HUMAN BREAST-CANCER GROWTH INHIBITED IN-VIVO BY A DOMINANT-NEGATIVE PLEIOTROPHIN MUTANT

Pleiotrophin (PTN) is a recently described 18- kDa heparin binding gro wth/differentiation factor. It also is a proto-oncogene; cells transfo rmed by the Ptn gene form highly angiogenic tumors when implanted into the nude mouse. PTN may be an important regulator of transformation i n other tumors, because constitutively high levels of expression of th e pleiotrophin (Ptn) gene are found in human breast cancer and other m alignant cell lines, and its levels of expression are high in many hum an tumor specimens. To determine whether PTN is an important regulator of the malignant phenotype of human breast cancer cells, we construct ed a mutant cDNA to encode a truncated PTN designed to heterodimerize with the product of the endogenous Ptn gene during processing, The mut ant gene product blocked transformation of NIH 3T3 cells by the wild t ype (wt) Ptn gene product, The mutant Ptn cDNA was then introduced int o human breast cancer MDA-MB-231 cells, and clonal lines that stably e xpress the mutant Ptn cDNA were selected. The truncated PTN was shown to form heterodimers with the endogenous Ptn gene product in these cel ls. Furthermore, the MDA-MB-231 cells that express the mutant Ptn gene were no longer transformed; they failed to form plaques or colonies i n soft agar and were unable to form tumors in the athymic nude mouse, The results establish an important role of PTN in the dysregulated gro wth of human breast cancer cells and suggest that constitutive express ion of PTN may be essential to the malignant phenotype of human breast cancers in vivo.