Hepatocytes prepare the soil for liver metastasis

ABSTRACT

The liver is the most common organ site of cancer metastasis. Molecular determinants of this organotropism, though, are poorly understood. We recently showed that hepatocytes regulate the formation of a “pro-metastatic” niche in the liver via their release of serum amyloid proteins.

Metastatic disease is the main cause of morbidity and mortality in cancer. Disseminated cancer cells may reach any organ in the body, but the liver is particularly susceptible. This is especially evident in pancreatic cancer, a lethal disease that presents with metastasis affecting the liver in nearly 60% of patients at the time of diagnosis. For these patients, the 5-year overall survival rate is only 3%.¹

The liver’s extensive vascular architecture² and genetic mutations in pancreatic cancer cells³ are implicated as major determinants of metastatic tropism to the liver. However, emerging evidence shows that metastatic tropism is also influenced by the formation of a pro-metastatic niche that provides a fertile “soil” for circulating cancer cells to “seed” and grow. This niche environment is underpinned by recruitment of myeloid cells, activation of fibroblasts, and deposition of matrix proteins that together establish an environment conducive to cancer cell colonization. In our studies, we have identified an intercellular network orchestrated by hepatocytes that forms the basis of this pro-metastatic niche and, in doing so, have defined key therapeutic targets.⁴

Using clinically relevant mouse models of cancer and human clinical specimens, we found that hepatocytes display activation of signal transducer and activator of transcription 3 (STAT3) signaling early during primary cancer development. This process is dependent on interleukin 6 (IL-6), a pro-inflammatory cytokine, that is released into the circulation by stromal cells residing within primary tumors. As a result, hepatocytes produce serum amyloid A proteins (SAA1 and SAA2, referred to collectively as SAA), which are acute-phase reactants that engender accumulation of myeloid cells and fibrosis within the liver. These changes in concert establish a pro-metastatic niche (Figure 1). Mice that have a primary tumor, compared to those without a tumor, in the pancreas display a higher number (seeding) and size (growth) of metastatic colonies in the liver. Intriguingly, we found that ectopic expression of IL-6 was sufficient for the formation of a liver pro-metastatic niche, suggesting that hepatocyte activation does not depend on the origin of IL-6. This finding carries a significant implication that any underlying condition that induces IL-6 expression in the body, including obesity, cardiovascular diseases, and inflammatory conditions, may predispose the liver to cancer metastasis. Effective prevention of liver metastasis, therefore, might need to address not only the primary malignancy but also non-malignant conditions that induce stromal and cellular changes in the liver.

Figure 1.

Formation of a pro-metastatic niche in the liver and therapeutic strategies. Early during pancreatic tumor development, non-malignant cells that reside within primary tumors release interleukin 6 (IL-6) into the circulation. IL-6 then activates signal transducer and activator of transcription 3 (STAT3) signaling in hepatocytes, which in turn produce serum amyloid A proteins (SAA) that orchestrate the formation of a pro-metastatic niche in the liver. This process presents multiple points of therapeutic intervention (shown in red). IL-6R, IL-6 receptor.

Hepatocyte-mediated formation of a pro-metastatic niche in the liver reveals multiple points of therapeutic intervention (Figure 1). Given that initiation of a pro-metastatic niche is dependent on IL-6 released by stromal cells within primary tumors, antibodies targeting IL-6 or IL-6 receptor (IL-6R) are a potential strategy to prevent this process. Using a mouse model of pancreatic cancer, we showed that administration of anti-IL-6R antibodies early during pancreatic cancer development inhibited the formation of a pro-metastatic niche in the liver and reduced metastatic colonization. In addition, Janus kinase 1/2 (JAK1/2) or STAT3 inhibitors that can disrupt IL-6–STAT3 signaling may offer an approach to inhibiting the formation of the pro-metastatic niche with high specificity. Our finding that genetic ablation of components of IL-6–STAT3–SAA signaling impaired metastatic colonization in the liver supports this approach. Therapeutic modalities targeting IL-6–STAT3–SAA signaling may be particularly beneficial to patients who are diagnosed at an early stage of the disease when elements of the pro-metastatic niche are still being formed. At present, patients who undergo surgical resection have a high likelihood (nearly 50%) of developing recurrence in the liver. For these patients, blockade of IL-6–STAT3 signaling in a neoadjuvant or adjuvant setting may provide an effective means to inhibiting cancer recurrence in the liver.

Beyond disrupting signaling steps that initiate a pro-metastatic niche in the liver, fibrosis and myeloid cell accumulation that define this niche may also be amenable to therapeutic intervention. Though our current study does not specify cell types that bind hepatocyte-derived SAA, recent studies point to hepatic stellate cells as a target of SAA and an important source of chemokines that may recruit myeloid cells into the liver.⁵ Given that focal adhesion kinase (FAK),⁶ vitamin D,⁷ and transforming growth factor β (TGF-β)⁸ are key activators of stellate cells, therapies targeting these molecules may complement strategies aimed at blocking initiation of a pro-metastatic niche in the liver. CD40 agonists may also offer yet another approach to degrade liver fibrosis. Our laboratory has previously shown that CD40 agonists decrease fibrosis within primary pancreatic tumors via recruitment of inflammatory monocytes that facilitate the release of matrix metalloproteinases.⁹ Demonstration of reversibility of the pro-metastatic niche in the liver would pave a way to effective treatments for metastatic disease.

In addition to investigating strategies for preventing and treating metastatic disease, SAA may serve as an important diagnostic and prognostic marker of liver metastasis. Increasing by >1,000-fold in response to inflammation, SAA is a sensitive marker of liver injury and inflammatory conditions that affect the liver. Our study showed that circulating levels of SAA correlate with cancer progression and worse overall survival in patients with metastatic pancreatic and lung cancer. This finding suggests that SAA may be evaluated in conjunction with other markers of gastrointestinal cancer, including cancer antigen 19-9 (CA19-9) and thrombospondin-2,¹⁰ for detection of liver metastasis and/or for monitoring responses to therapies. SAA may also be used to identify a subset of patients who are more prone to developing metastatic disease and require more rigorous monitoring and treatment regimens. Together, our study defines hepatocytes as key regulators of liver metastasis and identify IL-6–STAT3–SAA signaling as a novel therapeutic target.

Funding Statement

This work is supported by the National Institutes of Health grants F30 CA196106 (J.W.L.), T32 HL007439 (J.W.L), and R01 CA197916 (G.L.B.); the Pancreatic Cancer Action Network-AACR Career Development Award 15-20-25-BEAT supported by an anonymous foundation (G.L.B.); and the Stand Up to Cancer (SU2C) Innovative Research Grant SU2C-AACR-IRG 13-17 (G.L.B).

Disclosure of Potential Conflicts of Interest

G.L.B. is a consultant/advisory board member for Seattle Genetics, Aduro Biotech, AstraZeneca, Bristol-Myers Squibb, Genmab, Merck, and BiolineRx; reports receive commercial research grants from Incyte, Bristol-Myers Squibb, Verastem, Halozyme, Biothera, Newlink, Novartis, and Janssen. G.L.B. is an inventor of intellectual property and recipient of royalties related to CAR T cells that are licensed by the University of Pennsylvania to Novartis. No additional potential conflicts of interest were disclosed by J.W.L.