THE REGULATION OF PORCINE THECA CELL-PROLIFERATION INVITRO - SYNERGISTIC ACTIONS OF EPIDERMAL GROWTH-FACTOR AND PLATELET-DERIVED GROWTH-FACTOR

An important but poorly understood aspect of mammalian follicle development involves the regulation of theca cell proliferation. To investigate the premise that growth factors regulate theca cell proliferation, porcine theca cells were prepared by collagenase/DN'ase digestion of follicle linings after the removal of the granulosa cells and allowed to attach for 24 h. This method provided a monolayer of theca cells that had little if any granulosa cell contamination and which secreted high levels of androstenedione relative to granulosa cells during moderate-term culture (33-fold difference, P < 0.01). In medium containing fetal calf serum (10%), theca cells were significantly more responsive to platelet-derived growth factor (PDGF) than epidermal growth factor (EGF) in terms of proliferation (13.4 +/- 0.2-vs. 7.0 +/- 0.1-fold increases relative to the initial cell count, P < 0.05). This is in contrast to granulosa cells which were significantly more responsive to EGF than PDGF (7.1 +/- 0.1 vs. 4.0 +/- 0.2 fold-increases, P < 0.05). Since serum has been shown to contain both EGF and PDGF, proliferation studies were performed using plasma-derived serum (PDS) which is growth factor restricted to examine more closely the direct effects of growth factors. In medium containing 0.25% PDS and within experiments, PDGF (1-25 ng/ml) stimulated theca cell proliferation in a dose-dependent manner (2.3-fold increase relative to controls, P < 0.05) whereas EGF did not. EGF, however, markedly enhanced the proliferative action of PDGF (6.4-fold increase relative to controls, P < 0.05). Insulin-like growth factor I and low density lipoprotein, factors which enhance markedly the proliferative effects of EGF and PDGF in terms of granulosa cell proliferation, exhibited only a modest synergistic effect with respect to EGF and PDGF upon theca cells (9.5-fold increase vs. a 6.4-fold increase above controls, P < 0.05). Temporal studies in vitro indicate that theca cell proliferation is low during the first 3-day exposure to growth factors irrespective of treatment (a 2-fold increase over the seeding density). During the second 3-day exposure, however, theca cell proliferation increases 4- to 5-fold. The temporal pattern of theca cell proliferation stimulated by fetal calf serum supplemented with EGF or PDGF and PDS-containing medium supplemented with PDGF, EGF, insulin-like growth factor I, and low density lipoprotein is similar. These results suggest that PDGF is a major mitogen toward porcine theca cells and that EGF greatly enhances its activity. These results coupled with those demonstrating PDGF enhancement of EGF/TGF-alpha-stimulated granulosa cell proliferation suggest that these two growth factors may act directly and synergistically to promote proliferation of both theca and granulosa cells leading to follicle growth.