Summary
Increased levels of plasminogen activator inhibitor-1 (PAI-1) indicate an enhanced
risk of ischaemic/hypoxic cardiovascular events and a poor prognosis. The expression
of PAI-1 can be induced by various stimuli including hypoxia, insulin and insulin-like
growth factor 1 (IGF-1). The hypoxia-inducible factor-1 (HIF-1) is critical for hypoxia
or insulin/IGF-1 mediated PAI-1 induction, but the components involved in merging
the signals are not known so far. The adaptor/scaffold protein Ruk/CIN85 may be a
candidate since it plays important roles in the regulation of processes associated
with cardiovascular and oncological diseases such as downregulation of receptor tyrosine
kinases, apoptosis, adhesion and invasion. Therefore, it was the aim of this study
to investigate the involvement of Ruk/CIN85 in the regulation of PAI-1 expression.
It was found that Ruk/CIN85 induced PAI-1 mRNA and protein expression both under normoxia
and hypoxia. The induction of PAI-1 expression by Ruk/CIN85 occurred at the transcriptional
level since the half-life of PAI-1 mRNA was not affected in cells overexpressing Ruk/
CIN85 and reporter gene assays using wild-type and mutant human PAI-1 promoter luciferase
constructs showed that the hypoxia responsive element was responsible for Ruk/CIN85
effects. Further, knocking down HIF-1α abolished not only the hypoxia-dependent but
also the Ruk/CIN85-dependent PAI-1 induction. In addition, transient or stable overexpression
of Ruk/CIN85 also induced HIF-1α protein levels and HIF-1 activity and knocking down
Ruk/CIN85 reversed these effects. Thereby, Ruk/CIN85 interfered with the proline hydroxylation-dependent
HIF-1α protein destabilisation. Together, these results provide the first evidence
that Ruk/CIN85 induces PAI-1 expression via modulation of HIF-1α stability.
Keywords
Ruk/CIN85 - PAI-1 - hypoxia - HIF-1