Cytokines have gained much attention in biological sciences and medicine over the last three decades. Cytokines consist of several families of soluble molecules, such as interferons (IFNs) and interleukins (ILs), that transmit signals to the cell interior, eliciting various of responses from a cell. It is now widely known that these cytokines play critical roles in many biological systems, such as immunity and oncogenesis. However, until the late 1970’s, the structures and functions of these cytokines, as well as the mechanisms underlying the signal transmission and the regulation of their expression, remained elusive. Indeed, these issues were difficult to clarify because of the facts that these molecules are usually produced simultaneously at very low levels in many cell types, making it difficult to obtain them in their pure forms, and that many cytokines may have multiple biological activities. Furthermore, despite the prospects of clinical applications of cytokines, it was difficult to obtain a sufficient amount of each cytokine in the pure form.
My career in research on cytokines began with the characterization of the human fibroblast IFN gene (now referred to as IFN-β) in 1979 and elucidated its primary structure. In collaboration with Dr. Charles Weissmann and colleagues, we subsequently demonstrated that IFN-α and IFN-β genes constitute a gene family; this turned out to be the first of the numerous cytokine gene families to be identified later. We also identified and characterized a human interleukin gene, the IL-2 gene and generated recombinant IL-2, thereby enabling the study of the molecular basis of lymphocyte proliferation. The availability of these recombinant cytokines has made their clinical applications in the treatment of cancer, hepatitis and multiple sclerosis, and their use in studies of molecular signaling mechanisms possible.
Subsequently, we and others identified regulatory elements within the IFN-β and IL-2 gene promoters. Further research on cytokine gene expression and signaling led us to the discovery of a family of transcription factors, the interferon regulatory factors (IRFs). We demonstrated, in collaboration with other groups, the important and broad functions of IRF-1 and IRF-2 in the regulation of interferon responses and other immune functions, such as the CD4+ T cell response and differentiation of natural killer and dendritic cells. We also elucidated the general regulatory mechanisms of IFN-α/β gene induction involving two other members of the IRF family, IRF-3 and IRF-7.
Our achievements in the study of IRFs in oncogenesis include the demonstration that IRF-1 and IRF-2 mediate a broad spectrum of cancer-related actions. We found that IRF-1 regulates cell cycle arrest in cooperation with the tumor suppressor p53, that it is also involved in the regulation of apoptosis, and that the loss of IRF-1 exacerbates tumor development in p53-deficient mice. These findings collectively established important links between IRFs in oncogenesis and immunity. More recently, we have found novel links between interferon actions and the tumor suppressor p53. Although IFN-α/β has been used for the treatment of some human cancers, little was known about how IFNs elicit antitumor activity. We found that interferons induce the transcription of p53 mRNA via the activation of the IRF-9-containing transcription factor ISGF3, which is followed by an increase in p53 protein level. Indeed, this was the first demonstration of p53 gene induction by a cytokine. We also showed that the p53 protein is induced but not activated by IFNs, and that IFN boosts the p53 response to stress signals. We adduced evidence that IFN signaling contributes to tumor suppression via p53 induction, helping to explain the mechanism underlying the antitumor actions of IFNs. This study could have important implications for cancer therapy as it also demonstrates that, as a result of the enhanced accumulation of p53, IFN-treated cells become more susceptible to DNA-damaging chemotherapeutic drugs that activate p53. We further demonstrated that p53 is activated in cells by virus infection, and showed that this activation plays a role in the elimination of virus-infected cells by apoptosis. This study therefore demonstrated a new role of p53 in addition to its functions in oncogenesis.
The importance of the IRF family members has been further corroborated by the recent demonstration of their key roles in Toll-like receptor (TLR) signaling in innate and adaptive immune responses. We and others showed that both IRF-5 and IRF-7 interact with MyD88, the adaptor critical for TLR signaling. In collaboration with Drs. Tak W. Mak and Nubuaki Yoshida and their colleagues, we demonstrated that the MyD88-dependent activation of IRF-5 is essential for the TLR-mediated induction of pro-inflammatory cytokines, whereas the activation of IRF-7 is essential for the potent induction of IFN-α/β by TLR9. The elucidation of mechanisms underlying CD8+ cytotoxic T cell responses is essential for understanding anti-tumor immunity and, in this context, we showed that the MyD88-IRF-7 pathway is essential to the TLR9-mediated CD8+ cytotoxic T cell generation, and that this pathway is under spatiotemporal regulation.
It is remarkable that studies of the IFN-α/β system carried out by many scientists, significantly contributed to our understanding of the mechanisms underlying cytokine action, immune responses and oncogenesis. Indeed, the critical roles of the Janus family of protein tyrosine kinases (JAK kinases), signal transducers and activators of transcription (STATs), and IRFs have all been identified in the context of IFN induction and action, and their broad functions in other biological systems are widely appreciated by now. As for the future prospects, we envisage that a further understanding of the versatile functions of IRFs should also provide important insights into improvements of therapeutic interventions for numerous diseases associated with infection, immunity and cancer.
Acknowledgements:This work carried out in my laboratory was supported by a Grant-In-Aid for Scientific Research (KAKENHI) on Priority Areas from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. I thank Drs. Jan Vilcek, Henry Mihich, Charles Weissmann, Massimo Libonati, and Edward Barsoumian, and all of my colleagues for their long-lasting support and friendship and for sharing their enthusiasm for science.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]