BMP-9, a hepatic cytokine belonging to the TGF-β superfamily, has gathered attention
in recent studies due to its controversial role in cancer, including hepatocellular
carcinoma (HCC). Some investigations indicate BMP-9's promotion of cell proliferation
and epithelial-to-mesenchymal transition (EMT), while others suggest anti-proliferative
effects and a reduction in mesenchymal markers in HCC cells. The aim of this study
was therefore to better understand BMP-9/ALK1 signaling in HCC.
Starting with in silico data (TCGA) we conducted an extensive analysis of expression
levels for BMP-9 signaling pathway components in HCC tumour samples. ALK1 was very
heterogeneously expressed. We divided the HCC (LIHC) cohort into ALK1 high, intermediate
and low expressors and found that in ALK1 high tissues, endoglin (ENG, a BMP-9 co-receptor)
is also significantly enhanced. By further in silico analyses we found that high ALK1
in combination with high ENG is significantly associated with better survival times
of the patients. GSEA analyses of the ALK1 high versus low tumour samples further
revealed that low ALK1 expression is correlated with an enrichment of several hallmark
gene sets for enhanced cell proliferation whereas high ALK1 in HCC is related to inflammation.
To further characterize the hepatic BMP-9 response we used diverse HCC cell lines
and confirmed in vitro that in HLE cells BMP-9 promoted EMT and proliferation whereas
in Hep3B cells, as well as normal hepatocytes (HC), BMP-9 antagonized proliferation.
In Hep3B cells BMP-9 also inhibited the expression of several cancer stem cell markers.
Furthermore, we previously found that BMP-9 can enhance pro-inflammatory LPS-responses
in macrophages (MP) and now found that in wild-type bone marrow derived macrophages
(BMDM) BMP-9 induced IL-6 expression in vitro but in BMDM derived from mice with MP-specific
ALK1 deletion (LysM-Cre/ALK1-flox) this induction was lost. Implying that BMP-9/ALK1
responses in MP in addition to those in endothelial cells and/or HC seem to act in
concert and might lead to the pro-inflammatory actions of ALK1 signalling in HCC patients
in vivo.
We conclude that BMP-9 acts on the diverse liver cell types in parallel and that in
patients with ALK1-high HCC tissues BMP-9 possibly antagonizes tumour progression.
