Redirecting RNA splicing by SMAD3 turns TGF-β into a tumor promoter

Abstract

Transforming growth factor b (TGF-b) is a well-known growth inhibitor of normal epithelial cells, but it is also secreted by solid tumors to promote cancer progression. Our recent discovery of SMAD3-PCBP1 complex with direct RNA-binding properties has shed light on how this conversion is implemented by controlling pre-mRNA splicing patterns. Transforming growth factor b (TGF-b) is well known to be a potent growth inhibitor and tumor suppressor; however, at the time when TGF-b was discovered, it was first identified as an activity that transformed normal fibroblasts to form colonies in soft-agar in cooperation with epidermal growth factor (EGF) or TGF-a, both of which are ligands of EGF receptors (EGFR). 1 Fast forward 35 y to present time, it has been appreciated that the two seemingly opposing functions of TGF-b are cell-contextual dependent: in normal epithelial, endothelial, and hematopoietic cells. TGF-b strongly inhibits growth, but this inhibition is lost in late stages of carcinomas with elevated levels of signaling from the mitogen-activated protein kinase (MAPK) pathway sustained by oncogenic mutations. 2,3 Multiple mechanisms by which TGF-b inhibits cell growth have been unraveled, chief among them are transcriptional induction of cyclin dependent kinase (CDK) inhibitors and suppression of proto-oncogene MYC and apoptosis inducer B-cell lymphoma 2 (BCL2). On the other hand, advanced tumors are frequently found with high levels of TGF-b that promotes tumor progression by inducing epithelial to mesenchymal transition (EMT) and invasion. 4,5 Several key transcriptional targets of TGF-b including Snail Family Zinc Finger 1 (SNAl1), Zinc finger E-box binding homeobox 1 (ZEB1), high mobility group AT-Hook 2 (HMGA2), and forkhead box A1 (FOXA1) have been shown to play important roles in TGF-b-induced EMT, while other nontranscriptional mechanisms have also been reported. Despite a vast body of the literature on the nuts and bolts of how TGF-b signals, the mechanism that underpins the switch of TGF-b from a growth inhibitor to a tumor promoter still remains elusive. In searching for proteins that confer regulation of the key TGF-b pathway transcription effector SMAD3 via phosphorylation of threonine 179 (T179) in the linker region, we identified an RNA-binding protein poly(RC) binding protein 1 (PCBP1, also known as hnRNP E1), and serendipitously discovered that by partnering with PCBP1, SMAD3 is brought onto the pre-mRNA of a cancer stem cell marker gene CD44 to regulate its alternative splicing. 6 In addition to CD44, our global RNA-seq study revealed a plethora of cancers genes whose splicing patterns are altered by the SMAD3-PCBP1 interaction in favor of tumor progression. These findings let us to propose that regulation of alternative splicing by the concerted action of receptor-activated SMAD3 and PCBP1 is a key mechanism that propels TGF-b to a tumor promoter (Fig. 1).According to the current paradigm of TGF-b signaling, SMAD3 is activated by phosphorylation at the carboxyl terminus SSXS motif b...

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Transforming growth factor-β in tumour development

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