Delta-like 1 Contributes to Cell Growth by Increasing the Interferon-inducible Protein 16 Expression in Hepatocellular Carcinoma

Background: Delta-like 1 (DLK1), a fetal liver stem cell marker, is strongly expressed in human and rodent fetal liver, but not in adult liver. Notably, dysregulation of DLK1 was found in some human hepatocellular carcinomas (HCC). However, the effect of DLK1 on HCC cell growth and its underlying mechanism are still largely unknown.

Aims: To (i) assess the expression of DLK1 in human HCC and adjacent liver tissues and human HCC cell lines; (ii) evaluate the effect of DLK1 on SMMC-7721, Huh7 HCC cell growth in vitro and in vivo; and (iii) explore the potential mechanism of DLK1 that regulates HCC cell growth.

Methods: The expression of DLK1 mRNA and protein were detected using reverse transcriptase-polymerase chain reaction and immunohistochemistry respectively. The effect of DLK1 on the proliferation of SMMC-7721 and Huh7 cells was evaluated by colony formation and tumour xenograft assay. The differential expression profiles of DLK1-overexpressing SMMC-7721 cells and control cells were compared using HG-U133 Plus 2 Genechip. The cell cycle distribution of DLK1 forced expressing cells was comparatively analysed.

Results: Upregulation of DLK1 was observed in 41 of 57 (71.9%) human HCC samples. Ectopic expression of DLK1 promoted cell proliferation, colony formation and tumorigenicity in SMMC-7721 and Huh7 cells. DLK1 upregulated the expression of interferon-inducible protein 16 (IFI16) and its promoter transcriptional activity, decreased p21waf1/cip1 and induced cell cycle acceleration. However, silencing of IFI16 using small interfering RNA abrogated DLK1-induced proliferation in these cells.

Conclusions: IFI16 may be an essential downstream target of DLK1 in HCC cells and required for DLK1-induced cell proliferation.