Suppression of v-Src transformation by andrographolide via degradation of the v-Src protein and attenuation of the Erk signaling pathway

Elevated expression and aberrant activation of the src oncogene are strongly associated with cancer initiation and progression, thereby making Src a promising molecular target for anti-cancer therapy. Through drug screening using a temperature-inducible v-Src-transformed epithelial cell line, we found that andrographolide could suppress v-Src-induced transformation and down-regulate v-Src protein expression. In addition, actin cable dissolution and E-cadherin down-regulation, features of transformed phenotype, are perturbed by andrographolide. Moreover, andrographolide promoted v-Src degradation via a ubiquitin-dependent manner. Although andrographolide treatment altered the tyrosine phosphorylation pattern in v-Src-expressing cells, it did not directly affect the kinase activity of v-Src. Both the Erk and phosphatidylinositol 3-kinase signaling pathways were strongly inhibited in andrographolide-treated v-Src cells. However, only MKK inhibitors (PD98059 and U0126) were able to cause a non-transformed morphology similar to that of andrographolide-treated v-Src cells. Moreover, overexpression of constitutively active MKK1 in v-Src cells blocked andrographolide-mediated morphological inhibition. Interestingly, andrographolide treatment could also reduce the protein level of the c-Src truncation mutant (Src531), an Src mutant originally identified from human colon cancer cells. In summary, we demonstrated that andrographolide antagonized v-Src action through promotion of v-Src protein degradation. Furthermore, attenuation of the Erk1/2 signaling pathway is essential for andrographolide-mediated inhibition of v-Src transformation. Our results demonstrate that andrographolide can act as a v-Src inhibitor and reveal a novel action mechanism of andrographolide.