Loss of differentiation of 4NQO-induced rat malignant oral keratinocytes correlates with metastatic dissemination and is associated with a reduced cellular response to TGF-beta 1 and an altered receptor profile

This study examined the metastatic capacity of clonal populations of 4NQO-i nduced rat malignant oral keratinocytes following orthotopic transplantatio n to athymic mice. Polygonal and spindle cells formed well-differentiated s quamous cell carcinomas (keratin positive and vimentin negative) and undiff erentiated spindle cell tumours (keratin negative and vimentin positive), r espectively, in almost 100% of animals at the site of inoculation (floor of mouth). Transplantation of 5x10(6) cells of either cell type at high cell density resulted in approximately 50% of animals forming pulmonary metastas es. By contrast, inoculation of 2x10(6) differentiated polygonal cells resu lted in the formation of significantly fewer pulmonary metastases than the undifferentiated spindle cells. A single well-differentiated clone of polyg onal cells and 3 of 4 of the undifferentiated spindle cell lines produced c omparable levels of TGF-beta 1. One undifferentiated spindle cell line expr essed significantly more TGF-beta 1 and, following transplantation orthotop ically, fewer animals formed pulmonary metastases despite the formation of primary tumours in almost all grafted animals, suggesting that TGF-beta 1 c an act as a tumour suppressor in this cell type. All cell lines produced co mparable amounts of TGF-beta 2. The clones of polygonal cells were markedly inhibited and the spindle cells were only partially inhibited by exogenous TGF-beta 1. Both cell types expressed high-affinity TGF-beta cell surface receptors; the ratio of type I to type II TGF-beta receptors was 1.0:<3.0 i n the spindle cells and 1.0:17.9 in the polygonal clone. The results sugges t that differentiated rat malignant oral keratinocytes are less aggressive and have a decreased potential to metastasise than their undifferentiated s pindle cell counterparts. This may be attributable, in part, to a change in TGF-beta receptor profile leading to the partial loss of response to exoge nous TGF-beta 1.