Ribozyme-targeting elucidates a direct role of pleiotrophin in tumor growth

The growth and metastasis of solid tumors rely on the activities of polypeptide growth factors. However, numerous growth factors are expressed in tumors, and it is difficult to decipher which are essential for tumor progression. We found the secreted growth factor pleiotrophin (PTN) expressed at high levels in a number of human tumor cell lines as well as in tumor samples. To assess the role of PTN in tumor growth, we inactivated the PTN gene with PTN-targeted hammerhead ribozyme constructs. Cotransfection of PTN and of the ribozymes inhibited PTN-induced colony formation of PTN-responsive cells whereas a point mutant, catalytically inactive ribozyme was ineffective. Colony formation induced by transfections with a closely related growth factor (midkine) was not affected by the ribozymes. In human melanoma cells that express high levels of PTN mRNA, stable transfection with PTN-targeted ribozymes quenched production of PTN, inhibited colony formation of the cells, and prevented their tumor growth in mice. This demonstrates that expression of a growth factor can be a rate-limiting step for malignant progression and suggests that ribozymes could be used therapeutically to target tumor growth factors.