STAT proteins as novel targets for cancer drug discovery

Signal transducer and activator of transcription (STAT) proteins are latent cytoplasmic transcription factors that were discovered in the context of cytokine and growth factor signalling. Normal STAT signalling is tightly controlled with finite kinetics, which is in keeping with standard cellular responses. However, persistent STAT activation has also been observed and is frequently associated with malignant transformation. Constitutive activation of STAT proteins, notably of Stat3 and Stat5, is detected in many human tumour cells and cells transformed by oncoproteins that activate tyrosine kinase signalling pathways. It is well-established that constitutively active Stat3 is one of the molecular abnormalities that has a causal role in oncogenesis. Aberrant Stat3 promotes uncontrolled growth and survival through dysregulation of gene expression, including cyclin D1, c-Myc, Bcl-xL, Mcl-1 and survivin genes, and thereby contributes to oncogenesis. Moreover, recent studies reveal that persistently active Stat3 induces tumour angiogenesis by upregulation of vascular endothelial growth factor induction, and modulates immune functions in favour of tumour immune evasion. Overall, studies have validated Stat3 as a novel target for cancer therapy, and hence provided the rationale for developing small-molecule Stat3 inhibitors. This review will discuss current evidence for the critical role of aberrant STAT signalling in malignant transformation, and examine the validity as well as the therapeutic potential of Stat3 as a cancer target. An update on the efforts to develop novel Stat3 inhibitors for therapeutic application will also be provided.