Osteopontin (OPN) is an extracellular matrix (ECM) protein that binds to αvβ integrins and receptors of CD44 family to propagate cellular signals, and promotes induction of cell adhesion, chemotaxis, ECM degradation, angiogenesis, prevention of apoptosis, and indolent tumor growth. Our previous gene expression profiling study found OPN was overexpressed in metastatic HCC and identified OPN as one of the leading genes associated with metastasis of HCC.
A significantly elevated plasma OPN level was found in patients with HCC recurrence during the follow-up time compared with those without recurrence, and was one of the leading independent prognostic factors for HCC patients, even in those with early stage of HCC. Serum OPN levels significantly decrease after resection of HCCs. It could serve as a surrogate serologic biomarker for monitoring treatment response and tumor recurrence after HCC resection, including those AFP-negative ones.
To determine the effects of genetic variants of OPN on the progression and prognosis of HCC, OPN promoter polymorphisms were assessed in HCC tissues of 856 patients who underwent radical liver resection. Single nucleotide polymorphisms (SNPs) were identiﬁed at four loci (-1748, -616, -443, and -155). Only SNPs at -443 and their related haplotypes (Ht2, Ht3) were significantly associated with HCC prognosis. Ht2 and -443TT genotype could significantly increase the promoter transcriptional activity and expression level of OPN compared with Ht3 or -443CC genotype, and lead to an obvious increase both tumor growth and lung metastasis of HCC.
We also used microarray-based comparative genomic hybridization (array-CGH) to identify the difference of intratumor genomic aberrations between OPN-positive and OPN-negative HCC cells, and found that OPN-positive HCC cells have much more amplifications of chromosomal regions including 4q13.1-q13.3, 4q21.23-q22.1, and 13q32.1-q32.3 compared with the OPN-negative ones.
The abundance of clinical and experimental evidence regarding the link between OPN and HCC metastasis makes OPN an attractive potential therapeutic target for combating HCC metastasis. Based on both in vitro and in vivo investigations using antibody or Lentiviral-mediated miRNA against OPN, we found that OPN plays an important role in metastasis as well as HCC tumor growth. Moreover, the necessary level of OPN for tumor growth is much lower than that for HCC metastasis, and a very low level of OPN is sufficient for tumor growth. OPN might regulate tumor growth through activation of the MAPK pathway. Furthermore, inducing the NF-kB translocation and the production/activation of MMP-2 may be an important mechanism by which OPN is involved in HCC metastasis.
The serine protease thrombin interacts with OPN and can modify the biological activity of this molecule. We found that the thrombin level was strongly associated with the metastatic potential of HCC cells, and that thrombin was remarkably overexpressed in HCC tissues, and associated with tumor recurrence after HCC resection, particularly in those with elevated OPN levels. In vitro and in vivo assays demonstrated that thrombin promotes the proliferation and pulmonary metastases of OPN+ HCC cells, which were significantly suppressed by treatment with thrombin inhibitor.
In conclusions: OPN is an attractive potential therapeutic target for combating HCC metastasis, and thrombin may be an alternative target to inhibit HCC metastasis in OPN+ patients.