Because recurrence in head and neck squamous cell carcinoma (HNSCC) is strongly associated with poor survival, the ability to identify patients at high risk for relapse may provide significant clinical benefit. Thus, development of noninvasive biomarker assays to predict recurrence in patients with HNSCC could lead to heightened tumor surveillance, and the potential for early detection and treatment, resulting in an improvement in long-term survival. Detection of immune biomarkers in serum represents 1 promising approach to this goal. In this issue, Duffy and colleagues1 enrolled 444 newly diagnosed HNSCC patients to participate in the largest prospective cohort study to investigate levels of pretreatment serum interleukin (IL)-6, an important proinflammatory cytokine, as a predictor of recurrence and survival. The patients were followed over a 4-year period. The results of their study indicate that lowest pretreatment serum IL-6 levels had fewer recurrences and better survival compared with those in the highest quartile. These data strongly support IL-6 as a valuable biomarker for patients with HNSCC.
Cytokines are soluble proteins that modulate immune and inflammatory responses, representing potential candidates as biomarkers of HNSCC. IL-6 is a pleiotropic cytokine that has widespread effects on hematopoietic lineages, such as induction of immunoglobulin production in B cells and T-cell proliferation and differentiation into cytotoxic T cells.2 IL-6 also functions to induce the expression of several acute-phase proteins.2 In the context of HNSCC, IL-6 has been shown to activate the critical oncogene, signal transducer and activator of transcription (STAT)-3, by autocrine stimulation.3 IL-6–mediated STAT activation begins with receptor dimerization. Janus kinase (JAK) proteins aggregate and undergo reciprocal activation by transphosphorylation. These activated JAKs phosphorylate receptor tyrosine residues, which serve as docking sites for STAT3 Src homology 2 domains. Upon STAT binding to the receptor, JAKs phosphorylate a critical tyrosine residue on STAT-3 (tyrosine 705). Activated STAT-3 proteins homodimerize (or heterodimerize with activated STAT-1) then translocate to the nucleus to mediate gene transcription.
The role of STAT-3 in HNSCC tumorigenesis is multifaceted and complex. Constitutive STAT-3 activation contributes to both cellular transformation and down-modulation of the host immune surveillance program. STAT-3 can inhibit apoptosis in HNSCC through up-regulation of the antiapoptotic proteins such as Bcl-xL.4 STAT-3 also induces tumor angiogenesis directly by activating the vascular endothelial growth factor (VEGF) gene and indirectly through activating hypoxia-inducible factor-1α.5-8 Constitutive STAT-3 signaling has also been implicated in abrogating the secretion of proinflammatory cytokines and chemokines from tumor cells. B16 tumor cells transfected with a dominant negative STAT-3 inhibitor resulted in up-regulation of tumor necrosis factor-α, IL-6, interferon-β, and the chemokines, CCL5 and CXCL10.9 In addition, supernatants derived from tumor cells treated with STAT-3 inhibitors stimulate macrophages and neutrophils.9 Furthermore, STAT-3 overexpression in tumor cells causes secretion of IL-10 and VEGF that are immunosuppressive, proangiogenic, and inhibit dendritic cell maturation.9-11 When STAT-3 is ablated in hematopoietic cells, dendritic cell (DC) maturation and priming of naive antigen-specific T cells is enhanced, natural killer (NK) and neutrophil cytotoxicity is increased, stronger T-cell responses against tumor antigens are observed, and there is a decrease in regulatory T cells compared with their STAT-3–expressing counterparts.12, 13 In this way, STAT-3 signaling in both tumor and immune cells decreases both innate and adaptive antitumor immune responses, thereby contributing to HNSCC immune escape as well as its pro-oncogenic effects.
Whereas IL-6 has various roles, as described above, cytokines and chemokines generally act in concert to mediate immune and inflammatory functions. HNSCC cells are likely to play a crucial role as sources and targets of these cells in the process of carcinogenesis and tumor progression. Freshly isolated HNSCC and established cell lines have been found to secrete a variety of cytokines that have been implicated in tumorigenesis, angiogenesis, and metastasis such as IL-1α, IL-1β, IL-4, IL-6, IL-8, granulocyte-macrophage colony–stimulating factor, and VEGF.14–16 These findings have prompted further studies to establish whether cytokines measured in sera of patients with HNSCC could be useful to predict occurrence, recurrence, and survival. Chen and colleagues demonstrated that IL-6, IL-8, and VEGF serum concentrations in patients with HNSCC were significantly increased compared with age- and sex-matched control subjects.14 Several other studies consistently found elevated IL-6 serum concentration in patients with HNSCC when they were compared with healthy control subjects.17–19 Our group performed a case-control study whereby multiplexed serum cytokine profiles were compared between 112 patients with active HNSCC and 117 chronic tobacco smokers. Using individuals at high risk for HNSCC as control subjects provided a more realistic application of a biomarker screening test, and a group cured of their HNSCC for 3 years was included to determine profiles of recurrence after successful therapy. As found in other studies, our multiplex measurements showed elevated serum IL-6 levels in sera of patients with active HNSCC compared with controls.20 Despite ample evidence suggesting that a serum IL-6 level may serve as a useful biomarker for HNSCC and predict recurrence, a large prospective study has not been performed, until recently, as described by Duffy et al1 in this issue of Cancer.
In summary, findings by Duffy et al1 help to validate inflammatory dysregulation as a contributor of HNSCC development and provide further support for biomarkers to enable clinicians to identify patients at high risk for recurrence of disease, or possibly even second primary tumors. With increased surveillance of those patients, early detection and treatment may increase survival. Whether a single biomarker will ever be used to predict disease status in a cancer as heterogeneous as HNSCC is arguable, but certainly validation of such serum cytokines as IL-6 in the context of a noninvasive test may eventually find its way into clinical practice. Combination of IL-6 with other promising biomarkers, and investigations into the pathophysiologic role of inflammatory cytokines in HNSCC, should continue in earnest.