The STAT (signal transducers and activators of transcription) family of proteins are DNA-binding proteins that play a dual role in signal transduction and activation of transcription. Presently, there are six distinct members of the STAT family (STAT1, STAT2, STAT3, STAT4, STAT5, and STAT6) and several isoforms (STAT1•, STAT1•, STAT3• and STAT3•). The activities of the STATs are modulated by various cytokines and mitogenic stimuli. Binding of a cytokine to its receptor results in the activation of Janus protein tyrosine kinases (JAKs) associated with these receptors. This in turn, phosphorylates STAT, resulting in translocation to the nucleus and transcriptional activation of STAT responsive genes. Phosphorylation on a specific tyrosine residue on the STATs results in their activation, resulting in the formation of homodimers and/or heterodimers of STAT which bind to specific gene promoter sequences. Events mediated by cytokines through STAT activation include cell proliferation and differentiation and prevention of apoptosis.
The specificity of STAT activation is due to specific cytokines, i.e. each STAT is responsive to a small number of specific cytokines. Other non-cytokine signaling molecules, such as growth factors, have also been found to activate STATs. Binding of these factors to a cell surface receptor associated with protein tyrosine kinase also results in phosphorylation of STAT.
STAT3 (also acute phase response factor (APRF)), in particular, has been found to be responsive to interleukin-6 (IL-6) as well as epidermal growth factor (EGF) (Darnell, Jr., J. E., et al., Science, 1994, 264, 1415-1421). In addition, STAT3 has been found to have an important role in signal transduction by interferons (Yang, C.-H., et al., Proc. Natl. Acad. Sci. USA, 1998, 95, 5568-5572). Evidence exists suggesting that STAT3 may be regulated by the MAPK pathway. ERK2 induces serine phosphorylation and also associates with STAT3 (Jain, N., et al., Oncogene, 1998, 17, 3157-3167).
STAT3 is expressed in most cell types (Zhong, Z., et al., Proc. Natl. Acad. Sci. USA, 1994, 91, 4806-4810). It induces the expression of genes involved in response to tissue injury and inflammation. STAT3 has also been shown to prevent apoptosis through the expression of bcl-2 (Fukada, T., et al., Immunity, 1996, 5, 449-460).
Aberrant expression of or constitutive expression of STAT3 is associated with a number of disease processes. STAT3 has been shown to be involved in cell transformation.
It is constitutively activated in v-src-transformed cells (Yu, C.-L., et al., Science, 1995, 269, 81-83). Constitutively active STAT3 also induces STAT3 mediated gene expression and is required for cell transformation by src (Turkson, J., et al., Mol. Cell. Biol., 1998, 18, 2545-2552). STAT3 is also constitutively active in Human T cell lymphotropic virus I (HTLV-I) transformed cells (Migone, T.-S. et al., Science, 1995, 269, 79-83).
Constitutive activation and/or overexpression of STAT3 appears to be involved in several forms of cancer, including myeloma, breast carcinomas, prostate cancer, brain tumors, head and neck carcinomas, melanoma, leukemias and lymphomas, particularly chronic myelogenous leukemia and multiple myeloma. Niu et al., Cancer Res., 1999, 59, 5059-5063. Breast cancer cell lines that overexpress EGFR constitutively express phosphorylated STAT3 (Sartor, C. I., et al., Cancer Res., 1997, 57, 978-987; Garcia, R., et al., Cell Growth and Differentiation, 1997, 8, 1267-1276). Activated STAT3 levels were also found to be elevated in low grade glioblastomas and medulloblastomas (Cattaneo, E., et al., Anticancer Res., 1998, 18, 2381-2387).
Cells derived from both rat and human prostate cancers have been shown to have constitutively activated STAT3, with STAT3 activation being correlated with malignant potential. Expression of a dominant-negative STAT3 was found to significantly inhibit the growth of human prostate cells. Ni et al., Cancer Res., 2000, 60, 1225-1228.
STAT3 has also been found to be constitutively activated in some acute leukemias (Gouilleux-Gruart, V., et al., Leuk. Lymphoma, 1997, 28, 83-88) and T cell lymphoma (Yu, C.-L., et al., J. Immunol., 1997, 159, 5206-5210). Interestingly, STAT3 has been found to be constitutively phosphorylated on a serine residue in chronic lymphocytic leukemia (Frank, D. A., et al., J. Clin. Invest., 1997, 100, 3140-3148). In addition, antisense oligonucleotides to STAT3 have been shown to promote apoptosis in non small cell lung cancer cells (Song et al., Oncogene 22:4150, 2003) and prostate cancer cells (Mora et al., Cancer Res. 62:6659, 2002).
STAT3 has been found to be constitutively active in myeloma tumor cells, both in culture and in bone marrow mononuclear cells from patients with multiple myeloma. These cells are resistant to Fas-mediated apoptosis and express high levels of Bcl-xL. STAT3 signaling was shown to be essential for survival of myeloma tumor cells by conferring resistance to apoptosis. Thus STAT3 is a potential target for therapeutic intervention in multiple myeloma and other cancers with activated STAT3 signaling. There is a distinct medical need for novel therapies for chemoresistant myeloma. Velcade was approved in May 2003 with an 188 evaluable patient pivotal trial based on tumor shrinkage, not survival. 28% showed a partial tresponse. The data is currently under FDA review.
Catlett-Falcone, R., et al., Immunity, 1999, 10, 105-115. A gene therapy approach in a syngeneic mouse tumor model system has been used to inhibit activated STAT3 in vivo using a dominant-negative STAT3 variant. This inhibition of activated STAT3 signaling was found to suppress B16 melanoma tumor growth and induce apoptosis of B16 tumor cells in vivo. Interestingly, the number of apoptotic cells (95%) exceeded the number of transfected cells, indicating a possible antitumor “bystander effect” in which an inflammatory response (tumor infiltration by acute and chronic inflammatory cells) may participate in killing of residual tumor cells. Niu et al., Cancer Res., 1999, 59, 5059-5063. Constitutively activated STAT3 is also associated with chronic myelogenous leukemia.
STAT3 may also play a role in inflammatory diseases including rheumatoid arthritis. Activated STAT3 has been found in the synovial fluid of rheumatoid arthritis patients (Sengupta, T. K., et al., J. Exp. Med., 1995, 181, 1015-1025) and cells from inflamed joints (Wang, F., et al., J. Exp. Med., 1995, 182, 1825-1831).
Multiple forms of STAT3 exist, generated by alternative splicing. STAT3• is a short form of STAT3 (also, STAT3•) that differs predominately by the absence of 55 amino acid residues at the C-terminus. This domain contains the transactivation domain, and thus, STAT3• may act as a negative regulator of STAT3 function (Caldenhoven, E., et al., J. Biol. Chem., 1996, 271, 13221-13227). STAT3• has been found to be more stable and have greater DNA-binding activity than STAT3•, while STAT3• is more transcriptionally active.
There are currently several approaches for inhibiting STAT3 expression. U.S. Pat. Nos. 5,719,042 and 5,844,082 to Akira, S. and Kishimoto, T. disclose the use of inhibitors of APRF, including antibodies, antisense nucleic acids and ribozymes for the treatment of IL-6 associated diseases, such as inflammatory diseases, leukemia, and cancer. Schreiber, R. D., et al., in U.S. Pat. Nos. 5,731,155; 5,582,999; and 5,463,023, disclose methods of inhibiting transcriptional activation using short peptides that bind p91. Darnell, J. E., et al., in U.S. Pat. No. 5,716,622, disclose peptides containing the DNA binding domain of STATs, chimeric proteins containing the DNA binding domain, and antibodies to STATs for inhibiting STAT transcriptional activation.
The use of an antisense oligonucleotide targeted to the translation start region of human STAT3 has been disclosed (Grandis, J. R., et al., J. Clin. Invest., 1998, 102, 1385-1392). In this report, a phosphorothioate oligodeoxynucleotide complementary to the translation start region of STAT3 inhibited TGF-β stimulated cell growth mediated by the epidermal growth factor receptor (EGFR).
There remains an unmet need for therapeutic compositions and methods targeting expression of STAT3, and disease processes associated therewith.