TGF-β mediatesPTENsuppression and cell motility through calcium-dependent PKC-α activation in pancreatic cancer cells

Abstract

Transforming growth factor-beta (TGF-beta) suppresses growth via the TGF-beta-SMAD pathway but promotes growth in cancer cells with disrupted SMAD signaling and corresponds to an invasive phenotype. TGF-beta also downregulates the tumor suppressor PTEN that is rarely mutated in sporadic pancreatic cancer; this downregulation may mediate cell proliferation and invasiveness, but the mechanism is unknown. Here, we examined whether TGF-beta modulation of PTEN was mediated by protein kinase C (PKC). We have previously demonstrated that SMAD4-null BxPc-3 pancreatic cancer cells treated with TGF-beta1 (10 ng/ml) suppressed PTEN expression and increased cell proliferation. TGF-beta-treated cells were examined for PKC activation and its coupling to PTEN expression, utilizing pharmacological and knockdown methods. Calcium mobilization and cell migration were also examined. In BxPc-3 cells, only two PKC isoforms were activated by TGF-beta, and PTEN downregulation by TGF-beta was specifically mediated by PKC-alpha. In parallel, TGF-beta rapidly induced an increase in cytoplasmic free calcium from intracellular stores, consistent with subsequent PKC-alpha activation. The TGF-beta-induced increase in cell migration was blocked by knockdown of PKC-alpha. Thus calcium-dependent PKC-alpha mediates TGF-beta-induced transcriptional downregulation of PTEN, and this pathway promotes cell migration in a SMAD4-null environment. The TGF-beta-PKC-alpha-PTEN cascade may be a key pathway for pancreatic cancer cells to proliferate and metastasize.