Actions of growth hormone on cellular differentiation and proliferation

Actions of growth hormone on cellular differentiation and proliferation 

As has been discussed, some of the actions of GH involve the induction of expression of specific proteins, but the effect is reversed on removal of the hormone. In a few cases, however, GH appears to induce a more permanent differentiation. 

together with other hormones, can induce differentiation of cells of the 3T3 mouse fibroblast line into adipocytes in vitro [98,99]. The change involves morphological alterations, enzymic differentiation, increased hormone responsiveness, triglyceride accumulation and a limited mitogenic response. Somatomedin C has no effect on this transformation. 

GH also has actions on the differentiation of muscle. It stimulates the conversion of myoblasts to myotubes in vitro, with associated changes in the levels of creatine phosphokinase [99]. It is also possible that GH has direct effects on the differentiation of cartilage, acting synergistically with somatomedins to promote cartilage growth [72], 

GH also has direct effects on the proliferation of a variety of cells and tissues in culture, including chondrocytes [74] (though others have been unable to confirm this effect [100]), haematopoietic progenitor cells [101], normal and leukaemic human T cells [102], smooth muscle [103] and rat pancreatic B-cells [104]. The physiological significance of these effects is not clear, and it has not been clarified to what extent somatomedin C/IGF-I released from the same cells may be mediating the mitogenic effects. It is clear, however, that enhanced cell division in many tissues follows administration of GH in vivo [72] and this may be a consequence of direct actions rather than (or as well as) somatomedin C/IGF-I mediated effects. 

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