TGF-beta autocrine pathway and MAPK signaling promote cell invasiveness and in vivo mammary adenocarcinoma tumor progression

Abstract

Breast cancer progression and metastasis have been linked to abnormal signaling by transforming growth factor-beta (TGF-beta) cytokines. In early-stage breast cancers, TGF-beta exhibits tumor suppressor activity by repressing cell proliferation and inducing cell death, whereas in advanced-stage tumors, TGF-beta promotes invasion and metastatic dissemination. The molecular mechanisms underlying pro-oncogenic activities of TGF-beta are not fully understood. The present study validates the role of TGF-beta signaling in cancer progression and explores mediators of pro-oncogenic TGF-beta activities using the LM3 mammary adenocarcinoma cell line, derived from a spontaneous murine mammary adenocarcinoma. Expression of kinase-inactive TGF-beta receptors decreased both basal and TGF-beta-induced invasion. Analysis of signal transduction mediators showed that p38MAPK and MEK contribute to TGF-beta stimulation of cell motility and invasion. TGF-beta disrupted the epithelial actin structures supporting cell-cell adhesions, and increased linear actin filaments. Moreover, MEK and p38MAPK pathways showed opposite effects on actin remodeling in response to TGF-beta. Blockade of Raf-MEK signaling enhanced TGF-beta induction of actin stress-fibers whereas p38MAPK inhibitors blocked this effect. A novel observation was made that TGF-beta rapidly activates the actin nucleation Arp2/3 complex. In addition, TGF-beta stimulated matrix metalloproteinase MMP-9 secretion via a MAPK-independent pathway. Experiments using syngeneic mice showed that kinase-inactive TGF-beta receptors inhibit the first stages of LM3 tumor growth in vivo. Our studies demonstrate that autocrine TGF-beta signaling contributes to the invasive behavior of mammary carcinoma cells. Moreover, we show that both MAPK-dependent and -independent pathways are necessary for TGF-beta-induced effects. Therefore, MEK-ERK and p38 MAPK pathways are potential venues for therapeutic intervention in pro-oncogenic TGF-beta signaling.