Autocrine TGFβ1 Opposes Exogenous TGFβ1-Induced Cell Migration and Growth Arrest through Sustainment of a Feed-Forward Loop Involving MEK-ERK Signaling

Simple Summary Transforming growth factor (TGF) beta signaling is intimately involved in nearly all aspects of tumor development and is known for its role as both a tumor suppressor in benign tissues and a tumor promoter in advanced cancers. This dual role is also reflected by cancer cell-produced TGF beta that eventually acts on the same cell(s) in an autocrine fashion. Recently, we observed that endogenous TGFB1 can inhibit rather than stimulate cell motility in cell lines with high autocrine TGF beta production. The unexpected anti-migratory role prompted us to evaluate how autocrine TGF beta 1 impacts the cells' migratory and proliferative responses to exogenous (recombinant human) TGF beta. Surprisingly, endogenous TGFB1 opposed the migratory and growth-inhibitory responses induced by exogenous TGF beta 1 by driving a self-perpetuating feedforward loop involving MEK-ERK signaling. Our observation has implications for the use of TGF beta signaling inhibitors in cancer therapy. Autocrine transforming growth factor beta (aTGF beta) has been implicated in the regulation of cell invasion and growth of several malignant cancers such as pancreatic ductal adenocarcinoma (PDAC) or triple-negative breast cancer (TNBC). Recently, we observed that endogenous TGFB1 can inhibit rather than stimulate cell motility in cell lines with high aTGF beta production and mutant KRAS, i.e., Panc1 (PDAC) and MDA-MB-231 (TNBC). The unexpected anti-migratory role prompted us to evaluate if aTGF beta 1 may be able to antagonize the action of exogenous (recombinant human) TGF beta (rhTGF beta), a well-known promoter of cell motility and growth arrest in these cells. Surprisingly, RNA interference-mediated knockdown of the endogenous TGFB1 sensitized genes involved in EMT and cell motility (i.e., SNAI1) to up-regulation by rhTGF beta 1, which was associated with a more pronounced migratory response following rhTGF beta 1 treatment. Ectopic expression of TGFB1 decreased both basal and rhTGF beta 1-induced migratory activities in MDA-MB-231 cells but had the opposite effect in Panc1 cells. Moreover, silencing TGFB1 reduced basal proliferation and enhanced growth inhibition by rhTGF beta 1 and induction of cyclin-dependent kinase inhibitor, p21(WAF1). Finally, we show that aTGF beta 1 promotes MEK-ERK signaling and vice versa to form a self-perpetuating feedforward loop that is sensitive to SB431542, an inhibitor of the TGF beta type I receptor, ALK5. Together, these data suggest that in transformed cells an ALK5-MEK-ERK-aTGF beta 1 pathway opposes the promigratory and growth-arresting function of rhTGF beta 1. This observation has profound translational implications for TGF beta signaling in cancer.