Insulin-like growth factor-I (IGF-I) has been shown to stimulate myoblast proliferation for a limited time after which serum is required to reactivate IGF-I-stimulated myoblast proliferation. The aim of these studies was to determine whether IGF-I can stimulate myoblast proliferation and/or inhibit apoptosis alone or whether co-factors are necessary. This was achieved by investigating the proliferative response of L6 myoblasts to IGF-I and horse serum (HS) and by examining the status of cells in terms of cell number, substrate adherence, cell viability and DNA laddering following incubation with IGF-I and HS. L6 myoblasts proliferate in response to IGF-I after 36 h is not due to accumulation of waste products or lack of IGF-I. The addition of a low level (1% v/v) of HS restores the ability of myoblasts to proliferate in response to IGF-I and this supports the existence of a mitogenic competence factor. Furthermore, myoblasts failing to proliferate in response to IGF-I after 36 h regain the capacity to respond to IGF-I for a further period of 36 h when exposed to fetal bovine serum. Following the initial (36 h) phase of IGF-I-stimulated proliferation, removal of both IGF-I and HS led to a dramatic (60%) reduction in the number of cells fully attached to the culture vessel, with 60% of the completely detached cells dead. Agarose gel electrophoresis of extracts from these detached cells revealed higher levels of DNA laddering than extracts prepared from attached cells with IGF-I present. This suggests that IGF-I acts as a survival factor by protecting cells from apoptosis. In conclusion these experiments support the presence of a mitogenic competence factor in horse serum, which restores the ability of cells to proliferate in response to IGF-I. Unlike proliferation, protection against apoptosis is achieved by IGF-I or HS independently of each other.
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