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Circulating adiponectin levels are inversely associated with risk of various obesity-related cancers. However, the effect of adiponectin on carcinogenesis and progression of tongue squamous cell carcinoma (TSCC) remains unknown. We measured serum adiponectin levels in 59 patients with TSCC and 50 healthy controls. Expression of adiponectin and its receptors in paired tumor and paracancerous specimens were determined by immunohistochemical staining (n = 37) and western blot (n = 30), respectively. Serum adiponectin level was lower in patients than in controls (5.0 ± 2.4 vs 8.4 ± 3.5 μg/mL, P < 0.01), and was inversely associated with histological grade and lymph node metastasis but not tumor size. Local adiponectin levels in tumor tissue gradually decreased as tumor-node-metastasis stage increased, while the expression of adiponectin receptors was unchanged. In addition, serum adiponectin levels in the TSCC patients without metabolic and cardiovascular diseases, or without smoking and drinking habits, were still lower than in controls. Furthermore, adiponectin inhibited the migration, but not proliferation, of SCC15 cells in vitro. These results indicate that a decreased adiponectin level is associated with risk of TSCC. Hypoadiponectinemia might be used as a biomarker to predict an aggressive phenotype of TSCC.
Tongue squamous cell carcinoma (TSCC) is one of the most common cancers in the oral cavity and is characterized by rapid growth, diffuse invasion, high propensity for cervical nodal metastasis and high recurrence. Lymph node metastasis affects the probability of regional control and is the strongest prognostic factor for survival with TSCC. Despite refinement of surgical techniques and chemotherapy or radiotherapy, 5-year survival is still unsatisfactory. Considerable epidemiological evidence indicates that smoking, alcohol intake, chronic mechanical stimulation and betel quid chewing are associated with the incidence of TSCC,[4, 5] but the molecular mechanisms responsible for TSCC remain unclear. Furthermore, TSCC has become increasingly prevalent among young and middle-aged populations. Therefore, it is necessary to develop new strategies to improve the diagnosis and therapy for TSCC.
Adiponectin is an adipokine produced predominantly by adipocytes that circulates abundantly in plasma. Adiponectin acts through two different membrane-bound adiponectin receptors, AdipoR1 and AdipoR2, and functions as an anti-diabetic, anti-atherogenic, anti-inflammatory and anti-angiogenic hormone.[7, 8] Circulating adiponectin levels are lower in patients with obesity, type 2 diabetes and coronary artery disease.[8, 9] Hypoadiponectinemia may be a biomarker for metabolic and cardiovascular diseases. Recent epidemiological studies indicate that hypoadiponectinemia is associated with risk of various cancers, including breast, endometrial, and colorectal cancers.[10-12] In addition, adiponectin has anti-proliferative and pro-apoptotic effects on breast cancer cells. These results suggest that adiponectin plays a direct and/or indirect role in carcinogenesis and progression of obesity-related cancers. However, serum adiponectin level is unchanged in lung cancer and even increased in pancreatic and hepatocellular carcinoma.[14-16] Therefore, the exact roles of adiponectin in cancer development require further investigation.
Despite numerous reports indicating the important roles of adiponectin in various obesity-related cancers, a potential association between adiponectin and nonobesity-related cancers, such as TSCC, has not been explored. Specifically, the expression of adiponectin receptors in tongue tissue remains unknown. We hypothesized that changes in adiponectin and its receptors may be linked to carcinogenesis and progression of TSCC. Here, we investigated serum and local adiponectin levels as well as the expression of AdipoRs in TSCC, and evaluated their association with clinicopathologic features of TSCC. Moreover, we examined the effects of adiponectin on proliferation and migration of SCC15, a TSCC cell line.
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Our study presents three major novel findings. First, we demonstrated that serum adiponectin level was reduced in TSCC, and inversely associated with histological grade and lymph node metastasis of TSCC. Second, we identified the tongue as a new origin and target of adiponectin by characterizing the expression and distribution of adiponectin and its receptors in human tongue tissue and SCC15 cells. Moreover, the adiponectin levels in TSCC tissue were inversely associated with the clinical stages of TSCC, although adiponectin receptor levels remained unchanged. Third, adiponectin inhibited the migration of SCC15 cells in vitro. These results suggest that a decreased adiponectin level is associated with an aggressive phenotype of TSCC.
Although mainly secreted by adipose tissue, adiponectin levels are inversely correlated with BMI. Recently, adiponectin has been identified as a novel risk marker in various obesity-related cancers, such as breast, endometrial, renal, colon and prostate cancer.[10-12, 24, 25] However, only a few studies have focused on the effect of adiponectin on nonobesity-related cancers. Serum adiponectin was found to be lower in esophagus squamous cell carcinoma, but unchanged in lung cancer.[14, 26] In obese women, weight loss is associated with increased circulating adiponectin and reduced risk of breast cancer. However, so far, obesity has not been associated with TSCC. In fact, most patients with TSCC lose weight due to difficulty chewing, swallowing and appetite loss, especially at later stages. Here, we found that serum adiponectin level was only inversely correlated with weight, BMI, fasting blood glucose and insulin level in healthy controls, as reported previously,[24, 29] but not in TSCC patients. These results are consistent with reports on gastric and esophageal cancer, and the impairment of this correlation might be due to the cachexic status of cancer patients.[26, 30] Furthermore, serum adiponectin level was significantly lower in TSCC patients and inversely associated with the risk and severity of TSCC, suggesting that adiponectin has a potential role in the carcinogenesis and progression of this nonobesity-related cancer.
Serum adiponectin level is regulated by numerous physiological and pathological factors. Hypoadiponectinemia is closely associated with obesity, type 2 diabetes, dyslipidemia and cardiovascular disease.[8, 9] Medications including thiazolidinediones, angiotensin-converting enzyme inhibitors and angiotensin type-1 receptor blockers have been shown to increase circulating adiponectin levels.[31-33] To account for possible interferences of existing diseases and medication, we performed further analysis in a TSCC subgroup without metabolic and cardiovascular diseases. Serum adiponectin level in patients without the related diseases was still lower than in controls, and remained inversely associated with the risk of TSCC. In addition, hypoadiponectinemia was found in individuals with smoking and drinking habits, both as major risk factors of TSCC.[34, 35] However, hypoadiponectinemia still existed in patients without these habits. Consistently, numerous studies have indicated that hypoadiponectinemia is correlated with cancer risk even after adjustment for the potential confounders including BMI, diabetes, hypertension, smoking, or drinking in endometrial, colorectal[12, 17] and renal cancers. Taken together, these results indicate that hypoadiponectinemia is associated with TSCC, which might not be due to the effect of related diseases, habits or medication.
Circulating adiponectin exists as a full length protein (fAd) as well as a biological active globular C-terminal domain (gAd). gAd exists as a trimer, while fAd exists as a low-molecular weight (LMW) trimer, a middle-molecular weight (MMW) hexamer, and a high-molecular-weight (HMW) multimer. Recent studies show that a decreased HMW adiponectin level is related to increased risk of breast and liver cancers,[38, 39] while there is no clear association between HMW adiponectin level and the risk of colorectal cancer. Conversely, LMW level is inversely associated with risk of liver and colorectal cancers.[40, 41] In the present study, adiponectin detected by ELISA was total adiponectin including both fAd and gAd. Hence, the correlation of serum adiponectin subtypes with TSCC should be further explored.
Recently, growing evidence has revealed that adiponectin and its receptors are ubiquitously expressed in various cell types, including breast, endometrium and colon cancer cells. We identified the expression and distribution of adiponectin and its receptors in tongue tissue and SCC15 cells. Moreover, adiponectin could be secreted by SCC15. These results suggest that the tongue is a new origin and target of adiponectin, and that adiponectin might affect the metabolism and function of tongue tissue as via endocrine as well as paracrine and/or autocrine pathways. However, the exact effect of adiponectin on the tongue remains to be explored.
We evaluated the correlation of serum and local adiponectin levels with histopathologic features of TSCC. Hypoadiponectinemia was associated with moderately-differentiated and poorly-differentiated TSCC and lymph node metastasis. Of note, although important in the TNM system, tumor size was not significantly associated with serum adiponectin level. It is likely that the rate of lymph node metastasis was not associated with increased tumor size, but rather more closely associated with the degree of differentiation in oral cancer, as described previously. Compared with paracancer, TSCC tissues showed higher adiponectin levels at an early stage, when tumors were <4 cm and without lymph node metastasis. Studies of breast and colorectal cancers show similar results.[42, 44] However, the reason for increased local adiponectin level remains unknown and might be an early response to the alteration of tumor microenvironment. More importantly, tissue levels of adiponectin gradually decreased with increasingly clinical stage, which was consistent with the inverse association of serum adiponectin level and TSCC grade and N stage. The expression of AdipoR1 and AdipoR2 in TSCC tissue was unchanged. These results suggest that hypoadiponectinemia is correlated with histopathologic features of TSCC, and might be a new biomarker of aggressive phenotype in TSCC.
Despite the inverse correlation between adiponectin and various cancers, the underlying mechanisms of adiponectin in potential cancer suppression are not fully elucidated. Both gAd and fAd exhibit potent anti-inflammatory and anti-atherosclerotic effects, and play important roles in regulating glucose and lipid metabolism. gAd and fAd inhibit the proliferation of colorectal cancer cells.[46, 47] However, gAd and fAd behave differently in inhibiting the proliferation of esophageal adenocarcinoma, breast and prostate cancer cells.[48-50] In the present study, gAd significantly inhibited the migration of SCC15 cells, but had no effect on their proliferation in vitro. This suggests that adiponectin, at least in globular form, might have a direct protective role in progression of TSCC.
In conclusion, we have provided the first evidence that serum adiponectin level is inversely associated with risk of TSCC in human. The decreased serum and local adiponectin levels were associated with clinicopathological characteristics of TSCC, notably histological grade, lymph node metastasis and clinical stage. Adiponectin could directly inhibit the migration of SCC15 cells. These findings may improve our understanding of the roles of adiponectin in the carcinogenesis and progression of TSCC and provide insights for novel diagnostic and therapeutic targets in TSCC.