Tumor Microenvironment and Metastasis
Ningling Kang, PhD
Through the support of NIH grant, Hepatic Stellate Cell Regulation of Metastatic Growth in the Liver (R01CA160069), we explored a novel role of p300 acetyltransferase in hepatic stellate cell (HSC) activation in the last year. Liver is a common site for metastatic cancer cells, such as gastrointestinal cancer cells, to colonize and grow. In response to cancer invasion of the liver, cancer cells and other components of the liver microenvironment produce growth factors and cytokines to induce transdifferentiation of quiescent HSCs into myofibroblasts (MFs). In turn, activated-HSC/MFs promote liver metastasis by releasing growth factors, cytokines, extracellular matrix (ECM) proteins, and matrix metalloproteinases. Because the activated-HSC/MFs generate and deposit excessive ECM, they are considered a major contributor to desmoplasia and stiffness of liver metastases.
Although the role of stiffness on the biology of cancer cells has received intensive investigations, it is unknown if stiffness can in return regulate the biology of the activated-HSC/MFs. To address this question, we employed polyacrylamide gels with incremental stiffness as cell culture substrates for HSCs to investigate whether and how stiffness influences myofibroblastic activation of HSCs. Our data indeed show that a stiff substrate induces HSCs to express alpha-smooth muscle actin (α-SMA) and CTGF, markers of HSC activation, and concurrently nuclear accumulation of p300 acetyltransferase in HSCs. Perturbation of p300 by cre-loxP-mediated gene deletion, shRNA-based knockdown, or pharmacological inhibition abrogates stiffness-induced HSC activation. Mechanistically, stiffness activates a RhoA-Akt mechanosignaling to promote p300 phosphorylation at serine 1834 and subsequently targets it to the nucleus to epigenetically regulate gene transcription. RNA sequencing reveals that stiffness promotes HSCs to transcribe a panel of tumor-promoting paracrine factors, including CXCL12, IL11, IL6, VEGFA, PDGFA and B. In agreement with these data, conditioned medium of HSCs on a stiff substrate promotes colorectal tumor growth in vitro and in a subcutaneous tumor implantation in vivo model as compared to that of HSCs on a soft substrate. Additionally, portal vein injection of colorectal cancer cells into p300 conditional knockout in vivo models (p300F/F|collagen1A1-cre) leads to reduced liver metastatic growth as compared to control in vivo models.
Our study uncovers “an amplification loop” for liver metastatic growth whereby stromal stiffness, built by activated-HSC/MFs, in return acts on the activated-HSC/MFs to further enhance their activation and tumorpromoting effects via a HSC p300-dependent mechanism. The RhoA-Akt-p300 mechanosignaling of HSCs thus represents as a novel target for anti-liver metastasis therapy. A manuscript about this study has been submitted and it is currently under peer-review.