Abstract 4005: JAK/STAT activation following Src inhibition is mediated by the loss of SOCS2 expression

Systemic therapy that results in both decreased invasion and significant cytotoxicity would be ideal to treat head and neck squamous cell carcinoma (HNSCC). One promising therapeutic target is Src due to its well-defined roles in both invasion and survival. STAT3 is a key mediator of the oncogenic effects of Src. However we previously found that sustained Src inhibition resulted in only transient inhibition of STAT3. Likewise, Src inhibition resulted in a universal reduction in invasion, but a more variable effect on apoptosis. Restoring STAT3 inhibition enhanced apoptosis. Previously we showed that STAT3 reactivation was mediated by JAK2 kinase activation and altered JAK2-STAT3 binding. Reactivation of JAK2/STAT3 suggests a feedback loop, possibly via the loss of a negative regulator. Of the three known feedback loops that regulate JAK/STAT function, we considered the most likely candidate to be the suppressors of cytokine signaling (SOCS) proteins, which can compete for STAT binding and inhibit JAK kinase activity. To determine if SOCS proteins are involved in JAK/STAT3 reactivation, we measured RNA levels of all 8 SOCS family members in 5 HNSCC cell lines using quantitative PCR. The expression of most of the SOCS decrease after initial Src inhibition, but recover, consistent with their regulation by STAT3. However, the expression of SOCS2 decreased in all 5 lines. Likewise, we found that the addition of a molar stoichiometric amount of recombinant SOCS2 inhibits Jak2 kinase activity in vitro. To confirm that SOCS2 regulates STAT3 activity, we knocked-down SOCS2 by siRNA which resulted in the activation of STAT3. Similarly, transient overexpression of SOCS2 resulted in significant downregulation of activated STAT3. Others have reported that STAT5 can regulate SOCS2 expression. Consistent with this, we found that Src inhibition led to durable STAT5 inhibition. Moreover, the depletion of both isoforms of STAT5 (a and b) with siRNA resulted in a dramatic downregulation of the SOCS2 expression. To determine the biological significance of SOCS2 expression, we discovered that specific SOCS2 depletion reduced HNSCC cytotoxicity following Src inhibition. Our results demonstrate that sustained Src inhibition leads to the loss of SOCS2 expression via STAT5 inhibition. The loss of SOCS2 allows the recovery of JAK2 kinase activity, JAK2/STAT3 binding, STAT3 activation, and cell survival despite durable Src inhibition. To our knowledge, this feedback pathway has never been described previously. Given that pharmacologic Src and JAK inhibitors are currently being evaluated in clinical trials, these results have significant implications for cancer therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4005.