Insulin-like growth factor-I protects myoblasts from apoptosis but requires other factors to stimulate proliferation.
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.
Napier
J R
JR
AgResearch Ltd, Private Bag 3123, Hamilton, New Zealand.
Thomas
M F
MF
Sharma
M
M
Hodgkinson
S C
SC
Bass
J J
JJ
D000818:Animals; D017209:Apoptosis; D002417:Cattle; D002448:Cell Adhesion / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D053938:DNA Fragmentation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004587:Electrophoresis, Agar Gel; D005312:Fetal Blood; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D013268:Stimulation, Chemical
D007334:Insulin-Like Growth Factor I