Cells derived from an experimental luteinized ovarian
tumor are more sensitive to
GnRH endocrine action than control luteal cells. In an attempt to understand the possible causes of the differential sensibility to
GnRH action, we examined the number and affinity of
GnRH receptors and the second messenger response to
GnRH stimulation in both tissues. For
GnRH receptor studies membranes were obtained from 4- to 6-week-old ovarian
tumors (
luteoma) and ovaries from prepubertal rats treated with 25 IU PMSG and 25 IU hCG (SPO) and were incubated with [125I]
Buserelin. The number of
GnRH receptors were increased in
luteoma compared with that in SPO ovaries; dissociation constants were similar in both tissues.
GnRH stimulation of second messenger release was assessed in cells obtained from
luteoma and SPO ovaries by
collagenase treatment.
Buserelin (100 ng/ml) induced a significant 35%
calcium increase in SPO cells, as determined by the
fura-2 method; in
luteoma cells no response was observed after
buserelin stimulation, although a
calcium transient was induced by
thapsigargin (0.5 microM), an inhibitor of Ca2+-
adenosine triphosphatase associated with the endoplasmic reticulum. The effect of
buserelin on
inositol phosphates was evaluated after incubation of
luteoma and SPO cells with [3H]
myoinositol for 48 h.
Buserelin induced a 400% increase in
inositol trisphosphate in SPO cells. Again,
luteoma cells did not respond to
buserelin stimulation, although NaF (10 mM), an activator of
G proteins coupled to
phospholipase C, induced an 800% increase in
inositol trisphosphate. Although the number of
GnRH receptors is augmented in
luteoma cells, justifying an increased endocrine response, neither
inositol phosphates nor intracellular
calcium were released by a
GnRH analog, indicating the uncoupling of
GnRH receptors from
phospholipase C. These data provide evidence that the transformation of the ovary into a
luteoma implies the acquisition of novel characteristics in the
GnRH receptor second messenger-generating system.