Endocrine Abstracts

GH has diverse direct effects including the induction of lipolysis in adipocytes and mineralisation in bone. Interestingly, pluripotent mesenchymal stem cells can be isolated and cultured from adult bone marrow and induced to differentiate in vitro to both adipocytes and osteoblasts. GHR activation can be coupled to JAK2/STAT5 and the ERK (MAPK) pathway. We have therefore studied the evolution of GHR-associated cell signalling in adult rat BMSC’s during differentiation along adipocyte and osteoblast lineages.

BMSC’s were isolated from rat femur (10 wk male Wistar), cultured in control medium (alphaMEM 10% FCS) or adipocyte differentiation medium (containing the PPARgamma agonist pioglitazone) for 3–7 days or osteoblast differentiation medium (containing ascorbate and dexamethasone) for 20 days. Cultures were then stained for lipid with oil red O and for alkaline phosphatase or treated with GH (10 min) and PMA (10 min) to study GH induced activation of ERK and STAT5 in each of the cell types. ERK and STAT5 activation was assessed using western blotting probed with activation state (phospho)-specific antibodies.

Oil red O staining was only detectable in the adipocyte differentiated cells. GH stimulation of the ERK pathway was not detectable in undifferentiated or osteoblast differentiated cells but was detectable in 3 day differentiated adipocytes and at a higher level in the 7 day differentiated adipocytes. PMA was able to produce readily detectable activation of ERK in all cell types, demonstrating the presence of a functional ERK pathway. GH induced activation of STAT5 was detectable in all cell types.

In conclusion, despite the presence of both functional GH receptors as judged by GH-induced STAT5 activation and functional ERK (induced by PMA) in all cell types, GHR-activation is only coupled to the ERK pathway in adipocyte differentiated BMSC’s. Thus it is possible that the ERK pathway may be more important in mediating the effects of GH in fat than bone.