Expression of different phenotypes in cell lines from canine mammary spindle-cell tumours and osteosarcomas indicating a pluripotent mammary stem cell origin.

Mammary spindle-cell tumours and sarcomas seem to be restricted to dogs and humans. Two cell lines from spontaneous primary canine mammary spindle-cell tumours (CMT-U304 and CMT-U309) and two cell lines from spontaneous primary canine mammary osteosarcomas (CMT-U334 and CMT-U335) were established to study the mesenchymal phenotypes of mammary tumours in the female dog. The cells from the spindle-cell tumours expressed cytokeratin, vimentin and smooth muscle actin filaments. When these cells were inoculated subcutaneously into female and male nude mice they formed different types of mesenchymal tumours such as spindle-cell tumours, fibroma and rhabdomyoid tumours (n = 6/8). The cells from the osteosarcomas expressed vimentin filaments and also formed different types of mesenchymal tumours such as chondroid, rhabdomyoid, smooth muscle-like and spindle-cell tumours (n = 6/10). The cell lines CMT-U304, CMT-U309 and CMT-U335 had receptors for progesterone but none of the four cell lines had receptors for estrogen. All four cell lines and their corresponding primary tumours showed identical allelic patterns in microsatellite analysis. By in situ hybridization with genomic DNA we could verify that all formed tumours but one were of canine origin. Our results support the hypothesis that canine mammary tumours are derived from pluripotent stem cells.