Cardiovascular Effects of Growth Hormone Treatment: Potential Risks and Benefits

Cardiac telomere length in heart development, function, and disease

Telomeres are repetitive nucleoprotein structures at chromosome ends, and a decrease in the number of these repeats, known as a reduction in telomere length (TL), triggers cellular senescence and apoptosis. Heart disease, the worldwide leading cause of death, often results from the loss of cardiac cells, which could be explained by decreases in TL. Due to the cell-specific regulation of TL, this review focuses on studies that have measured telomeres in heart cells and critically assesses the relationship between cardiac TL and heart function. There are several lines of evidence that have identified rapid changes in cardiac TL during the onset and progression of heart disease as well as at critical stages of development. There are also many factors, such as the loss of telomeric proteins, oxidative stress, and hypoxia, that decrease cardiac TL and heart function. In contrast, antioxidants, calorie restriction, and exercise can prevent both cardiac telomere attrition and the progression of heart disease. TL in the heart is also indicative of proliferative potential and could facilitate the identification of cells suitable for cardiac rejuvenation. Although these findings highlight the involvement of TL in heart function, there are important questions regarding the validity of animal models, as well as several confounding factors, that need to be considered when interpreting results and planning future research. With these in mind, elucidating the telomeric mechanisms involved in heart development and the transition to disease holds promise to prevent cardiac dysfunction and potentiate regeneration after injury.

The E-domain region of mechano-growth factor inhibits cellular apoptosis and preserves cardiac function during myocardial infarction

Insulin-like growth factor-1 (IGF-1) isoforms are expressed via alternative splicing. Expression of the minor isoform IGF-1Eb [also known as mechano-growth factor (MGF)] is responsive to cell stress. Since IGF-1 isoforms differ in their E-domain regions, we are interested in determining the biological function of the MGF E-domain. To do so, a synthetic peptide analog was used to gain mechanistic insight into the actions of the E-domain. Treatment of H9c2 cells indicated a rapid cellular uptake mechanism that did not involve IGF-1 receptor activation but resulted in a nuclear localization. Peptide treatment inhibited the intrinsic apoptotic pathway in H9c2 cells subjected to cell stress with sorbitol by preventing the collapse of the mitochondrial membrane potential and inhibition of caspase-3 activation. Therefore, we administered the peptide at the time of myocardial infarction (MI) in mice. At 2 weeks post-MI cardiac function, gene expression and cell death were assayed. A significant decline in both systolic and diastolic function was evident in untreated mice based on PV loop analysis. Delivery of the E-peptide ameliorated the decline in function and resulted in significant preservation of cardiac contractility. Associated with these changes were an inhibition of pathologic hypertrophy and significantly fewer apoptotic nuclei in the viable myocardium of E-peptide-treated mice post-MI. We conclude that administration of the MGF E-domain peptide may provide a means of modulating local tissue IGF-1 autocrine/paracrine actions to preserve cardiac function, prevent cell death, and pathologic remodeling in the heart.

Effects of GH treatment in GH-deficient adults on adiponectin, leptin and pregnancy-associated plasma protein-A

OBJECTIVE

GH deficiency (GHD) in adults is associated with adverse effects on metabolism and increased cardiovascular risk. Pregnancy-associated plasma protein-A (PAPP-A) is a protease that promotes IGF-I availability in vascular tissues. PAPP-A levels appear to correlate with carotid intima-media thickness and have been proposed as an early predictor of cardiac events. The aim of our study was to evaluate PAPP-A levels in GHD adults at baseline and after GH replacement and correlate them with changes in body composition, lipid profile, glucose homeostasis, inflammatory markers and in leptin and adiponectin.

PATIENTS AND METHODS

Fourteen GHD adults were evaluated at baseline and after 1 year of GH therapy. All patients were compared at baseline with 28 age-, sex- and body mass index (BMI)-matched control subjects.

RESULTS

At baseline, GHD adults showed higher PAPP-A levels (P=0.03) and higher leptin (P=0.04), fibrinogen (P=0.002) and highly sensitive C-reactive protein (P=0.01) values than controls. Therapy with GH reduced PAPP-A (P=0.03) and fibrinogen levels (P=0.002) while increased BMI (P=0.01) and reduced waist-hip ratio (WHR; P=0.05) were observed. Insulin and homeostasis model assessment of insulin resistance index increased after treatment (P<0.004/P=0.007), without changes in leptin or adiponectin levels. PAPP-A values correlated positively with BMI and WHR and negatively with adiponectin before and after treatment, with no correlation with glucose homeostasis parameters, lipid profile or leptin.

CONCLUSIONS

Our study suggests that PAPP-A expression is increased in GHD adults, and that 1 year of GH replacement therapy is able to reduce PAPP-A levels in this population. However, further studies are required to determine whether this decrease correlates with an improvement in atherosclerosis.

Protein kinase cascades in the regulation of cardiac hypertrophy

In broad terms, there are 3 types of cardiac hypertrophy: normal growth, growth induced by physical conditioning (i.e., physiologic hypertrophy), and growth induced by pathologic stimuli. Recent evidence suggests that normal and exercise-induced cardiac growth are regulated in large part by the growth hormone/IGF axis via signaling through the PI3K/Akt pathway. In contrast, pathological or reactive cardiac growth is triggered by autocrine and paracrine neurohormonal factors released during biomechanical stress that signal through the Gq/phospholipase C pathway, leading to an increase in cytosolic calcium and activation of PKC. Here we review recent developments in the area of these cardiotrophic kinases, highlighting the utility of animal models that are helping to identify molecular targets in the human condition.