* Context.-Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in diverse human cancers and plays a critical role in tumor cell survival, proliferation, migration, invasion, angiogenesis, and inhibition of apoptosis. The phosphorylated active form of STAT3 (pSTAT3) mediates its effects via nuclear transcriptional activity. However, it was recently observed that the nonphosphorylated, cytoplasmic, inactive form of STAT3 is involved in cell motility and consequently tumor invasion. It appears that, although STAT3 is not absolutely required for tumor formation, tumors that develop in the presence of STAT3 become dependent on its expression for their survival, making it a potential therapeutic target.

Objective.-To investigate the possible utility of STAT3 as a future therapeutic target in non-small cell lung carcinoma (NSCLC) and malignant mesothelioma (MM).

Design.-Immunohistochemical expression of MIB-1, STAT3, and pSTAT3 was assessed in 303 NSCLC and 44 MM archival cases.

Results.-A more conspicuous expression of inactive STAT3 (91.44% in NSCLC and 79.5% in MM cases) was present compared with the nuclear activated form pSTAT3 (60.53% in NSCLC and 61.4% in MM cases). MIB-1 did not correlate with the expression of STAT3 or pSTAT3.

Conclusions.-The strong expression of cytoplasmic inactive STAT3 in NSCLC and MM cases implies a major role for STAT3 in tumor motility, invasion, and metastasis via a nontranscriptional pathway. We conclude that STAT3 and pSTAT3 are up-regulated in a high percentage of NSCLCs and MMs, regardless of tumor type, age, sex, smoking status, stage and grade of tumor, or survival, providing a basis for therapeutic intervention.

(Arch Pathol Lab Med. 2007;131:1350-1360)

Signal transducer and activator of transcription (STAT) proteins are latent cytoplasmic molecules (Figure 1, A), which are activated in response to many cytokines (eg, interleukin 6) and growth factor signals1 (eg, epidermal growth factor, platelet-derived growth factor, and vascular endothelial growth factor) (Figure 1, B) and have the ability to transduce a signal from a cell surface receptor into the nucleus and promote the transcription of specific genes. Although these transcription factors are often not mutated themselves, their overactivation by mutations in other genes leads to the gene expression changes that are hallmarks of malignant disease. The activation of cytoplasmic, latent STATs is dependent on tyrosine phosphorylation (Figure 1, C), which induces dimerization (Figure 1, D) via interaction between 2 STAT molecules.2 Many tyrosine kinases including Janus kinases (Figure 1, E) and receptor tyrosine kinases (Figure 1, F) can mediate phosphorylation (activation) of STAT proteins. Activated STATs are translocated to the nucleus (Figure 1, G), where they bind to consensus sequences on the promoter of target genes and activate their transcription2 (Figure 1, H). In normal cells, STAT tyrosine phosphorylation is transient, lasting from minutes to several hours, and the tyrosine dephosphorylated STAT proteins are shuttled back to the cytosol from the nucleus3 (Figure 1, I). However, in numerous cancer-derived cell lines or in primary tumors, STAT proteins are persistently tyrosine phosphorylated.4 There are at least 6 STAT proteins involved in this complex signaling pathway. Among these STATs, STAT3 stands out by its constitutive activation in most human neoplasms.2,5 Activation of STAT3 is detected in solid tumors (breast, lung, head and neck [squamous cell carcinoma]; ovarian and pancreatic cancers; melanoma; hepatocellular carcinoma; cholangiocarcinoma; and prostate, endometrial, and cervical cancers) as well as in hematopoietic tumors (acute myelogenous leukemia, multiple myeloma, Hodgkin disease, non-Hodgkin lymphoma, B-cell lymphoma, and cutaneous T-cell lymphoma).1,2,5-7 It has been demonstrated that STAT3 in tumor cells promotes cell growth and cell survival through the transcriptional up-regulation of target genes7 (eg, c-myc, bcl-xL, bcl-2, cyclin D1, VEGF), whose products promote cell proliferation, cell migration, invasion, angiogenesis, and inhibition of apoptosis8 (Figure 1, J). It has been described that the inhibition of STAT3 function in non-small cell lung carcinoma (NSCLC) cells increases apoptosis of these cells, suggesting that STAT3 is necessary for their survival.9,10 It is believed that STAT3 mediates its effects through the nuclear localized transcriptional activity. However, evidence for a cytoplasmic nontranscriptional role for STAT3 in cell motility and consequently in tumor invasion has accumulated and a direct effect of nonphosphorylated cytoplasmic STAT3 on cell motility through a nontranscriptional protein interaction mechanism has been recently demonstrated11,12 (Figure 2, A through E). STAT3's control of cell motility, in addition to its cell proliferation-promoting and antiapoptotic activities, directly contributes to its role in tumorigenesis and tumor progression.8 In addition, STAT3 has been described as a component of focal adhesions (sites of cell contact with the extracellular matrix) that may contribute to the invasiveness of cancer cells13 (Figure 2, C). There is an unmet need to develop new treatment modalities for lung cancer. Molecular targets for treating cancer have been particularly difficult to develop because of the multiplicity of genetic mutations in cancer cells. Targeting 1 mutation at a time appears to be ineffective. Recently, however, it has been realized that genetic mutations in cancer often converge on central regulators: the transcription factors.5 New research is now pointing to STAT3 as a promising target for the development of a global anticancer therapy agent. To investigate the possible utility of STAT3 as a future therapeutic target in NSCLC and malignant mesothelioma (MM), we evaluated the presence of the inactive form of STAT3, the active phosphorylated form of STAT3 (pSTAT3), and the proliferation marker MIB-1. In NSCLC cases, the results were correlated with age, sex, grade, stage, smoking status, and survival.