Upcoming expression of ALK1 in HCC determines the switch from tumour-suppressive to tumour-promoting BMP-9 signalling

Question Bone morphogenetic protein (BMP)-9 is a hepatic cytokine that belongs to the TGF-β superfamily. Recent studies have shown that the role of BMP-9 in hepatocellular carcinoma (HCC) is rather controversial. BMP-9 was described to promote cell proliferation and epithelial-to-mesenchymal transition (EMT) in some studies, whereas in others it acted anti-proliferative and decreased mesenchymal markers in HCC cells. HCC is still the third leading cause of cancer death worldwide and new therapeutic targets to improve therapeutic strategies are urgently needed. Aim of this study was therefore to better understand BMP-9 signalling in HCC.

Methods Public databases such as The Cancer Genome Atlas (TCGA) and The Cancer Proteome Atlas (TCPA) were used to extensively analyse available expression data for BMP-9 signalling pathway components. Expression of ALK1, ALK2, Activin A Receptor Type 2A (ACVR2A), Activin A Receptor Type 2B (ACVR2B), BMPRII (BMPR2), endoglin (ENG), Smad1 (SMAD1) and others were assessed. Smad-1-phosphorylation, cell -proliferation and -migration upon BMP-9 stimulation were determined in two human HCC cell lines (HLE and Hep3B) by Western blot, real-time PCR, proliferation- and wound closure assays. Freshly collected samples from 6 HCC patients were additionally analysed for validation of the in silico findings.

Results In silico results show that Alk1 is upregulated in HCC patient samples and that presence of Alk1 is associated with attributes of cancer progression. In line with this, BMP-9 induced proliferation, migration and EMT in HLE cells, which display high levels of Alk1, but not in epithelial Hep3B cells with low Alk1 expression. Over-expression of Alk1 in Hep3B could partially induce tumorigenice effects. Finally, we show that Alk1 expression leads to decreased Smad-1 phosphorylation and an enhanced tumour-promoting expression pattern, whereas non-Alk1 BMP-9 signalling (e.g. via Alk2) uses the canonical Smad-1-pathway for tumour-suppressive signalling.

Conclusions These results suggest that in tumour cells presence of Alk1 promotes tumour progression whereas non-malignant hepatocytes (and some HCC cell lines) do not express Alk1 and respond to BMP-9 via the Smad-1 pathway, thereby stabilizing the differentiated (non-proliferative) parenchymal phenotype and acting tumour-suppressive. Thus, individualized determination of Alk1 expression in HCC patients should help to predict potential responsiveness to BMP-9 targeted therapies.

Publication History

Article published online:
04 January 2021