Growth hormone signalling and its regulation: preventing too much of a good thing

Growth hormone resistance in chronic heart failure and its therapeutic implications

BACKGROUND

In recent years the administration of recombinant human growth hormone (GH) has received great attention. This review compares the potential of this therapeutic intervention in heart failure with that in other diseases where wasting is commonly seen. The pathophysiologic importance of GH and insulin-like growth factor (IGF)-I in these conditions will be discussed.

METHODS AND RESULTS

Abnormalities of the GH-IGF-I axis play an important role in the development of cachexia in chronic illnesses. GH resistance is a major determinant of the wasting process, acting through several different mechanisms: increased catabolism, impaired anabolism, and enhanced apoptosis in peripheral tissues. GH therapy has been evaluated in chronic heart failure (CHF); acquired GH resistance may explain the general lack of therapeutic success in the majority of studies. The assessment of plasma levels of GH, IGF-I, and, in particular, GH binding protein may help to guide dosing of GH for CHF patients.

CONCLUSIONS

GH resistance might be overcome by use of intermittent or higher doses of GH, or alternatively by combining GH with IGF-I. Randomized studies of GH therapy in catabolic states, with targeted dosing and longer duration of treatment are required to fully assess the safety and efficacy of this treatment approach.

Protein tyrosine phosphatase 1B attenuates growth hormone-mediated JAK2-STAT signaling

Protein tyrosine phosphatase-1B (PTP-1B) attenuates insulin, PDGF, EGF, and IGF-I signaling by dephosphorylating tyrosine residues located in the tyrosine kinase domain of the corresponding receptors. More recently, PTP-1B was shown to modulate the action of cytokine signaling via the nonreceptor tyrosine kinase JAK2. Transmission of the growth hormone (GH) signal also depends on JAK2, raising the possibility that PTP-1B modulates GH action. Consistent with this hypothesis, GH increased the abundance of tyrosine-phosphorylated JAK2 associated with a catalytically inactive mutant of PTP-1B. GH-induced JAK2 phosphorylation was greater in knockout (KO) than in wild-type (WT) PTP-1B embryonic fibroblasts and resulted in increased tyrosine phosphorylation of STAT3 and STAT5, while overexpression of PTP-1B reduced the GH-mediated activation of the acid-labile subunit gene. To evaluate the in vivo relevance of these observations, mice were injected with GH under fed and fasted conditions. As expected, tyrosine phosphorylation of JAK2 and STAT5 occurred readily in the livers of fed WT mice and was almost completely abolished during fasting. In contrast, resistance to the action of GH was severely impaired in the livers of fasted KO mice. These results indicate that PTP-1B regulates GH signaling by reducing the extent of JAK2 phosphorylation and suggest that PTP-1B is essential for limiting the action of GH during metabolic stress such as fasting.

Dimerization, ubiquitylation and endocytosis go together in growth hormone receptor function

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of the clathrin-coated endocytosis is sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, several adapter systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system both in endocytic and lysosomal membrane traffic. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. In this review, we will discuss the complexity and implications of this mechanism for membrane trafficking with emphasis on the growth hormone receptor.