs), we consistently discovered large numbers of GFP+ disseminating tumor cells (DTCs) in the surrounding normal-looking lung tissues. Our further analyses have indicated that MTCs and DTCs are two biologically distinct tumor cell populations. There is a complete lack of intercellular organization of DTCs compared to MTCs and they proliferated at a much lower rate than MTCs. Most importantly, when MTCs and DTCs were isolated from the same lung and re-implanted in the liver of a new host, we consistently found that MTCs retained very limited ability to re-metastasize to the lung upon re-implantation even though they were isolated from lung metastases. Instead, MTCs formed large tumors at the implantation site and developed extensive intrahepatic metastases. High-grade invasions into surrounding abdominal organs are also evident. On the contrary, DTCs grew significantly slower and less aggressively when implanted back into the liver. However, they exhibited much higher metastatic ability than MTCs by rapidly forming large multifocal lung metastases and killed animals within a shorter time of period. In fact, those secondary lung metastases formed by DTCs are the largest among all the lung metastases identified in the entire cohort of this liver cancer allograft model (n>50). This striking capacity of DTCs to form metastases that is superior to that of MTCs has strongly suggested the necessity to target both cell populations in late-stage liver cancer metastasis in order to effectively prevent the growth and further spreading of metastases. We are currently conducting systematic molecular dissection of DTCs and MTCs by RNA-seq gene expression profiling and proteomics to map their molecular features and to identify potential druggable molecular targets specifically tailored to each population. Citation Format: Liyuan Li, Melissa Johnson, Christopher Calabrese, David Finkelstein, Armita Bahrami, Dolores Lopez-Terrada, Liqin Zhu. Identification of highly metastatic disseminating tumor cells in late-stage liver cancer lung metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr B38.