Molecular Mechanisms Associated with Sustained Urokinase Plasminogen Activator Expression in Metastatic Breast Cancer Cells

Abstract

Elevated levels of urokinase plasminogen activator (uPA) are detected in various aggressive cancer types and are closely associated with poor prognosis of cancer. While uPA can be transiently upregulated by diverse extracellular stimuli, only sustained uPA expression contributes to breast cancer invasion/metastasis. However, how sustained uPA expression is regulated and achieved in invasive breast cancer cells is not understood. The overall goal of this study is to elucidate the mechanism responsible for sustained uPA expression. Here, we show that sustained uPA expression is regulated in a mechanism distinct from transiently induced uPA expressions. Interleukin enhancerbinding factor 3 (ILF3) facilitates uPA expression by both activating uPA gene transcription and inhibiting the processing of uPA mRNA-targeting pri-miRNAs. Another part of this study is to investigate the role of the miRNA system in sustained uPA expression and cancer cell invasion. Knockdown of Drosha, DGCR8, or Dicer, key components of miRNA processing machinery led to substantially higher uPA expression and increased in vitro invasion in invasive breast cancer cells, although it was unable to increase the uPA level in non-invasive breast cancer cells. In fact, we identified that uPA mRNA was a direct target of miR-193a/b and miR-181a. Interestingly, we found that the levels of mature miR-193a, miR-193b, and miR-181a, but not their respective primary miRNAs, were lower in high uPA-expressing cells compared to low uPA-expressing cells. Furthermore, the high levels of mature miR-193a, miR-193b, and miR-181a in partly attributed to lower Drosha/DGCR8 expression in high uPA-expressing cells. To identify mechanism pertinent to ILF3 regulation of sustained uPA expression, we found that sustained uPA expression is sensitive to pan-PKC inhibitor and c/nPKC inhibitor, but not cPKC inhibitor, suggesting that ore or more nPKC isotypes are critical for sustained uPA expression. With the aid of siRNAs, we showed that PKCδ and PKCη were involved in sustained uPA expression. To functionally connect PKCδ/η and ILF3, We revealed that the knockdown of PKCδ or PKCη led to ILF3 accumulation in the cytoplasm, the reduction of primary miRNA binding ability, and enhanced production of mature miR-193a/b and miR-181a. These results indicate that ILF3 is a linker between PKCδ/η and uPA expression, and the PKCδ/η-ILF3 signaling axis is important for sustained uPA expression in breast cancer cells.