TGF-β: an emerging player in drug resistance

Abstract

The transforming growth factor β (TGF-β) pathway acts as a double-edged sword in tumorigenesis. By constraining epithelial cell growth, TGF-β is a potent tumor suppressor. However, TGF-β also acts as a key player in the induction of epithelial-to-mesenchymal transition (EMT), thereby enhancing invasiveness and metastasis. Furthermore, TGF-β signaling has recently been correlated with resistance against both targeted and conventional anticancer agents. Here, we present data demonstrating a role for TGF-β in chemotherapy resistance in colorectal cancer (CRC). We discuss these results in the context of recent findings indicating TGF-β signaling as an emerging player in cancer drug resistance.

Keywords: EMT; TGF-β; chemotherapy; drug resistance.

Figure 1. MED12 knockdown-induced TGF-β signaling confers chemotherapy resistance. (A and B) Knockdown of MED12 was assessed on protein and RNA level (C) MED12 knockdown results in enhanced expression of a panel of TGF-β target genes (D–F) Cell viability assay of SKCO-1 cells treated with cisplatin, oxaliplatin, or 5-FU. Cell viability was measured by the conversion of resazurin into resorufin after 72 h drug treatment (n = 3).

Figure 2. C-type CRC tumors display higher p-SMAD2/3 levels in both tumor and stroma. Representative images of p-SMAD2/3 staining in A-type CRC tumors (A) and C-type tumors (B). Quantitative analysis of p-SMAD2/3 staining in tumor tissue (C). A-type vs B-type (ns), A-type vs C-type (***), B-type vs C-type (ns). (D) Quantitative analysis of p-SMAD2/3 staining in CAFs. A-type vs B-type (ns), A-type vs C-type (****), B-type vs C-type (ns). (E) Spearman correlation between p-SMAD2/3 staining in tumor cells and CAFs.

Figure 3. TGF-β signaling prevents chemotherapy induces apoptosis (A) SKCO-1 cells were treated for 24 h with 1 or 2 μM of oxaliplatin and cell lysates were harvested for western blot analysis. MED12 knockdown activates TGF-β signaling resulting in increased pSMAD2 and pERK levels and prevents apoptosis, measured by cleaved PARP (n = 3). Equal results are observed upon TGF-β treatment (B). SKCO-1 cells are treated with 10 μM oxaliplatin, 10 μM Cisplatin, or 5 μM ABT-263 and induction of apoptosis was analyzed in real time by using CellPlayer Caspase 3/7 reagent (n = 3). (D) Representative image of apoptosis induction (green) in control or MED12KD cells upon 32 h cisplatin treatment. (E) Cell viability assay of SKCO-1 cells treated with ABT-263. Cell viability was measured by the conversion of resazurin into resorufin after 72 h drug treatment (n = 3).