Hypoxia Selectively Increases a SMAD3 Signaling Axis to Promote Cancer Cell Invasion

SIMPLE SUMMARY: While 90% of cancer patient deaths are caused by cancer dissemination through metastasis, no current therapy selectively targets this process due to the lack of selective inhibitors. Various intratumoral factors are known to promote metastasis, notably low oxygen concentration, called hypoxia, and various growth factors, including transforming growth factor β (TGFβ). Our group previously demonstrated that hypoxia enhances TGFβ-induced cancer cell invasion processes. The present study further characterizes the mechanisms involved in hypoxia-induced cancer cell invasion using in vitro, in vivo and in silico approaches and identifies a HDAC6- SMAD3 pathway that can be pharmacologically targeted to inhibit tumor progression. ABSTRACT: Transforming growth factor β (TGFβ) plays a paradoxical role in cancer, first inhibiting then promoting its progression, a duality that poses a real challenge for the development of effective TGFβ-targeted therapies. The major TGFβ downstream effectors, SMAD2 and SMAD3, display both distinct and overlapping functions and accumulating evidence suggests that their activation ratio may contribute to the dual effect of TGFβ. However, the mechanisms responsible for their selective activation remain poorly understood. Here, we provide experimental evidence that hypoxia induces the pro-invasive arm of TGFβ signaling through a selective increase in SMAD3 interaction with SMAD-Anchor for Receptor Activation (SARA). This event relies on HDAC6-dependent SMAD3 bioavailability, as well as increased SARA recruitment to EEA1+ endosomes. A motility gene expression study indicated that SMAD3 selectively increased the expression of ITGB2 and VIM, two genes that were found to be implicated in hypoxia-induced cell invasion and associated with tumor progression and metastasis in cohorts of cancer patients. Furthermore, CAM xenograft assays show the significant benefit of selective inhibition of the SMAD3 signaling pathway as opposed to global TGFβ inhibition in preventing tumor progression. Overall, these results suggest that fine-tuning of the pro-invasive HDAC6-SARA-SMAD3 axis could be a better strategy towards effective cancer treatments.