Human ortholog of Drosophila Melted impedes SMAD2 release from TGF-β receptor I to inhibit TGF-β signaling

Drosophila melted encodes a pleckstrin homology (PH) domain-containing protein that enables normal tissue growth, metabolism, and photoreceptor differentiation by modulating Forkhead box O (FOXO), target of rapamycin, and Hippo signaling pathways. Ventricular zone expressed PH domain-containing 1 (VEPH1) is the mammalian ortholog of melted, and although it exhibits tissue-restricted expression during mouse development and is potentially amplified in several human cancers, little is known of its function. Here we explore the impact of VEPH1 expression in ovarian cancer cells by gene-expression profiling. In cells with elevated VEPH1 expression, transcriptional programs associated with metabolism and FOXO and Hippo signaling were affected, analogous to what has been reported for Melted. We also observed altered regulation of multiple transforming growth factor-β (TGF-β) target genes. Global profiling revealed that elevated VEPH1 expression suppressed TGF-β-induced transcriptional responses. This inhibitory effect was verified on selected TGF-β target genes and by reporter gene assays in multiple cell lines. We further demonstrated that VEPH1 interacts with TGF-β receptor I (TβRI) and inhibits nuclear accumulation of activated Sma- and Mad-related protein 2 (SMAD2). We identified two TβRI-interacting regions (TIRs) with opposing effects on TGF-β signaling. TIR1, located at the N terminus, inhibits canonical TGF-β signaling and promotes SMAD2 retention at TβRI, similar to full-length VEPH1. In contrast, TIR2, located at the C-terminal region encompassing the PH domain, decreases SMAD2 retention at TβRI and enhances TGF-β signaling. Our studies indicate that VEPH1 inhibits TGF-β signaling by impeding the release of activated SMAD2 from TβRI and may modulate TGF-β signaling during development and cancer initiation or progression.