Colorectal cancer (CRC) is a major cause of cancer mortality worldwide1. More than half of the CRC patients develop hepatic metastasis that occurs through the portal vein dissemination1. Currently, there are no effective therapeutics that can cure advanced liver metastasis, and most patients succumb to metastatic disease.
The metastatic niche or tumor microenvironment plays a key role in engraftment and growth of disseminated CRC cells2. Cancer-associated fibroblasts (CAFs), a prominent component of the tumor microenvironment, promote or restrain cancer progression through secreting growth factors, remodeling the extracellular matrix (ECM), and modulating immune landscapes and angiogenesis3,4,5. CAFs also confer resistance to chemotherapies and immunotherapies3. Moreover, CAFs regulate initiation and progression of CRC liver metastasis and predict prognosis in patients with CRC3,6,7,8. Thus, CAF-related factors could be exploited for the development of therapeutic strategies to inhibit CRC liver metastasis. However, the lack of satisfactory mouse models to study the metastatic tumor stroma has been a major obstacle to developing stroma-targeted therapies.
Currently, animal models to study CRC liver metastasis include primary CRC models that spontaneously develop hepatic metastasis and cancer cell transplantation models into the liver. Primary CRC mouse models, such as genetically engineered mouse models and colonic injection of cancer cells, rarely show metastasis to the liver9,10,11,12. Moreover, even if a liver metastasis is observed, these models show long latency from the primary tumor induction to metastasis, and potentially die of primary tumor burden12. To efficiently generate CRC liver metastases, cultured CRC cells are transplanted into the liver using three injection approaches: intra-splenic injection, direct intra-parenchymal injection into the liver, and portal vein injection. Intra-splenically injected cancer cells spread into the splenic vein, the portal vein, and ultimately to the liver13,14. However, the intra-splenic injection yields a lower tumor take ratio compared with other transplantation models15,16. With intra-splenic injection, surgical removal of the spleen is performed to avoid cancer growth in the spleen, which can potentially compromise immune cell maturation17. Furthermore, intra-splenic injection can also result in unintended tumor growth in the spleen and abdominal cavity18, complicating liver metastasis analyses. Direct intra-parenchymal injection into the liver efficiently induces hepatic metastasis16,19,20. Nonetheless, this approach does not fully recapitulate a biological step of liver metastasis that naturally occurs through portal vein dissemination. Using direct injection into the liver, entry of cancer cells into a non-portal, but systemic circulation can also result in multiple large lung metastases16. Although a majority of patients with CRC liver metastasis show multiple tumor nodules in the liver21, direct injection into a specific liver lobe generates a single tumor mass19,20. Portal vein injection or mesenteric vein injection, though technically challenging, allows efficient delivery of tumor cells into the liver in a manner that recapitulates the growth patterns seen in patients17. This strategy can minimize the possibility of secondary-site metastases and enables rapid growth of cancer cells in the liver, simplifying mouse survival analyses.
Historically, colorectal cancer cell lines such as mouse MC-38, human HT-29, and SW-620 were used to generate mouse models of hepatic metastasis22,23. However, these colorectal cancer cell lines do not induce a desmoplastic stromal reaction. Low stromal content in the tumors makes it difficult to investigate the biological roles of cancer-associated fibroblasts. Recent advances in CRC organoids and their transplantation have offered useful platforms to assess vital roles of the stroma in cancer progression24. Liver transplantation of CRC organoids generates a fibroblast-rich tumor microenvironment and has provided novel insights into stromal research6,25. Currently, portal or mesenteric vein injection of organoids has become a gold standard approach to generate CRC liver metastasis6,25,26,27,28. Nonetheless, to our knowledge, no previous papers have described detailed methods for the portal vein injection of colorectal tumoroids. Here, we present a methodology for using portal vein injection of CRC organoids to develop novel adeno-associated virus (AAV)-mediated stroma-directed therapy.
Hepatocytes are an important constituent of the metastatic tumor microenvironment in the liver and play a critical role in metastatic cancer progression29. Inspired by the success of AAV gene therapy approaches to induce protein expression in hepatocytes in non-neoplastic patients30,31, we investigated a similar approach but aimed at modifying the liver tumor microenvironment in CRC25. As such, we also describe herein the tail vein injection of AAV8 to induce expression of anti-tumorigenic proteins to modify the liver tumor microenvironment. The AAV8 serotype, designated by the choice of viral capsid protein during virus production, leads to high transduction efficiency specifically of hepatocytes (i.e., targeted gene expression in the liver tumor microenvironment)32. We have previously shown that Islr (immunoglobulin superfamily containing leucine-rich repeat) is a CAF-specific gene that induces bone morphogenetic protein (BMP) signaling, reduces CRC tumoroid growth, and promotes Lgr5+ intestinal stem cell differentiation25. We tested whether AAV8-mediated overexpression of the cancer-restraining stromal gene, Islr, in hepatocytes could attenuate hepatic metastasis progression by performing portal vein injection of CRC tumoroids in AAV8-Islr-treated mice.
In this paper, we first describe the tail vein injection procedure of liver tropic AAV. Then, we describe a method for tumoroid cell preparation and portal vein injection into the AAV-treated mice. Finally, we present approaches to monitor metastatic tumor progression to assess the efficacy of stroma-directed therapeutics.