Splicing factor proline- and glutamine-rich (SFPQ) protein causes transcriptional repression of SNAIL to counteract TGF-β signaling.

TGF-β is known to regulate several embryonic and adult signaling pathways. Moreover, this signaling pathway regulates several cellular functions including differentiation, cell division, angiogenesis, hematopoiesis, and cell migration. However, studies suggest that an uncontrolled activation of TGF-β signaling may contribute to many human diseases. Therefore, counter-regulatory mechanism(s) to restrain abrupt TGF-β activation during cellular homeostasis is necessary to maintain an adequate balance of TGF-β downstream signaling. TGF-β through Smad complex activation causes transcriptional regulation of many transcription factors including Snail which act as an immediate-early response gene in TGF-β signaling. Herein, for the first time, we report that Splicing factor proline- and glutamine-rich (SFPQ), an RNA binding paraspeckles-associated protein works as a transcriptional repressor of Snail. We first assessed SFPQ levels in a pro-fibrotic rat model of subtotal nephrectomy and observed a significant reduction in its expression. In endothelial cells (ECs), TGF-β treatment decreased SFPQ protein levels without affecting transcript levels, indicating post-translational regulation. Inhibition studies revealed ubiquitination-dependent proteasomal degradation of SFPQ upon TGF-β stimulation. Notably, TGF-β induced cytosolic translocation of SFPQ, leading to reduced nuclear levels. Functionally, SFPQ knockdown enhanced, while its overexpression suppressed, TGF-β-induced Snail expression. Although SFPQ interacted with transcription factors such as Smad2/3, Smad4, and N1-ICD, TGF-β did not alter these associations. Instead, ChIP-qPCR analysis demonstrated SFPQ enrichment at the E-box promoter and regions proximal to the Snail transcription start site. Collectively, these findings support a role for SFPQ in regulating Snail expression and modulating TGF-β-associated transcriptional responses in the experimental models examined in this study.