Background: Integrins, cell-surface receptors that mediate adhesive interactions between cells and the extracellular matrix (ECM), play an important role in cancer progression. Expression of the vitronectin receptor alphavbeta3 integrin correlates with increased invasive and metastatic capacity of malignant melanomas, yet it remains unclear how expression of this integrin triggers melanoma invasion and metastasis.
Results: Two melanoma cell lines C8161.9 and M14 both express high levels of alphavbeta3 integrin and adhere to vitronectin. However, only the highly metastatic C8161.9 cells are capable of invading vitronectin-enriched Matrigel in an alphavbeta3-depenent manner. Elevated levels of PKCalpha and PKCdelta, and activated Src were detected specifically in the highly metastatic melanoma cells, but not in the low metastatic M14 cells. Inhibition of Src or PKC activity suppressed alphavbeta3-dependent invasion. Furthermore, over expression of Src or PKCalpha and PKCdelta was sufficient to confer alphavbeta3-dependent invasiveness to M14 cells. Stress fiber formation and focal adhesion formation were almost completely absent in C8161.9 cells compared to M14 cells. Inhibition of Src signaling was sufficient to restore normal actin architecture, and resulted in decreased p190RhoGAP phosphorylation and enhanced RhoA activity. Src had no effect on Rac activity. Loss of PKCalpha expression, but not PKCdelta, by siRNA inhibited Rac and PAK activity as well as invasiveness. Loss of PKCalpha restored focal adhesion formation and partially restored stress fiber formation, while loss of PKCdelta primarily restored stress fibers.
Conclusion: The misregulated expression of PKCalpha and PKCdelta and elevated Src activity in metastatic melanoma cells is required for efficient alphavbeta3-mediated invasion. PKCalpha and Src enhance alphavbeta3-mediated invasion in part by increasing the GTPase activity of Rac relative to RhoA. PKCalpha influences focal adhesion formation, while PKCdelta controls stress fibers.