The MXT
tumor is an
experimental mammary neoplasm which is maintained by serial
transplantation using B6D2F1 mice, and which contains significant amounts of
estrogen and
progesterone receptors. The aim of the present study is to examine the effects of
ovariectomy (OVX) or
ovariectomy plus
hypophysectomy (OVX-HX) on both the macroscopic growth and the cell proliferation of this
tumor. This cell proliferation was evaluated by means of in vivo tritiated
thymidine autoradiography. In addition, we investigated the effects of a
GnRH analog (
Gonadorelin: HRF, 5-oxo-Pro-His-Trp-Ser-Tyr-
Gly-Leu-
Arg-Pro-Gly-hydrochloride) on MTX
tumor cell proliferation on 7 day-OVX and 5 day-HX (OVX-HX) mice. The uterine
luminal epithelium was chosen to monitor the methodology. Our data clearly demonstrate that there is a delay in the growth of MXT
tumors grafted into hypophysectomized animals and, to a lesser degree, ovariectomized animals. With respect to proliferation,
castration induced a dramatic decrease of the
thymidine labelling index (TLI) in the tissue used to monitor the methodology (the uterine
luminal epithemium); in contrast, no cell proliferation was induced by
hypophysectomy or HRF administration. In 4-week-old MXT
tumors,
ovariectomy also markedly decreased the TLI within a few days of its taking place. However
hypophysectomy, performed on castrated animals, induced a significant and transient increase of cell proliferation in this
neoplasm, an increase which lasted from the 2nd to the 5th day following the operation. The high basal level of MXT cell proliferation recorded in OVX-HX animals decreased dramatically after the administration of HRF between 12 and 48 h prior to the sacrifice of the animals. It is concluded that the HRF exerts a direct effect on the MXT
tumor cells, and this HRF might be essential for their growth.