Targeting the ATF-1/CREB Transcription Factors by Single Chain Fv Fragment in Human Melanoma: Potential Modality for Cancer Therapy

Abstract

Activating transcription factor-1 (ATF-1) and cAMP-responsive element (CRE)-binding protein (CREB) have been implicated in cAMP and Ca2+-induced transcriptional activation. The expression of the transcription factors ATF-1 and CREB is upregulated in metastatic melanoma cells. However, how overexpression of ATF-1/CREB contributes to the acquisition of the metastatic phenotype is unclear. Previously we demonstrated that quenching of CREB activity in metastatic melanoma cells by means of a dominant-negative form of CREB (KCREB) led to a decrease in their tumorigenicity and metastatic potential in nude mice. We identified two mechanisms that explain how overexpression of CREB/ATF-1 contributes to the metastatic phenotype. The first is one in which CREB/ATF-1 play an essential role in invasion by regulating the CRE-dependent expression of the metalloproteinase MMP-2 and the adhesion molecule MCAM/MUC18 genes. In the second mechanism, CREB and ATF-1 act as survival factors for human melanoma cells. Here, the effect of disrupting ATF-1 activity was investigated using intracellular expression of an inhibitory anti-ATF-1 single chain antibody fragment (ScFv). Intracellular expression of ScFv anti-ATF-1 in MeWo melanoma cells caused significant reduction in CRE-dependent promoter activation. In addition, expression of ScFv anti-ATF-1 in melanoma cells suppressed their tumorigenicity and metastatic potential in nude mice. ScFv anti-ATF-1 rendered the melanoma cells susceptible to thapsigargin-induced apoptosis in vitro and caused massive apoptosis in tumors transplanted subcutaneously into nude mice, confirming that AFT-1/CREB act as survival factors for human melanoma cells. These studies demonstrate the potential usage of ScFv anti-ATF-1 as an inhibitor of tumor growth and metastasis of solid tumors in vivo.