Summary
Plasminogen activation by urokinase-type plasminogen activator (uPA)is implicated
in tumor invasion and metastasis by the breakdown of extracellular matrix. We have
recently demonstrated the inhibitory effect of cAMP on uPA gene transcription in RC-K8
human lymphoma cells (Biochim Biophys Acta 1268: 293-9, 1995). Prostacyclin produced
by endothelial cells is shown to increase cellular cAMP levels by activating adenylate
cyclase. We, therefore, examined the effect of a stable analogue of prostacyclin,
Beraprost, on uPA production in RC-K8 cells. uPA activity gradually increased in the
conditioned medium with time. Beraprost (0.1 nM-1.0 μM) inhibited uPA accumulation
in a dose-dependent manner without affecting cell viability. Fibrin-zymography demonstrated
that high and low molecular forms of uPA were present in the conditioned medium and
that after Beraprost-treat-ment all forms of uPA decreased and no PA/PA inhibitor
complex was present. Northern blot analysis revealed that after exposure to Bera-prost,
uPA mRNA levels increased transiently and then rapidly decreased to below control
levels. Treatment with Beraprost resulted in a rapid activation of cellular cyclic
AMP-dependent protein kinase (PKA). Beraprost completely negated uPA gene expression
induced by phorbol myristate acetate, an activator of protein kinase C (PKC). These
results suggest that Beraprost inhibits uPA production by suppressing uPA gene expression
through the PKA pathway and that PKA-mediated signals are dominant in uPA gene expression
as compared to those medicated by PKC. This inhibition of uPA expression by a prostacyclin
analogue may be an important fact to explain the mechanism of anti-metastatic effects
of prostacyclin.
