STAT3 isoforms: Alternative fates in cancer?

Signal transducer and activator of transcription (STAT) 3 is the main mediator of IL-6-type cytokine signaling and an important transcriptional regulator of cell proliferation, maturation and survival. It has been described as a key player in cancer development and progression. However, under certain circumstances, STAT3 is also considered a potent tumor suppressor. This heterogeneity partially depends on its expression as different iso-forms. Alternative splicing gives rise to two STAT3 isoforms, STAT3 alpha and its truncated version STAT3 beta. Both isoforms are transcriptionally active and display distinct functions under physiological and pathological conditions. In fact, while STAT3 alpha is widely described as an oncogene, STAT3 beta has gained attention as a potential tumor suppressor. This review provides a concise overview of the current knowledge on STAT3 during tumorigenesis, with special emphasis on the unique and complex roles of its alternatively spliced isoforms.