Targeting Endogenous Transforming Growth Factor β Receptor Signaling in SMAD4-Deficient Human Pancreatic Carcinoma Cells Inhibits Their Invasive Phenotype 1 (original) (raw)

Transforming growth factor-␤ (TGF-␤) suppresses tumor formation by blocking cell cycle progression and maintaining tissue homeostasis. In pancreatic carcinomas, this tumor suppressive activity is often lost by inactivation of the TGF-␤-signaling mediator, Smad4. We found that human pancreatic carcinoma cell lines that have undergone deletion of MADH4 constitutively expressed high endogenous levels of phosphorylated receptor-associated Smad proteins (pR-Smad2 and pR-Smad3), whereas Smad4-positive lines did not. These elevated pR-Smad levels could not be attributed to a decreased dephosphorylation rate nor to increased expression of TGF-␤ type I (T␤R-I) or type II (T␤R-II) receptors. Although minimal amounts of free bioactive TGF-␤1 and TGF-␤2 were detected in conditioned medium, treatment with a pan-specific (but not a TGF-␤3 specific) TGF-␤-neutralizing antibody and with anti-␣ V ␤ 6 integrin antibody decreased steady-state pSmad2 levels and activation of a TGF-␤-inducible reporter gene in neighboring cells, respectively. Thus, activation of TGF-␤ at the cell surface was responsible for the increased autocrine endogenous and paracrine signaling. Blocking T␤R-I activity using a selective kinase inhibitor (SD-093) strongly decreased the in vitro motility and invasiveness of the pancreatic carcinoma cells without affecting their growth characteristics, morphology, or the subcellular distribution of E-cadherin and F-actin. Moreover, exogenous TGF-␤ strongly stimulated in vitro invasiveness of BxPC-3 cells, an effect that could also be blocked by SD-093. Thus, the motile and invasive properties of Smad4deficient pancreatic cancer cells are at least partly driven by activation of endogenous TGF-␤ signaling. Therefore, targeting the T␤R-I kinase represents a potentially powerful novel therapeutic approach for the treatment of this disease.

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