Interleukin-6 (IL-6) is a pleiotropic inflammatory cytokine that regulates growth and differentiation of various types of malignant tumors, including head and neck squamous cell carcinoma (HNSCC). Compared with healthy controls, the levels of IL-6 are elevated in sera of patients with HNSCC. In this study, the role of IL-6 in the progression of HNSCC was evaluated.
The effects of IL-6 on cell growth were evaluated by expressing IL-6 cDNA into the human tongue cancer cell line (SAS), resulting in stable high-producer clones (IL-6OE) and low-producer clones (IL-6LE). In vitro, cell viability and clongenic assay were used for testing the growth difference among the clones. In vivo, the effects of tumor-derived IL-6 on growth were studied by subcutaneous inoculation of the clones on the flank of each severe combined immune-deficient (SCID) mice. Clinically, specimen from 111 surgically treated HNSCC patients were studied. IL-6 and p-STAT3 levels were measured by immunohistochemical staining and correlated with clinicopathological variables and patient survival.
Treatment of recombinant IL-6 and overexpression of IL-6 by intorducing IL-6 into SAS cells significantly increased clonogenic ability and cell proliferation in vitro compared with the parental SAS cells and the vector controls. In the animal model, the average tumor weight of IL-6OE was significantly higher than the IL-6LE groups (IL-6LE: 0.504g, IL-6OE: 0.859g, P=0.045) . Clinical data revealed that high level of IL-6 correlated with later stage, higher bone invasion activity, higher recurrence rate and poor survival rate. Meanwhile, the expression of IL-6 and p-STAT3 were positively correlated (R=0.667, P<0.01). The high level of p-STAT3 correlated with higher recurrence rate, and poor survival rate, too.
These findings strongly support the hypothesis that IL-6 promotes oral cancer cell growth and is associated with poor prognosis in patients with HNSCC.