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

TGF-{beta} 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-{beta} signaling may contribute to many human diseases. Therefore, counter-regulatory mechanism(s) to restrain abrupt TGF-{beta} activation during cellular homeostasis is necessary to maintain an adequate balance of TGF-{beta} downstream signaling. TGF-{beta} through Smad complex activation causes transcriptional regulation of many transcription factors including Snail which act as an immediate-early response gene in TGF-{beta} 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 confirmed a significant reduction in the expression level of SFPQ in the kidney glomeruli of rats that underwent subtotal nephrectomy. Endothelial cells (EC) treated with TGF-{beta} exhibited loss of SFPQ protein level without altering its transcript level. Inhibition of proteasomal or autophagosome-lysosome pathway revealed ubiquitination-dependent proteasomal degradation of SFPQ upon TGF-{beta} challenge. Prior to degradation, TGF-{beta} treatment resulted in the cytosolic export of SFPQ thereby diminishing nuclear SFPQ level. Knockdown of SFPQ augmented TGF-{beta}-dependent increase in Snail level while overexpression of SFPQ reversed TGF-{beta} induced Snail expression. Although SFPQ exhibited association with many transcription factors including Smad2/3, Smad4, and N1-ICD which regulate Snail gene expression, TGF-{beta} failed to alter the association of SFPQ with these transcription factors. Instead, through ChIP-qPCR analysis, we confirmed the enrichment of SFPQ in E-box promoter region and coding region proximal to TSS of the Snail gene. This study is the first to report SFPQ as a transcriptional repressor of Snail thereby regulating TGF-{beta} signaling during cellular homeostasis.