Abstract
Simple SummaryPTEN is a protein highly altered in endometrial cancer. PTEN mutation or deficiency leads to the activation of other downstream proteins that are important to the development of cancers. In this study, we have identified the SMAD2/3 proteins as targets of PTEN deficiency. We have found that loss of PTEN in endometrial cells leads to SMAD2/3 activation. To investigate the role of SMAD2/3 activation downstream of PTEN deficiency, we have used endometrial cells lacking both PTEN and SMAD2/3 proteins. These cells display even more tumorigenic potential than cells lacking only PTEN. These results suggest that SMAD2/3 acts as an obstacle for cancer development triggered by PTEN loss.TGF-β has a dichotomous function, acting as tumor suppressor in premalignant cells but as a tumor promoter for cancerous cells. These contradictory functions of TGF-β are caused by different cellular contexts, including both intracellular and environmental determinants. The TGF-β/SMAD and the PI3K/PTEN/AKT signal transduction pathways have an important role in the regulation of epithelial cell homeostasis and perturbations in either of these two pathways’ contributions to endometrial carcinogenesis. We have previously demonstrated that both PTEN and SMAD2/3 display tumor-suppressive functions in the endometrium, and genetic ablation of either gene results in sustained activation of PI3K/AKT signaling that suppresses TGF-β-induced apoptosis and enhances cell proliferation of mouse endometrial cells. However, the molecular and cellular effects of PTEN deficiency on TGF-β/SMAD2/3 signaling remain controversial. Here, using an in vitro and in vivo model of endometrial carcinogenesis, we have demonstrated that loss of PTEN leads to a constitutive SMAD2/3 nuclear translocation. To ascertain the function of nuclear SMAD2/3 downstream of PTEN deficiency, we analyzed the effects of double deletion PTEN and SMAD2/3 in mouse endometrial organoids. Double PTEN/SMAD2/3 ablation results in a further increase of cell proliferation and enlarged endometrial organoids compared to those harboring single PTEN, suggesting that nuclear translocation of SMAD2/3 constrains tumorigenesis induced by PTEN deficiency.
Highlights
TGF-β is a multimodal factor that participates in many biological and physiological processes
To ascertain whether the increase of SMAD2/3 expression was due to an increase of its expression, we performed an Reverse Transcriptase−Polymerase Chain Reaction (RT-PCR) analysis of SMAD2, SMAD3 and SMAD4 mRNA of Cre:ER+/−;PTENfl/fl treated with tamoxifen to induce PTEN deficiency
To further analyze subcellular distribution of SMADs after PTEN deletion, Cre:ER+/−;PTENfl/fl organoid cultures were treated with tamoxifen to induce PTEN deletion, and SMAD2/3 and SMAD4 localization was assessed by immunofluorescence
Summary
TGF-β is a multimodal factor that participates in many biological and physiological processes. TGF-β signaling pathways are triggered by its interaction to the TGF-β type II receptor (TGFβRII) that, in turn, interacts with the TGF-β type I receptor (TGFβRI or ALK5). TβRII phosphorylates TGFβRI and activates downstream effectors that transduce TGF-β signaling. The canonical TβRs signaling is conducted by the SMAD transcription factor family [2,3,4]. In addition to canonical SMAD signaling, TGF-β triggers other signaling pathways frequently referred as “non-SMAD” branch of TGF-β signaling [6,7]. These non-canonical TGF-β pathways include Rho-like GTPase signaling pathway, MAP kinase pathway and the Phosphatidylinositol-3 kinase/AKT (PI3K/AKT) signaling pathway
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