THE PRIMARY TUMOR IS THE PRIMARY SOURCE OF METASTASIS IN A HUMAN-MELANOMA SCID MODEL - IMPLICATIONS FOR THE DIRECT AUTOCRINE AND PARACRINE EPIGENETIC REGULATION OF THE METASTATIC PROCESS

Although previous autopsy and experimental studies had indicated that metastases can metastasize, the question of whether metastases from me tastases increasingly contribute to the overall metastatic burden is c rucial to the basic question of whether the metastatic process is more directly regulated by genetic or by epigenetic mechanisms. The highly metastatic human C8161 melanoma was transfected with either pSV2neo o r pSV2hygro and clones of neo-C8161 and hyg-C8161 were injected intrav enously and subcutaneously in SCID mice. In combination experiments, b oth the timing and the size of inoculum of tumor cells were titrated t o ensure that the hematogenously injected cells disseminated almost ex clusively to the lungs and that the overall pulmonary burden was equal to the primary tumor. In s.c. injection experiment, no spontaneous me tastases ever developed when the primary tumor was extirpated before i t had grown to more than 0.5 cm in diameter. When the primary tumor ap proached 1 cm in diameter, widely-disseminated metastases developed wi thin lungs, liver, subcutaneous sites and other internal viscera. In t he combination-injection experiments, while large numbers of both hema togenously and spontaneously metastatic clones were recovered from the lungs, a vast excess of only the latter clones was recovered from ext rapulmonary sites. Both hematogenously and spontaneously metastatic pu lmonary clones recovered showed similar levels of Matrigel invasion an d collagenases by substrate gel electrophoresis, but significantly dec reased levels when compared to the cell line. primary tumor clones, in contrast, demonstrated increased invasion and increased collagenases. Our findings argue for the importance of paracrine (orthotopic) and a utocrine (size) epigenetic mechanisms in the regulation of metastasis. (C) 1996 Wiley-Liss, Inc.