Regulation in vivo of the growth of Leydig cell tumors by antisense ribonucleic acid for parathyroid hormone-related peptide.

PTH-related peptide (PTHrP) has been shown to be the major mediator of hypercalcemia of malignancy, but may also exert effects on cell growth and differentiation. The Leydig cell tumor H-500, when implanted in Fischer rats, produces abundant PTHrP and eventually causes the death of the host animal. In the present study we have used antisense RNA technology to block the effects of PTHrP in H-500 Leydig tumor cells in vivo. The full-length rat PTHrP complementary DNA encoding amino acid -36-->141 was subcloned as an EcoRI-BglII insert in the antisense orientation into the mammalian expression vector pRc/CMV to produce the plasmid pRc-PAS. This plasmid was then stably transfected into the H-500 Leydig tumor cells with a Lipofectin reagent. After selection with the neomycin derivative G-418, a stable cell line, H-500-PTHrP-AS, was obtained which showed 80% inhibition of endogenous PTHrP messenger RNA compared to wild-type or vector-only transfected H-500 cells. Conditioned culture medium from these experimental cells showed a marked decrease in PTHrP immunoreactivity and in the ability of the medium to stimulate adenylate cyclase in UMR-106 rat osteosarcoma cells. Furthermore, inhibition of PTHrP production resulted in a significant increase in the doubling time of the H-500 cells. Transfection of the experimental plasmid into Rat-2 fibroblasts, which do not produce PTHrP, had no effect on cell growth. Control and experimental cells were then implanted sc into male Fischer rats. Animals were killed at timed intervals, and their tumor volumes were determined. Experimental animals receiving cells transfected with antisense PTHrP plasmid showed near-normal levels of plasma calcium and decreased expression of tumoral PTHrP messenger RNA. These animals also showed a 30-70% lower tumor volume during the course of the experiment compared to control animals. These studies have demonstrated that PTHrP can play a role as a promoter of tumor growth in vitro and in vivo.