Nuclear Localization of Signal Transducer and Activator of Transcription 3 (STAT3) in Head and Neck Squamous Cell Carcinoma Is Associated with a Better Prognosis

Abstract

Purpose

A high frequency of head and neck squamous cell cancers (HNSCC) contain constitutively activated STAT3. To further elucidate the prognostic role of STAT3 in HNSCC, the expression pattern of STAT3 was correlated with outcome in two independent data sets.

Experimental Design

STAT3 protein expression analysis was performed on a test cohort of 102 patients with HNSCC recruited between 1992 and 2005. Automated quantitative analysis (AQUA) was used to assess STAT3 protein expression. We evaluated associations with clinicopathological parameters and survival prognosis. Associations were validated in a second, independent cohort of 58 patients with confirmed HNSCC enrolled in Early Detection Research Network (EDRN) sponsored study who underwent surgical resection with curative intent at the University of Pittsburgh Medical Center between 2000 and 2004.

Results

STAT3 displayed mixed nuclear and cytoplasmic staining. Survival analysis showed that high nuclear STAT3 expression (top tertile versus the rest) was associated with longer progression-free survival (PFS) (n = 70, mean survival 88.9 vs. 46.7 months, p = 0.012 for the first cohort; n = 37; mean survival 60.3 vs. 33.0 months, p = 0.009 for the second cohort). After best model selection in the multivariable analysis context, only STAT3 was significant, revealing a lower risk of progression and death for patients with high nuclear STAT3-expressing tumors (HR = 0.28, 95% CI 0.10 to 0.82, p = 0.019 and HR = 0.23, 95% CI 0.07 to 0.76, p = 0.016 respectively).

Conclusions

Our results indicate that high nuclear STAT3 expression levels by AQUA are associated with favorable outcome in HNSCC.

INTRODUCTION

Head and neck squamous cell cancer (HNSCC) is the sixth leading cause of cancer-related deaths worldwide. Despite advances in diagnosis and treatment, morbidity and mortality rates of patients with HNSCC remain high. Advances in molecular biology have led to the identification of molecules that promote the malignant properties of tumor cells and induce resistance to chemotherapy and radiotherapy. Targeting these molecules has become a major goal in cancer therapy and may eventually increase cure rates.

The signal transducer and activator of transcription (STAT) proteins are transcription factors that have been implicated in signaling by oncoproteins, cytokine receptor-associated kinases, growth factor receptor tyrosine kinases, and non-receptor tyrosine kinases, originally defined as the signaling mechanism for interferon (IFN) receptors. After activation, STAT proteins dimerize and translocate to the nucleus, where they bind to DNA-response elements and alter transcription of genes controlling normal cell functions such as growth, apoptosis and differentiation(2). Among the 7 STAT members, STAT3 is particularly interesting due to its constitutive phosphorylation in several human cancers(3), its essential role in cell transformation by the Src oncoprotein(4) and its ability to induce malignant transformation(5–7).

Constitutive activation of STAT3 has been described in HNSCC and is involved in deregulation of cell cycle, increased proliferation and inhibition of apoptosis (8–9). STAT3 is activated frequently in matched tumor/normal mucosa samples from HNSCC patients compared to normal mucosa from patients without cancer(8). Constitutive activation of STATs was detected in extracts from human head and neck squamous carcinoma cells and was dependent on TGF-induced activation of the Epidermal Growth Factor Receptor (EGFR) tyrosine kinase(9).

Despite the diversity in STAT3 activation signals, all result in translocation of STAT3 to the nucleus, where it is involved in regulation of genes that promote the malignant phenotype. These findings suggest that activated STAT3 may provide a sensitive marker for detecting activation of oncogenic signaling pathways (i.e. EGFR) and aid in the selection of treatment modalities that may result in more efficient eradication of disease.

Although the role of constitutive activation of STAT3 in head and neck carcinogenesis has been well documented, its prognostic value for patients with HNSCC has not been rigorously studied. Studies(10,11) evaluating the prognostic significance of STAT3 are limited by the technical difficulties inherent in assessing pSTAT3 with conventional immunohistochemistry such as variability in immunohistochemical techniques, different methods of pathologist-based scoring and the semiquantitative nature of the assay. To overcome this problem, a method of in situ automated quantitative analysis (AQUA) has been developed(12) which allows measurements of protein expression within subcellular compartments that results in a number directly proportional to the number of molecules expressed per unit area. Thus, we avoid biases introduced from the arbitrary cutoff points used in conventional immunohistochemistry studies while at the same time preserving spatial and morphologic information that techniques such as Western blotting lose.

In the present study, we hypothesized that assessment of nuclear STAT3 levels by AQUA would be a more reliable indicator of STAT3 activation status. We therefore determined the impact of nuclear STAT3 protein levels on patient outcome in two independent cohorts of patients with HNSCC.

PATIENTS AND METHODS

Patient population

For Cohort 1, inclusion criteria were histologically confirmed primary squamous cell carcinomas of the head and neck treated at Yale-New Haven Hospital and Aristotle University Hospital between 1992 and 2005, and therapy with either external beam radiotherapy (EBRT) or gross total surgical resection and postoperative radiotherapy (n=102). Exclusion criteria included presentation with metastatic or recurrent disease or failure to receive a full course of radiation therapy. Patients with incomplete clinical-pathological data or those lost to follow-up were also excluded. For Cohort 2, patients with confirmed HNSCC (oral cavity, oropharynx, hypopharyx, or larynx) who were enrolled in an Early Detection Research Network (EDRN) sponsored study and who underwent surgical resection with curative intent at the University of Pittsburgh Medical Center between 2000 and 2004 were considered for inclusion in this study (n=161). Tumor specimens from 58 of these HNSCC patients, 50 with paired adjacent histologically normal adjacent mucosa, were used to construct a tissue microarray (TMA), using triplicate tissue cores. HNSCC tumors included in the Cohort 2 TMA did not differ significantly by tumor site, clinical stage, or patient gender or age from those HNSCC tumors not incorporated into the TMA. Tissues were collected under the auspices of a tissue bank protocol approved by the University of Pittsburgh Institutional Review Board.

Tissue microarray construction

Two tissue microarrays (TMAs) were constructed: (1) a TMA consisting of tumors from patient cohort I was constructed at the Yale University Tissue Microarray Facility including 102 cases and (2) a TMA consisting of HNSCC tumors from patient cohort II including 58 cases. Following institutional review board approval, the tissue microarrays were constructed as previously described(13). Tissue cores 0.6 mm in size were obtained from paraffin-embedded formalin-fixed tissue blocks from the archives of the Yale University and Aristotle University of Thessaloniki Department of Pathology or from the University of Pittsburgh Head and Neck Department of Pathology. Hematoxylin- and eosin-stained slides from all blocks were first reviewed by a pathologist to select representative areas of invasive tumor to be cored. The cores were placed on the recipient microarray block using a Tissue Microarrayer (Beecher Instrument, Silver Spring, MD). All tumors were represented with at least two-fold redundancy. Previous studies (ref) have demonstrated that the use of tissue microarrays containing one to two histospots provides a sufficiently representative sample for analysis by immunohistochemistry. Addition of a duplicate histospot, while not necessary, does provide marginally improved reliability(14). Cores from HPV16-positive SiHa cell lines fixed in formalin and embedded in paraffin were selected for positive controls and included in the Cohort 1 array.

Quantitative Immunofluorescent Staining

Tissue microarrays were deparaffinized with xylene and stained as previously described(15). Briefly, slides were rehydrated and antigen retrieval was achieved by pressure-cooking for 20 minutes in citrate buffer (pH = 6). Slides were preincubated with 0.3% bovine serum albumin in 0.1mol/L of Tris-buffered saline (pH = 8) for 30 minutes at room temperature. Slides were then incubated with primary antibody to STAT3 (1:1000, clone 124H6, #9139; Cell Signaling Technology, Beverly, MA) and a wide-spectrum rabbit anticow cytokeratin antibody (1:100, Z0622; Dako) at 4°C overnight. The STAT3 antibody has been extensively validated in previous studies using immunohistochemistry and Western blot analysis of neoplastic tissue and tumor cell lines. Subsequently, slides were incubated with Cy3-conjugated goat anti-rabbit secondary antibody (A11010; Molecular Probes, Eugene, OR) diluted 1:100 in mouse EnVision reagent (K4001; Dako Corporation, Carpinteria, CA) for 1 hr at room temperature. Cy5 directly conjugated to tyramide (FP117; Perkin-Elmer, Boston, MA) at a dilution was used as the fluorescent chromagen for STAT3 detection. Cy5 (red) was used because it is well outside the green-orange spectrum of tissue autofluorescence. Prolong mounting medium (ProLong Gold, P36931; Molecular Probes, Eugene, OR) containing 4', 6-diamidino-2-phenylindole (DAPI) was used to identify nuclei.

Automated Image Acquisition and Analysis

Automated image acquisition and analysis using automated in situ quantitative measurement of protein analysis (AQUA) has been described previously(12). In brief, monochromatic, high-resolution (1,024 × 1,024 pixel; 0.5μm) images were obtained of each histospot. We distinguished areas of tumor from stromal elements by creating a mask from the cytokeratin signal. DAPI signal was used to identify nuclei, and the cytokeratin signal was used to define cytoplasm. Overlapping pixels (to a 99% confidence interval) were excluded from both compartments. The signal (AQUA score) was scored on a normalized scale of 0 to 255 expressed as pixel intensity divided by the target area. AQUA scores for each subcellular compartment (nuclear and cytoplasmic STAT3) as well as the tumor mask were recorded. AQUA scores for duplicate tissue cores were averaged to obtain a mean score for each tumor.

Statistical Analysis

Histospots containing <5% tumor as assessed by mask area (automated) were excluded from further analysis. The AQUA STAT3 score was assessed by tertiles. Progression free survival and overall survival were assessed by Kaplan-Meier analysis with log-rank score for determining statistical significance. Comparisons of STAT3 expression with gender, TNM stage, histologic grade (only available in Cohort I) and tumor site were made by chi-square test or Fisher's exact test. Relative risk was assessed by Cox proportional hazards regression by multivariable analysis for these variables. In cohort 1, backward selection of the best model was used with removal criterion p>0.10. The prognostic ability of this model was evaluated in Cohort 2. All calculations and statistical analyses were performed by SPSS 15.0 for Windows (SPSS, Inc., Chicago, IL).

RESULTS

Clinical and Pathologic Variable Analysis

In cohort 1, there were 70 patients with primary squamous cell carcinoma of the head and neck who met inclusion criteria and for whom we had complete STAT3 expression data. We excluded from the analysis 32 cases (among 102) missing STAT3 expression information. These cases did not differ from the ones included in the analysis with respect to patient gender, tumor site, TNM stage, histologic grade, progression-free and overall survival as assessed by Fisher's exact test, and log rank test, respectively. For cohort 2, 37 of the 58 arrayed HNSCC tumors were successfully evaluated for nuclear STAT3 levels using AQUA. Unsuccessful assessment was due to tumor core loss; cases with lost cores did not differ from successfully evaluated tumors with respect to all parameters. Of those tumors successfully evaluated for nuclear STAT3 protein expression, tumors with high versus intermediate/low nuclear STAT3 levels did not differ by patient gender, tumor TNM stage or tumor site for Cohort 1 or Cohort 2 (Table 1).

Table 1.

Demographic, clinical and pathologic data by nuclear STAT3 status (Cohorts 1 and 2)

	Nuclear STAT3 expression status
	Cohort 1				Cohort 2
	n	Low (n=47) %	High (n=23) %	P	n	Low (n=25) %	High (n=12) %	P
Gender
Male	58	80.9	87.0	0.738	29	76.0	83.3	1.00
Female	12	19.1	13.0		8	24.0	16.7
TNM stage
I	5	6.8	8.7	0.696	2	5.0	8.3	0.492
II	9	15.9	8.7		6	20.0	16.7
III	18	29.5	21.7		4	20.0	0.0
IV	35	47.7	60.9		20	55.0	75.0
Unknown	3				5
Tumor Site
Oral cavity	8	18.2	0	0.100	14	39.1	41.7	0.950
Larynx	31	40.9	59.1		12	34.8	33.3
Oropharynx	25	38.6	36.4		7	17.4	25.0
Hypopharynx	2	2.3	4.5		2	8.7	0.0
Unknown	4				2
Tumor grade
Well differentiated	10	12.8	23.8	0.407
Moderately differentiated	32	59.0	42.9
Poorly differentiated	18	28.2	33.3
Unknown	10

Quantitative Immunohistochemistry for STAT3 Protein Expression

As visualized by fluorescent immunohistochemistry, STAT3 displayed mixed nuclear and cytoplasmic staining (Fig. 1). Normalized AQUA scores for STAT3 expression in tumors ranged from 1 to 100. Nuclear, cytoplasmic and tumor STAT3 expression followed a skewed distribution as expected for a cancer tissue biomarker (Fig. S1). To assess for intratumor heterogeneity of STAT3 expression and control for reproducibility of the assay, we compared AQUA scores from redundant tumor cores and observed significant correlation (Fig. S2; R = 0.89). AQUA scores in the nuclear compartment were averaged between the two histospots and final scores ranging from 3.92 to 97.92 were obtained for 70 patients.

Figure 1.

Fluorescent immunohistochemistry for automated analysis (AQUA).

A) Cytokeratin was used to identify tumor.

B) Binary gating of cytokeratin expression created the tumor mask (white).

C) Pseudocolored co-localization image demonstrating compartment assignment. Cytokeratin-Cy3 (green) was used to define the non-nuclear compartment; DAPI (blue) was used to define the nuclear compartment D) Cy5 (red) was used to identify STAT3. Original magnification × 20.

Univariate Survival Analysis

Progression-free survival

The status of nuclear STAT3 expression was evaluated for association with progression-free survival (PFS) using the log-rank statistic to determine significance. High nuclear STAT3 expression is associated with superior PFS. Patients in Cohort 1 with high nuclear STAT3 had a higher cumulative survival at 5 years compared to patients with intermediate or low STAT3. The cumulative PFS at 5 years was 75.2% for high nuclear STAT3 expressors vs. 35.4% for others. The median PFS has not been reached and the mean PFS is 88.9 months for high expressors, compared with a median of 21.3 and mean of 46.7 months for those with intermediate or low nuclear STAT3 expression by AQUA (Fig. 2A). As shown in Table 2, this difference in PFS was significant (p=0.012; log rank test). For Cohort 2, elevated nuclear expression of STAT3 was also associated with improved PFS (high STAT3: median 81.0, mean 60.3 months; low STAT3: median 17.2, mean 33.0 months, p=0.035). A greater proportion of patients in Cohort 2 with high nuclear STAT3 levels remained progression-free compared to patients with intermediate or low nuclear STAT3 levels (Table 2).

Figure 2.

A. Kaplan-Meier survival curve comparing progression free survival estimation between low and high expressing nuclear STAT3 groups. Patients with high nuclear STAT3 expression exhibit a higher probability of PFS (Cumulative PFS at 5 years 75.2% vs. 35.4%). B. Kaplan-Meier survival curve comparing overall survival estimation between low and high expressing nuclear STAT3 groups. Patients with high nuclear STAT3 expression exhibit a higher probability of OS (Cumulative OS at 5 years 72.4% vs. 38.3%).

Table 2.

Univariate survival analysis (Kaplan-Meier log-rank) (Cohorts 1 and 2)

	Mean Survival (95% CI) (months)	Median Survival (95% CI) (months)	Cumulative survival or recurrence (number of events) at 5 years	P-value
Cohort 1
PFS *				0.012
High nuclear STAT3 (N=23)	88.91 (67.33–110.49)	n.e (n.e-n.e)	75.2% (4)
Low nuclear STAT3 (N=47)	46.65 (32.09–61.21)	21.30 (12.95–29.65)	35.4% (24)
OS †
High nuclear STAT3 (N=23)	119.92 (86.13–153.71)	n.e (n.e-n.e)	72.4% (3)	0.009
Low nuclear STAT3 (N=47)	57.30 (38.48–76.11)	32.3 (18.24–46.36)	38.3% (23)
Cohort 2
PFS *				0.035
High nuclear STAT3 (N=12)	60.3 (41.2–79.5)	81.0 (n.e-n.e)	75.0% (3)
Low nuclear STAT3 (N=25)	33.0 (20.2–45.8)	17.2 (4.4–30.0)	39.3% (15)
OS †
High nuclear STAT3 (N=12)	63.1 (47.1–79.1)	81.0 (n.e-n.e)	68.8% (3)	0.244
Low nuclear STAT3 (N=25)	45.3 (31.2–59.4)	62.7 (0–135.9)	51.4% (12)

^*PFS: Progression-Free Survival

^†OS: Overall Survival

Overall Survival

The expression status of STAT3 was also evaluated for association with OS. Kaplan-Meier analysis demonstrated that there was a significant correlation between high nuclear STAT3 expression and improved OS. Patients in Cohort 1 with high nuclear STAT3 levels had significantly better OS (median not reached, mean 119.9 months) while patients with intermediate or low nuclear STAT3 levels had a shorter OS (median 32.3, mean 57.3 months), (p=0.009; log rank test). Patients with high nuclear STAT3 expression had an OS at 5 years of 72.4% compared with 38.3% for patients with intermediate or low STAT3 (Fig. 2B). A non-significant trend was observed for Cohort 2: patients with high nuclear STAT3 levels had a higher cumulative survival at 5 years than patients with intermediate or low STAT3 levels, 68.8% and 51.4% respectively.

Multivariable Survival Analysis

Using the Cox proportional hazards model, we carried out multivariable analysis to assess the prognostic value of nuclear STAT3 expression by AQUA for both PFS and OS in Cohort 1. We included the following prognostic variables in the regression model: gender, TNM stage, and tumor grade and tumor site. After backward selection, the model for PFS including only STAT3 is chosen. A 77% decrease in risk of a progression-defining event was estimated for patients with STAT3 protein expression in the upper tertile (HR = 0.23, 95% CI 0.06 to 0.91, p = 0.037) which for OS corresponds to an 87% decrease in risk (HR = 0.13, 95% CI 0.03 to 0.67, p = 0.014) (Table 3). For Cohort 2, even though the number of events is small, the significant association is sustained for PFS (HR=0.31, p=0.044) while it is not for OS (HR=0.54, p=NS).

Table 3.

Multivariable progression-free (PFS) and overall survival (OS) analysis, by Cox regression

Variable	Hazard Ratio	95% CI	P
PFS *
Male gender
Female gender	3.76	1.09–12.98	0.036
TNM Stage I
TNM Stage II	1.32	0.11–15.46	0.827
TNM Stage III	0.96	0.10–8.90	0.973
TNM Stage IV	2.59	0.30–22.52	0.388
Histology, well differentiated
Histology,moderately differentiated	0.39	0.10–1.62	0.196
Histology, poorly differentiated	0.24	0.05–1.23	0.086
Tumor Site, oral cavity
Tumor Site, larynx	1.52	0.31–7.46	0.604
Tumor Site, oropharynx	1.95	0.48–8.01	0.352
Nuclear STAT3 AQUA Score	0.23	0.06–0.91	0.037
OS †
Male gender
Female gender	2.53	0.78–8.23	0.123
TNM Stage I
TNM Stage II	1.05	0.06–17.89	0.973
TNM Stage III	0.80	0.07–8.93	0.849
TNM Stage IV	3.99	0.41–39.11	0.234
Histology, well differentiated
Histology,moderately differentiated	0.17	0.03–0.84	0.030
Histology, poorly differentiated	0.14	0.02–0.85	0.032
Tumor Site, oral cavity	0.79	0.17–3.62	0.762
Tumor Site, larynx	1.33	0.38–4.65	0.658
Tumor Site, oropharynx
Nuclear STAT3 AQUA Score	0.13	0.03–0.67	0.014

^*PFS: Progression-Free Survival

^†OS: Overall Survival

DISCUSSION

To our knowledge, this is the first study to perform quantitative assessment of nuclear STAT3 in head and neck cancer in association with patient prognosis. We used AQUA, a method that allows quantitative measurement of protein expression within subcellular compartments. Because activation of STAT3 signaling involves translocation to the nucleus, we hypothesized that assessment of nuclear STAT3 expression could act as a marker of activation of a STAT3-mediated signaling pathway. Our findings show higher nuclear STAT3 level is predictive of favorable clinical outcome. Furthermore, nuclear STAT3 was the only identified prognostic factor when adjusting for other commonly used prognostic markers and pathological parameters.

We previously reported the prognostic impact of tyrosine-phosphorylated STAT3 immunoexpression on survival in two independent cohorts of patients with HNSCC and evaluated pSTAT3, transforming growth factor-alpha (TGF-alpha), epidermal growth factor receptor (EGFR), and gastrin-releasing peptide receptor (GRPR) expression in matched tumor and lymph node metastases in one of these cohorts(11). Immunoexpression of pSTAT3 alone did not correlate with clinical outcome in either cohort. Masuda et al(10), analyzed by immunohistochemistry the levels of pSTAT3 in 51 primary squamous cell carcinomas of the tongue and found that p-STAT3 was an independent predictor of poor prognosis. P-STAT3 levels were also correlated with cyclin D1 levels, nodal metastasis and clinical stage. However, in those studies immunohistochemistry was performed on archived surgical specimens that were not processed using a specific protocol for evaluation of phospho-proteins. Routine formalin-fixed tissue from surgical resections does not provide the degree of preservation required to analyze functional proteins, particularly such transient events as phosphorylation. In addition, these studies are limited by the technical difficulties inherent in assessing pSTAT3 with conventional immunohistochemistry. Shah et al(16), analyzed 135 oral squamous cell carcinomas for cytoplasmic or nuclear STAT3 expression by immunohistochemistry. The authors found that nuclear STAT3 was an independent predictor for poor outcome in early-stage patients and concluded that STAT3 activation is an early event in head and neck carcinogenesis.

Our finding that high levels of nuclear STAT3 are associated with better outcome in HNSCC is subject to numerous interpretations. Tumors that activate these pathways appear less aggressive than tumors that progress even in the absence of STAT3 activation, perhaps because the later rely on alternate pathways more tightly related to invasiveness or treatment-resistance. Activated STAT3 in early stage HNSCC may play a role in tumor development by sparing cells from apoptotic death or through cooperation with other oncogenes. In our cohorts, the majority of tumors were of advanced stage. Thus, at advanced stage, tumors probably become independent of STAT3-mediated signaling. An alternative explanation suggests that STAT3 functions as a tumor suppressor protein. STAT3 plays a pleomorphic role in signal transduction and it is possible that it functions as an oncoprotein or a tumor suppressor protein depending on the setting. In mouse mammary gland, STAT3 is activated both during the apoptotic phase and during the highly proliferative state of early pregnancy (20). There are also a number of examples where activated Stat3 appears to play a role in differentiation and promoting apoptosis (17–19). Conditional knockout studies in murine models have shown that STAT3 function is essential in facilitating mammary gland involution by inducing extensive epithelial cell apoptosis through upregulating IGFBP-5 (17, 22). De La Iglesia et al(23), reported that STAT3 plays a pro-oncogenic or tumor suppressive role in glioblastoma depending on the mutational profile of the tumor. Deficiency of PTEN triggers a cascade that inhibits STAT3 signaling in murine astrocytes and human glioblastoma tumors. Accordingly, PTEN knockdown induces efficient malignant transformation of astrocytes upon knockdown of the STAT3 gene. In contrast to the tumor suppressive function of STAT3 in the PTEN pathway, STAT3 forms a physical complex with the pro-oncogenic protein EGFRVIII in the nucleus and mediates EGFRVIII-induced glial transformation. STAT3 may play a dual role in head and neck carcinogenesis depending on the genetic background of the tumor similar to glioblastoma.

A number of studies in patients' samples in different sets of tumors have demonstrated similar results to the present study. Dolled-Filhart et al (23), reported that breast tumors with positive STAT3 nuclear expression had a significantly improved survival (p = 0.03) at 5 years of follow-up. Hsiao JR et al(24), studied STAT3 and STAT5 expression patterns in relation to survival in nasopharyngeal cancer using immunohistochemistry. Strong intranuclear staining, or intranuclear and cytoplasmic staining, were noted in 31 of the 43 specimens classified as `positive' for significant constitutive STAT3 activation and in 32 of the 38 specimens classified as `positive' for significant constitutive STAT5 activation. The authors found that following radiotherapy, patients with constitutive STAT5 activation, or activation of both STAT3 and STAT5, had better progression-free survival and overall survival than those without activated STAT5. Nuclear STAT3 staining was inversely correlated with the development of distant metastases in a cohort of prostate cancer patients treated with either radiotherapy alone or radiotherapy plus short-term androgen blockade(25). To the contrary, STAT3 expression has been associated with worse outcome in several studies (26–28).

One limitation of our study is that our assay cannot differentiate between phosphorylated and unphosphorylated nuclear STAT3. It is known that STAT3 can enter the nucleus independently of its phosphorylation state. Unphosphorylated STAT3 enters the nucleus by binding to distinct importins(29). Although phosphorylation is not a prerequisite for STAT3 nuclear import, the requirement for phosphorylation for the transcriptional activity of STAT3 remains a controversial issue (5, 30–31). Yang et al(30), reported that a STAT3 mutated on Tyr⁷⁰⁵ retained its transcriptional activity but activated a different set of genes compared to wild-type STAT3. Nuclear phosphorylation of STAT3 by a nuclear tyrosine kinase has been reported in breast cancer and may represent an alternative STAT3 activation mechanism, contributing to its oncogenic potential(32).

To conclude, the present study shows that nuclear STAT3 protein expression in the upper tertile as assessed by AQUA is associated with better PFS in two independent cohorts of patients with HNSCC.

Statement of Translational Relevance.

STAT3 is activated by a variety of signals that all converge into nuclear translocation of STAT3 to the nucleus, where it is involved in regulation of genes that promote the malignant phenotype. Therefore, nuclear STAT3 may represent a sensitive marker for detecting activation of oncogenic signaling pathways (i.e. Epidermal Growth Factor Receptor) and aid in the selection of treatment modalities that may result in more efficient eradication of disease. Although the role of constitutive activation of STAT3 in head and neck carcinogenesis has been well documented, its prognostic value for patients with HNSCC has not been rigorously studied. Here, using AQUA we correlated nuclear STAT3 protein levels with patient outcome in two independent cohorts of patients with HNSCC. We found that high nuclear STAT3 expression levels by AQUA are associated with favorable outcome in HNSCC.