309. STAT5 Phosphorylation in Malignant Melanoma Is Mediated through SRC and JAK-1 Kinases and Exerts Anti-Apoptotic Effects

Abstract

Altered signaling pathways are key regulators of cellular functions in tumor cells. Constitutive activation of signal transducer and activator of transcription (STAT) proteins, particularly STAT3 and STAT5, may be involved in tumor formation and progression. In the present study, we have investigated the role of STAT5 in cutaneous metastases of human malignant melanoma. In 13 of 21 (76%) human melanoma metastases STAT5 was activated in comparison to normal human melanocytes (NHM). Stat5b transcripts were upregulated approximately 4-fold in the examined samples. The STAT5 target genes BCL-xL, BCL-2 and c-MYC were frequently upregulated in melanoma metastases. The investigation of the underlying mechanisms in melanoma cell lines revealed specific STAT5 activation by recombinant human epidermal growth factor (rEGF). rEGF-induced activation of STAT5 occurred in vitro particularly through the non-receptor tyrosine kinases SRC and JAK1. Stimulation with rEGF led to increased STAT5 DNA-binding acitivity, suggesting an important role of the EGF-receptor in STAT5 signaling and malignant progression of melanoma. Transfection with dominant-negative Stat5b led to enhancement of cell death and G1 arrest in A375 melanoma cells and correlated with reduced expression of anti-apoptotic BCL-xL. Our study suggests that activated STAT5 acts as a survival factor in the development and growth of human melanoma and may hold potential for molecular therapy.