Stat5a and Stat5b proteins have essential and nonessential, or redundant, roles in cytokine responses

A single amino acid in the DNA binding regions of STAT5A and STAT5B confers distinct DNA binding specificities

STAT5A and STAT5B are two highly related transcription factors encoded by two distinct genes. STAT5A and STAT5B are activated by a broad range of cytokines and growth factors. Although they can be differentially activated, the functional difference between these two molecules relative to their structure is not known. Here we demonstrated that STAT5A and STAT5B homodimers have distinct DNA binding preferences. Chimeric STAT5 molecules allowed us to identify a region between amino acid 420 and 545 responsible for the DNA binding specificity. This region is located in the previously characterized DNA binding region of STAT proteins. Sequence comparison between STAT5A and STAT5B from different species showed a difference of 5 amino acids in the region 420-545 between STAT5A and STAT5B. Substitution of these amino acids demonstrated that a glycine residue at position 433 in STAT5B and a glutamic residue at a similar position in STAT5A determined the DNA binding specificity. These data indicate that STAT5A and STAT5B homodimers may have distinct function and probably regulate the expression of common as well as distinct genes.

Stat5b is essential for natural killer cell-mediated proliferation and cytolytic activity

We have analyzed the immune system in Stat5-deficient mice. Although Stat5a-/- splenocytes have a partial defect in anti-CD3-induced proliferation that can be overcome by high dose interleukin (IL)-2, we now demonstrate that defective proliferation in Stat5b-/- splenocytes cannot be corrected by this treatment. Interestingly, this finding may be at least partially explained by diminished expression of the IL-2 receptor beta chain (IL-2Rbeta), which is a component of the receptors for both IL-2 and IL-15, although other defects may also exist. Similar to the defect in proliferation in activated splenocytes, freshly isolated splenocytes from Stat5b-/- mice exhibited greatly diminished proliferation in response to IL-2 and IL-15. This results from both a decrease in the number and responsiveness of natural killer (NK) cells. Corresponding to the diminished proliferation, basal as well as IL-2- and IL-15-mediated boosting of NK cytolytic activity was also greatly diminished. These data indicate an essential nonredundant role for Stat5b for potent NK cell-mediated proliferation and cytolytic activity.

Role of cytokines and extracellular matrix in the regulation of haemopoietic stem cells

The understanding of molecular mechanisms regulating the formation, growth and differentiation of haemopoietic stem cells has advanced considerably recently. Particular progress has been made in defining the cytokines, chemokines and extracellular matrix components which retain and maintain primitive haemopoietic cell populations in bone marrow. Furthermore, signal transduction pathways that are critical for haemopoiesis, both in vivo and in vitro, and that are activated by cytokines have also been identified and further characterised. The importance of these processes has, this year, been exemplified by the phenotypes of mice deficient in key signal transduction proteins and the discovery that mutations in the component proteins of some signalling pathways are linked to human diseases. Significant advances in understanding the molecular mechanisms for mobilisation of stem cells from bone marrow have also been made this year; this has potential importance for bone marrow transplantation.

The interleukin-4 receptor activates STAT5 by a mechanism that relies upon common gamma-chain

Interleukin (IL)-4 signaling proceeds via cytoplasmic activation of the Janus kinases JAK1 and JAK3 and the signal transducer and activator of transcription STAT6. We show that the IL-4 receptor, like other cytokine receptor systems utilizing the common receptor gamma-chain (gammac), is also connected to a signaling pathway that involves STAT5. Both STAT5a and STAT5b become tyrosine-phosphorylated and acquire specific DNA-binding properties in response to IL-4 receptor stimulation in the murine pro-B cell line Ba/F3. In preactivated human T cells, STAT5 became activated in an IL-4-dependent fashion as assayed by IL-4-induced STAT5 translocation from the cytoplasm to the cell nucleus and by binding to cognate DNA. Moreover, stimulation of preactivated human T cells by IL-4 led to specific transcriptional up-regulation of STAT5 target genes. IL-4 receptor-mediated STAT5 activation is dependent on the presence of gammac and JAK3 within the receptor complex. In COS-7 cells, the JAK/STAT pathway leading from the IL-4 receptor to STAT5-dependent regulation of a reporter gene relied largely on coexpression of JAK3. In Ba/F3 cells, studies on signal transduction evoked by directed specific receptor homo- or heterodimerization revealed that STAT5 activation can be triggered exclusively by IL-4R heterodimers containing gammac.

The use of antibodies against the IL-2 receptor in transplantation

Humanized monoclonal antibodies that recognize the alpha chain of the IL-2 receptor (e.g. daclizumab) have been used to prevent allograft rejection, since this chain is expressed by T cells participating in allograft rejection but not by resting T cells. In a randomized trial, when added to standard cyclosporin-based immunosuppression, daclizumab significantly reduced the frequency of acute rejection of renal transplants.

The CIS family: negative regulators of JAK-STAT signaling

A family of cytokine-inducible SH2 proteins (CISs) has recently been identified and the members are growing in number. In this family, the central SH2 domain and approximately 40 amino acids at the C-terminus (CIS homology domain; CH domain) are well conserved, while the N-terminal region shares little similarity and varies in length. Most CISs appear to be induced by several cytokines and at least three of them (CIS1, CIS3 and JAB) negatively regulate cytokine signal transduction. Forced expression of CIS1 inhibits STAT5 activation by binding of CIS1 to cytokine receptors, and CIS3 and JAB directly bind to the kinase domain of JAKs, thereby inhibiting kinase activity. Therefore, these CIS family members seem to be present in a classical negative feedback loop of cytokine signaling. They may also play a role in the mutual suppression of cytokine actions frequently found in immune and inflammatory responses. Precise molecular mechanisms of the signal inhibition and their physiological functions will be addressed in the near future. The CH domain is also found in several interesting genes containing WD-40 repeats, SPRY domains, ankyrin repeats, and GTPases. However, the function of the CH domain remains to be determined.

Protein-DNA interactions in interferon-gamma signaling

The ability to rapidly activate new genes is essential for the biological effects mediated by IFN-gamma. Studies directed at understanding how these genes are induced by this ligand led to the identification of the STAT family of transcription factors. STATs are rapidly activated at the receptor, whereupon they translocate to the nucleus and bind to a unique enhancer found in the promoter of target genes. The ability to identify this IFN-gamma response element and the proteins that bind it was critical for the elucidation of this pathway. These techniques are the focus of this review.