The vertebrate JAK signaling pathway is an essential component of cellular response to a wide array of cytokines and growth factors. The JAK cascade is reutilized for signaling events in numerous tissues and at multiple stages of mammalian development [reviewed by [1–3]]. Many interleukins, interferons, and growth factors are among the ligands that stimulate signaling through the JAK pathway. The pathway can also be stimulated through activation of some receptor tyrosine kinases, including epidermal growth factor receptor (EGFR). As a result of its broad utilization, JAK signaling is essential for many developmental events.
Though the JAK pathway is vital to many developmental processes, strict control of JAK signaling is equally important. As with other signaling pathways, mechanisms must be in place to balance the activation of JAK pathway activity. Regulation serves to "reset" the pathway so that it will be responsive to subsequent signals and it restricts the level or duration of the signal so that it is properly interpreted by the cell. Inappropriate JAK activation is the direct cause of a specific form of acute lymphocytic leukemia (ALL) [4–6]. In addition, JAK/STAT activation has been strongly correlated with a variety of cancers, including many blood cell and immune cell transformations [reviewed by [7–9]]. Furthermore, in cell culture, constitutive activation of c-Eyk, v-src, or v-abl results in the constitutive activation of specific STATs or JAKs [10–13]. These examples highlight the necessity of regulating JAK/STAT activation.
Because of the need to limit JAK activity, it is not surprising that there are several conserved protein families that regulate JAK activation [reviewed by [3, 14, 15]]. These include phosphatases, Protein Inhibitors of Activated STATs (PIAS), and, the best characterized, the suppressors of cytokine signaling (SOCS) family. In mammals, eight different SOCS genes have been found . These SOCS proteins have a distinctive modular architecture: a central SH2 domain followed by a carboxyl terminal SOCS domain, while the amino termini are quite divergent. Biochemical investigations have revealed that SOCS proteins use multiple mechanisms to regulate activity of the JAK pathway [see reviews, [3, 9]]. First, the SOCS SH2 domain can bind to the phosphorylated receptor, thereby prohibiting access to positive effectors of the pathway. Second, at least some SOCS can specifically inhibit the catalytic activity of JAKs. Lastly, SOCS binding to activated JAK pathway components may target those proteins for degradation. The SOCS motif interacts with the elongins B and C, which bind to cullins and are E3 ubiquitin ligases [17, 18]. Addition of ubiquitin to the bound proteins would target them for proteasomal degradation. Therefore, the negative influence of SOCS on its substrates may be due to multiple distinct mechanisms.
Use of the JAK signaling pathway for developmental processes is not restricted to mammals. Indeed, the JAK cascade is evolutionarily conserved, and can be found as an intact signaling pathway even in insects [3, 19–21]. In Drosophila, the JAK pathway is involved in embryonic patterning, sex determination, blood cell development, patterning of adult structures, planar polarity of photoreceptor clusters, maintenance of stem cells in spermatogenesis, and follicle cell patterning and function [see reviews [19, 21]]. Furthermore, the fly JAK pathway must also be properly regulated to avoid deleterious effects. As in vertebrates, hyperactive JAK signaling has also been shown to directly cause neoplastic cell growth in Drosophila. Two dominant gain-of-function alleles of hopscotch result in hypertrophy of the larval lymph glands, the hematopoietic organ, and melanotic masses [22–24]. Excess activity in the blood system causes overproliferation and differentiation of the macrophage-like blood cells, creating leukemia-like effects. Inappropriate activity in the developing tissues of the adult fly can also cause alteration of the development of the adult thorax, wing veins, head, eyes, and ovaries [22, 25–27].
Of the eight mammalian SOCS, four have been studied extensively (CIS, SOCS1-3). These genes have been shown to respond to JAK pathway activation and subsequently are able to downregulate its activity as described above, completing a classical negative feedback loop. In comparison, very little is known of the remaining four. Here we present the identification and characterization of Drosophila Socs44A. It contains the same modular domain architecture as mammalian SOCS and shows greatest sequence similarity to the relatively uncharacterized SOCS6 and SOCS7. We show that, unlike the previously studied Drosophila Socs36E [28, 29], Socs44A expression in embryogenesis is independent of JAK pathway activity. However, Socs44A is able to regulate the JAK cascade in embryogenesis, but not in oogenesis. Finally, Socs44A genetically interacts with and upregulates the EGFR/MAPK pathway. The characteristics of Socs44A that distinguish it from the canonical Socs36E may be representative of features that are shared with the class of less-defined mammalian SOCS genes.