ACTIVATION OF JAR3, BUT NOT JAK1, IS CRITICAL TO INTERLEUKIN-4 (IL4) STIMULATED PROLIFERATION AND REQUIRES A MEMBRANE-PROXIMAL REGION OF IL4 RECEPTOR-ALPHA

The tyrosine kinases JAK1 and JARS have been shown to undergo tyrosine phosphorylation in response to interleukin-2 (IL), IL4, IL7, and IL9, cytokines which share the common IL2 receptor gamma-chain (IL2R gamma ), and evidence has been found for a preferential coupling of JAK3 to IL2R gamma and JAK1 to IL2R beta. Here we show, using human premyeloid TF-1 cells, that IL4 stimulates JAK3 to a larger extent than JAK1, ba sed upon three different evaluation criteria. These include a more vig orous tyrosine phosphorylation of JAK3 as measured by anti-phosphotyro sine immunoblotting, a more marked activation of JAK3 as determined by in vitro tyrosine kinase assays and a more manifest presence of JAK3 in activated IL4-receptor complexes. These observations suggest that I L4 receptor signal transduction does not depend on equimolar heterodim erization of JAK1 and JAK3 following IL4-induced heterodimerization of IL4R alpha and IL2R gamma. Indeed, when human IL4R alpha was stably e xpressed in mouse BA/F3 cells, robust IL4-induced proliferation and JA K3 activation occurred without detectable involvement of JAK1, JAK2, o r TYK2. The present study suggests that JAK1 plays a subordinate role in IL4 receptor signaling, and that in certain cells exclusive JARS ac tivation may mediate IL4-induced cell growth. Moreover, mutational ana lysis of human IL4R alpha showed that a membrane-proximal cytoplasmic region was critical for JARS activation, while the I4R motif was not, which is compatible with a role of JARS upstream of the recruitment of the insulin receptor substrate-1/4PS signaling proteins by IL4 recept ors.