Mature adipocytes originate from fibroblast-like precursor cells, preadipocytes, which differentiate to obtain the characteristics of adipocytes. Our aim was to investigate how differentiation of human preadipocytes affects the distribution of insulin receptors (IR) and IGF-I receptors (IGF-IR) and other cell characteristics. Preadipocytes were differentiated using indomethacine, dexamethasone, isobutyl-methylxantine (IBMX) and high concentration of insulin. Gene expression was quantified by real-time RT-PCT in preadipocytes (PA), differentiated preadipocytes (dPA) and mature adipocytes (mAD). The amount of expressed receptor protein was analyzed using receptor specific ELISAs and Western blot. We also studied DNA synthesis with radiolabeled thymidine incorporation and glucose accumulation with radiolabeled glucose. Differentiation of PA increased gene expression of IR but not IGF-IR, GLUT4, growth hormone receptor (GHR) and adiponectin appeared or increased. In PA and dPA only IR-A was expressed whereas also IR-B was detected in mAD. By Western blot and ELISA, IR and IGF-IR was phosphorylated by their own ligant at 1 nM and in dPA the acitivation of both receptors was stimulated by IGF-I, but not insulin, at 1 nM. Accumulation of glucose in PA was increased by insulin at 10 nM and by IGF-I at 1 nM and 10 nM. DNA synthesis was increased by insulin and IGF-I at 10 nM.

In conclusion, both IR and IGF-IR are present in human preadipocytes and adipocytes. Differentiation is characterized by an increased IR/IGF-IR ratio.

We compared insulin and IGF effects in adipocytes expressing IR (insulin receptors), and preadipocytes expressing IR and IGF-IR (IGF-I receptors). Treatment of adipocytes with insulin, IGF-II or IGF-I resulted in phosphorylation of IR. Order of potency was insulin greater thanIGF-IIgreater than IGF-I. In preadipocytes IR, IGF-IR and insulin/IGF-I hybrid receptors (HR) were detected. Treatment of preadipocytes with IGF-I and IGF-II 10(-8) M resulted in activation of IGF-IR and IR whereas insulin was more potent in activating IR, with no effect on IGF-IR. In adipocytes glucose transport was 100-fold more sensitive to insulin than to IGFs and the maximal effect was higher with insulin. In preadipocytes glucose accumulation and DNA synthesis was equally sensitive to insulin and IGFs but the maximal effect was higher with IGF-I. In conclusion, insulin and IGF-I activate their cognate receptors and IGF-I also HR. IGF-II activates IR, IGF-IR and HR. Insulin and IGF-I are partial agonists to each others receptors.

Diabetes is associated with coronary microcirculatory dysfunction and heart failure as well as changes in insulin and IGF-I levels. Our aim was to study the role of insulin receptors and IGF-I receptors in metabolic, mitogenic and anti-inflammatory responses to insulin and IGF-I in human cardiac microvascular endothelial cells (HMVEC-C) and, for comparison, also human umbilical vein endothelial cells (HUVEC). Insulin receptor (IR) and IGF-I receptor (IGF-IR) gene expression was studied with real-time RT-PCR. Receptor protein expression and phosphorylation was determined with Western blot and ELISA. The metabolic and mitogenic effects were measured as glucose accumulation and thymidine incorporation. An E-selectin ELISA was used to investigate the anti-inflammatory responses. IGF-IR was more abundant than IR both regarding gene expression and protein in HMVEC-C and HUVEC. Immunoprecipitation with anti-IGF-IR antibody and immunoblotting with anti-IR antibody and vice versa, showed insulin/IGF-I hybrid receptors in these cells. IGF-I 10^-8 M significantly stimulated phosphorylation of both IGF-IR and IR in HMVEC-C. In HUVEC IGF-I 10^-8 M phosphorylated IGF-IR. IGF-I also stimulated DNA synthesis at 10^-8 M and glucose accumulation at 10^-7 M. TNF-α significantly increased E-selectin expression whereas no effects were found by insulin, IGF-I or high glucose.

We conclude that HMVEC-C express more IGF-I receptors than insulin receptors and at physiological concentrations of insulin and IGF-I mainly reacts to IGF-I probably due to the predominance of IGF-I receptors and insulin/IGF-I hybrid receptors. TNF-α has a pronounced pro-inflammatory effect in HMVEC-C which is not counteracted by insulin or IGF-I.

Insulin and IGF-I are related peptides with similar structure. They both signal via their cognate receptors, the insulin receptor (IR) and the insulin-like growth factor (IGF)-I receptor (IGF-IR).

Our aim was to simultaneously measure the amount of insulin and IGF-I receptors in different human tissues and also the IR-A and IR-B isoforms to study tissue specific expression

Renal artery intima-media, myometrium, skeletal muscle or liver tissue samples were obtained from patients undergoing surgery. IR, IGF-IR, IR-A and IR-B gene expression was investigated with real-time RT-PCR and expression of IR and IGF-IR protein was examined by Western blot and ELISA.

Renal arteries and myometrium expressed the IGF-IR gene to a higher extent than the IR gene, liver expressed more IR than IGF-IR and skeletal muscle expressed almost equal amounts of both receptors. IR-B was the most abundant isoform in all tissues. With Western blot we could detect IR in skeletal muscle, liver and myometrium. With ELISA we found that, normalized to total protein, the highest levels of IGF-IR were found in renal arteries and myometrium and low levels in skeletal muscle and liver. The highest levels of IR were found in liver.

In conclusion there is a large variation in the quantity and ratio of insulin receptors and IGF-I receptors expressed in different tissues, the extremes being arterial intima media with predominantly IGF-I receptors and liver with predominantly insulin receptors. This suggests that differential expression of insulin and IGF-I receptors is a key mechanism in regulation of growth and metabolism.