RAC1b Collaborates with TAp73α-SMAD4 Signaling to Induce Biglycan Expression and Inhibit Basal and TGF-β-Driven Cell Motility in Human Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer type characterized by a marked desmoplastic tumor stroma that is formed under the influence of transforming growth factor (TGF)-beta. Data from mouse models of pancreatic cancer have revealed that transcriptionally active p73 (TAp73) impacts the TGF-beta pathway through activation of Smad4 and secretion of biglycan (Bgn). However, whether this pathway also functions in human PDAC cells has not yet been studied. Here, we show that RNA interference-mediated silencing of TAp73 in PANC-1 cells strongly reduced the stimulatory effect of TGF-beta 1 on BGN. TAp73-mediated regulation of BGN, and inhibition of TGF-beta signaling through a (Smad-independent) ERK pathway, are reminiscent of what we previously observed for the small GTPase, RAC1b, prompting us to hypothesize that in human PDAC cells TAp73 and RAC1b are part of the same tumor-suppressive pathway. Like TAp73, RAC1b induced SMAD4 protein and mRNA expression. Moreover, siRNA-mediated knockdown of RAC1b reduced TAp73 mRNA levels, while ectopic expression of RAC1b increased them. Inhibition of BGN synthesis or depletion of secreted BGN from the culture medium reproduced the promigratory effect of RAC1b or TAp73 silencing and was associated with increased basal and TGF-beta 1-dependent ERK activation. BGN also phenocopied the effects of RAC1b or TAp73 on the expression of downstream effectors, like the EMT markers E-cadherin, Vimentin and SNAIL, as well as on negative regulation of the ALK2-SMAD1/5 arm of TGF-beta signaling. Collectively, we showed that tumor-suppressive TAp73-Smad4-Bgn signaling also operates in human cells and that RAC1b likely acts as an upstream activator of this pathway.