STAT-3 inhibitors: state of the art and new horizons for cancer treatment

The signal transducers and activators of transcription (STATs) include a class of cytoplasmic signaling proteins whose role in the regulation of cell growth and survival is mediated by phosphorylation of a critical tyrosine residue within the STAT protein. This occurs in response to cytokines and growth factors modulating the expression of specific target genes. In particular, phosphorylation induces STAT:STAT dimer formation between two monomers, via reciprocal phosphoTyr (pTyr)-SH2 domain interactions. To date, seven members of the STAT family, all with different roles, have been identified in mammals. After dimerization, phosphorylated STATs enter the nucleus and, working co-ordinately with other transcriptional co-activators and transcription factors, induce increased transcriptional initiation. In healthy human and animal cells, ligand-dependent activation of STATs is a transient process, lasting for several minutes to several hours. In contrast, in many cancerous cell lines and tumors, where growth factor dysregulation is frequently at the heart of cellular transformation, the STAT proteins (in particular STAT1, 3 and 5) are persistently tyrosine-phosphorylated or activated; abnormal levels of STAT3 activation have been observed in breast, ovarian, prostate, hematological and head and neck cancer cell lines. Thus, in this review, we examine the most important classes of agents designed to disrupt STAT3 signaling, with particular regard to STAT3 dimerization inhibitors, which could play a significant role in the future of cancer and adjuvant cancer therapies.