Abstract 101: JAK-independent STAT5 signaling as a node of therapeutic vulnerability in glioblastoma

Abstract

Abstract Glioblastoma (GBM), the most common primary brain tumor in adults, comprises all WHO Grade IV gliomas of astrocytic origin. Despite extensive characterization of the molecular and genetic features of GBM, there remains a paucity of viable treatment options, and this disease confers a dismal 5% 5-year survival rate. Genomic amplification of epidermal growth factor receptor (EGFR) occurs in 40-60% of primary GBM. Half of all EGFR-amplified cases of GBM also harbor EGFRvIII, a constitutively active, truncated variant of EGFR. EGFR and EGFRvIII transduce oncogenic signals via cytoplasmic mediators, including the JAK/STAT signal transduction pathway. The oncogenic role of STAT3 has been well-studied in CNS malignancies; STAT3 has been found to induce the expression of target genes involved in the proliferative and invasive phenotypes of GBM. However, little is known about the mechanism and role of STAT5 activation in GBM progression. Microenvironmental cues and EGFRvIII signaling, lost in adherent cell culture, are required for STAT5 activation, making STAT5 difficult to study by conventional cell culture methods. Our recent data from patient-derived xenograft (PDX) cultures demonstrates that activation of STAT5 is important in GBM cell invasion and survival. Further, STAT5 activation is present predominantly at the invasive periphery of the tumor. In this study, we seek to characterize the mechanism and role of STAT5 activation downstream of EGFRvIII in GBM. RNA sequencing of PDX tumors expressing stable shRNA knockdown of STAT3 or STAT5 indicate both redundant and distinct transcriptional changes induced by STAT3 and STAT5 in GBM. We also find that while STAT3 activation is dependent upon JAK1/2, STAT5 activation is JAK-independent downstream of EGFRvIII in GBM. Depletion of JAK1/2 expression by siRNA oligonucleotides and pharmacologic inhibition of JAK1/2 suppressed STAT3 activation but not STAT5. In fact, targeting JAK2 with kinase inhibitor WP1066 suppressed STAT3 activation, but increased activation of STAT5 in GBM cells expressing EGFR/EGFRvIII. Inhibition of Src family kinases (SFK), a family of upstream signaling effectors of the JAK/STAT pathway, attenuates both STAT3 and STAT5 activation. These results suggest that activation of STAT5 may provide a therapeutic escape mechanism to JAK-STAT3 inhibition in GBM, necessitating dual STAT3 and STAT5 inhibition to successfully inhibit the distinct oncogenic signals transmitted by STAT3 and STAT5 to promote GBM cell survival and resistance. Citation Format: Mylan Blomquist, Serdar Tuncali, Christopher Sereduk, Jean Kloss, Joseph Loftus, Nhan Tran. JAK-independent STAT5 signaling as a node of therapeutic vulnerability in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 101.