Expression of c-kit protooncogene is stimulated by cAMP in differentiated F9 mouse teratocarcinoma cells

Cyclic AMP in ovarian cancer cells both inhibits proliferation and increases c-KIT expression

C-KIT encodes a tyrosine kinase receptor (KIT) that, when activated by its ligand (KL), stimulates proliferation, differentiation, migration, and survival. Greater than 70% of epithelial ovarian cancers coexpress c-KIT and KL. C-KIT and KL expression levels have been shown to be up-regulated by cAMP in some cell types. Additionally, cAMP is well-recognized for its anti-proliferative effects in cancer cells. The goal of these experiments was to investigate these seemingly contradictory consequences of cAMP treatment by: (1) confirming the growth inhibitory actions of cAMP on ovarian cancer cells; (2) investigating the ability of cAMP to affect c-KIT and KL expression in these cells; and (3) examining the possible role of endogenous and/or cAMP-regulated c-KIT and KL expression in ovarian cancer cell proliferation. HEY cells, an ovarian cancer cell line which expresses c-KIT and KL, were treated with dibutyryl cyclic AMP (dbcAMP), 8-bromo-cAMP, and cholera toxin over a range of concentrations. With all treatments, stimulation of cAMP signaling caused a dose-dependent inhibition of HEY cell proliferation by up to 40, 62, and 38%, respectively. This inhibition of proliferation correlated with a dose-dependent increase in c-KIT mRNA expression, yielding 4- to 7-fold elevations in transcript abundance; there were no changes in steady-state levels of KL transcripts. In order to determine whether KIT expression/activity was responsible for the observed decrease in proliferation, dbcAMP-treated HEY cells were exposed either to anti-KIT neutralizing antibodies or to the KIT inhibitor STI571. These experiments demonstrated that KIT inhibition did not alter the growth rate of cells or reverse the dbcAMP-induced inhibition of proliferation. These results suggest that cAMP signaling pathways regulate both cell proliferation and c-KIT expression in ovarian cancer cells; however, KIT is not assuming its well-established role as a growth factor.

Lack of expression of c-KIT in ovarian cancers is associated with poor prognosis

The c-KIT protooncogene encodes a tyrosine kinase receptor, KIT, that is expressed in many normal and cancerous tissues. In this study, we have examined the expression of c-KIT and its ligand, stem cell factor (SCF), in human epithelial ovarian tumors, in normal ovaries and in cultured ovarian surface epithelium (OSE). Cultured cells, normal tissues and tumors were analyzed by Northern and Western blot analyses, reverse transcription-polymerase chain reaction and immunohistochemistry. Normal OSE expressed SCF, but not c-KIT; however, epithelial invaginations and inclusion cysts often expressed KIT protein. Of 15 benign ovarian tumors and tumors of low malignant potential, 87% expressed c-KIT, and 92% of these co-expressed SCF, suggesting the possibility of autocrine growth regulation. Of 35 malignant ovarian cancers, 71% expressed c-KIT (92% co-expressed SCF), with a trend for decreased c-KIT expression in advanced stage disease. Of 34 patients with malignant tumors for whom follow-up information was available (median follow-up time of 24 months), 9 had tumors that did not express c-KIT, 8 (89%) of whom have died and the remaining 1 has recurrent disease. Of the 25 patients with tumors expressing c-KIT, 56% are still alive. Eight of the patients have no evidence of disease and all had KIT-expressing tumors. Statistical analysis indicated that patients whose tumors did not express c-KIT had a significantly shorter (p < 0.05) disease-free survival time than patients who had KIT-expressing tumors. Our results suggest that c-KIT may play a role in early ovarian tumorigenesis, and that loss of c-KIT expression is associated with poor prognosis.

Transforming growth factor-beta regulates Kit ligand expression in rat ovarian surface epithelial cells

In preparation for ovulation, paracrine communication between the preovulatory follicle and overlying theca/stromal cells and ovarian surface epithelium (OSE) must take place to facilitate the degradative and apoptotic events associated with ovulation. Kit tyrosine kinase receptors and their ligand, kit ligand (KL) are expressed within ovarian follicles, and ligand-induced receptor activation appears to account for some of the cell - cell interactions important for oocyte development. We investigated the expression of Kit receptors and KL in OSE cells and the possibility that modulation of their expression could affect OSE cell activity. KL mRNA and protein were detected in the OSE cell layer of rat ovaries, and primary cultures of rat OSE as well as the immortalized rat OSE cell line, ROSE 199, expressed KL, but not Kit receptors. Both primary and immortalized OSE cells preferentially expressed KL-1, rather than KL-2, transcripts, suggesting that these cells produce predominantly the soluble form of KL. Activation of the cAMP signalling pathway using dibutyryl cAMP decreased proliferation of ROSE 199 cells and elicited a threefold increase in KL expression. TGF-beta similarly inhibited ROSE 199 cell proliferation, but strongly inhibited dibutyryl cAMP-induced KL expression, indicating that changes in KL expression were not directly associated with OSE cell proliferation. The expression of mostly soluble KL in the surface epithelium suggests that this cytokine may be acting in a paracrine fashion, perhaps interacting with nearby Kit receptor-bearing theca cells.

Combined action of stem cell factor, leukemia inhibitory factor, and cAMP on in vitro proliferation of mouse primordial germ cells

In the present paper we investigated the effects of stem cell factor/mastocyte growth factor (SCF/MGF), leukemia inhibitory factor/differentiating inhibitory activity (LIF/DIA) (two growth factors known to affect primordial germ cell growth in vitro) and forskolin (FRSK) (an activator of adenylate cyclase in many cell types) alone or in combination on the survival and proliferation of primordial germ cells (PGCs) obtained from 8.5, 10.5, and 11.5 days post coitum (dpc) mouse embryos and cultured without pre-formed cell feeder layers. The results showed that both at 1 and 3 days of culture the addition of 100 ng/ml SCF, 20 microM FRSK, or in some instances 20 ng/ml LIF alone caused a significant increase of PGC number as compared with controls. The highest effects were obtained when SCF and/or LIF were used together with FRSK. Moreover, we found that FRSK elevated cAMP levels in purified 11.5 dpc PGCs and that this compound, but not SCF and LIF, stimulated PGC proliferation, as assessed by 5-bromo-2'-deoxyuridin (BrdU) incorporation. These results suggest a mechanism of combined action of cAMP with SCF and/or LIF in the control of proliferation of mouse PGCs in vitro.