Acromegaly Case File

Acromegaly Case File

Eugene C.Toy, MD, William E. Seifert, Jr., PHD, Henry W. Strobel, PHD, Konrad P. Harms, MD

❖ CASE 47

A 36-year-old male comes into the physician’s office because his hands and feet are “swelling,” and his face has coarse features with oily skin. On examination, he is above the 95th percentile for height at his age. He is noted to have some coarse facial features including large nose, large tongue, and frontal bossing of his forehead. His hands are enlarged with soft tissue swelling, and his heel pad is thickened. He is noticed to have a slightly enlarged liver and spleen. The remainder of the examination was otherwise normal.

◆ What is the most likely diagnosis?

◆ What is the biochemical mechanism of this disorder?

ANSWERS TO CASE 47: ACROMEGALY

Summary: A 36-year-old male comes into the physician’s office with height greater that the 95th percentile, coarse facial features (frontal bossing, macroglossia, large nose), oily skin, organomegaly, and thickened soft tissue of the hand and feet.

◆ Diagnosis: Acromegaly

◆ Biochemical mechanism: Excess and autonomous secretion of growth hormone

CLINICAL CORRELATION

Acromegaly is a disorder with excessive growth hormone (GH), usually as a result of autonomous secretion from a nonmalignant anterior pituitary tumor. When acromegaly affects individuals prior to bone growth plate closure, giantism may result; after the bone growth plates close, the patients usually develop coarse features and large hands and feet. The diagnosis is confirmed by demonstrating the failure of GH suppression within 1 to 2 hours of an oral glucose load (75 gm), because GH is usually decreased with glucose. As a consequence of the pulsatility of GH secretion, a single random GH level is not useful for diagnosis. Morbidity can ensue because of diabetes or hypertension. The most feared complications are cardiac in nature, which may affect up to 30 percent of patients. Cardiac arrhythmias, cardiomyopathy, left ventricular hypertrophy, hypertension, and coronary heart disease can be present. Arrhythmias can be associated with cardiomyopathy or coronary heart disease. The primary therapy of acromegaly is surgical, but there is often only partial response. Thereafter somatostatin analogues or dopamine agonists are used, and as a last line of therapy, radiation.

APPROACH TO GROWTH HORMONE ABNORMALITIES

Objectives

1. Be familiar with growth hormone regulation and function.

2. Know about the association with other hormonal problems (diabetes mellitus [DM], thyroidism, etc.).

Definitions

Ghrelin: A 28-amino acid peptide that is octanoylated on serine-3. Ghrelin is synthesized and secreted by endocrine cells in the stomach and will bind to somatotrophic cells in the anterior pituitary to promote the release of growth hormone (somatotropin).

GHRH: Growth hormone–releasing hormone; a peptide synthesized and secreted by the hypothalamus. GHRH stimulates the synthesis and secretion of GH.

IGF-1: Insulin-like growth factor-1, also called somatomedin C; a 70-amino acid peptide that shares structural homology with proinsulin. IGF-1 is synthesized and released by the liver in response to growth hormone binding to receptors on the hepatocyte.

JAK2: Janus kinase 2; an enzyme that will phosphorylate tyrosine residues on target proteins. JAK2 is bound to the growth hormone receptor but is inactive until the receptor binds the hormone. When hormone binds the receptor, it triggers dimerization and activates JAK2.

Somatotropin: Growth hormone; a 191-amino acid polypeptide hormone that is synthesized and secreted from the somatotroph cells of the anterior pituitary. It acts primarily on hepatocytes, although muscle and adipose cells also have receptors for growth hormone.

DISCUSSION

Growth hormone (somatotropin) is a polypeptide synthesized and secreted by somatotrophs in the anterior pituitary. Growth hormone synthesis and secretion is stimulated by the hypothalamic peptide growth hormone– releasing hormone (GHRH). Somatostatin, a polypeptide produced by several tissues including the hypothalamus, opposes GHRH stimulation of growth hormone release. High levels of insulin-like growth factor-1 (IGF-1) directly suppress growth hormone production in somatotrophs and also act indirectly by stimulating somatostatin release from the hypothalamus. Furthermore, growth hormone itself directly feedback inhibits its synthesis in the somatotroph and also inhibits GHRH production by the hypothalamus. A potent stimulator of growth hormone release is ghrelin, a polypeptide hormone secreted by the stomach that acts directly on somatotrophs. In general, many environmental factors including stress, exercise, nutrition, and sleep influence circulating levels of growth hormone. The overall effect is a pulsatile pattern of growth hormone release reaching a maximum in children and young adults shortly after the onset of deep sleep. Levels of growth hormone fluctuate dramatically and are suppressed in normal individuals after oral glucose administration.

Growth hormone plays a major role in cell proliferation and in regulating protein, lipid, and carbohydrate metabolism. It stimulates protein synthesis and the accompanying amino acid uptake in many tissues. In adipocytes, it increases fat utilization by stimulating triglyceride breakdown and oxidation. It opposes effects of insulin by suppressing its ability to stimulate glucose uptake and by stimulating gluconeogenesis. Interestingly, injection of growth hormone stimulates insulin secretion, leading to hyperinsulinemia.

In most of these examples, growth hormone acts at two levels. Growth hormone acts directly by binding its receptors in the plasma membrane of target cells to influence cell proliferation and metabolism. Binding of growth hormone to its receptor triggers receptor dimerization. In the absence of bound hormone, the receptor is a monomer and is bound on its cytoplasmic side to an inactive protein kinase (Janus kinase 2 [JAK2]). The JAK2 protein kinase is a different polypeptide from the receptor and the receptor itself lacks protein kinase activity. After hormone binding and dimerization, JAK2 is activated by crossphosphorylation. Active JAK2 phosphorylates target proteins and itself on specific tyrosine residues to activate them, thus initiating an autocatalytic regulatory cascade.

Growth hormone also exerts important indirect effects by stimulating the liver and other tissues to secrete IGF-1. IGF-1 stimulates the proliferation of chondrocytes (cartilage cells) leading to bone growth. It also stimulates myoblast proliferation, leading to increased muscle mass. The IGF-1 receptor contains a tyrosine kinase activity in its cytoplasmic domain that is activated autocatalytically after IGF-1 binding, triggering activation of downstream signaling molecules. In this case, tyrosine kinase activity resides on the same polypeptide as hormone-binding activity.

Normal proliferation of somatic cells requires both thyroid hormone and growth hormone. Thyroid hormone stimulates growth hormone secretion, and many thyroid hormone actions on the insulin-like growth factor (IGF) system can be explained by this mechanism.

The profound physiologic role of growth hormone is revealed by conditions resulting from either its deficiency or excess. For example, mutations in growth hormone or its receptor lead to dwarfism. By contrast, excessive secretion leads to giantism, if expressed before the growth plates have closed, or acromegaly, if overproduction is initiated in the adult. Usually, growth hormone overproduction in the adult is the result of a noncancerous pituitary tumor. Overgrowth (thickening) of bones and connective tissues leads to the characteristic features of acromegaly, with accompanying enlargement of other tissues including the heart. In women, breast milk secretion may result. Left untreated, acromegaly may lead to DI (glucose intolerance), hypertension, heart failure, and sleep apnea.

COMPREHENSION QUESTIONS

[47.1] A 30-year-old female of normal weight was recently diagnosed with type II diabetes and hypertension. Menstrual cycles were irregular. In appearance she had unusually coarse features; a noticeable enlargement of the tongue, hands, and feet; and a deep voice. Although not pregnant or nursing, she unexpectedly began producing breast milk (galactorrhea). Which one of the following possibilities is most likely to explain all of these symptoms?

A. Hyperinsulinemia and insulin resistance

B. Pituitary tumor and growth hormone overproduction

C. Testosterone overproduction

D. Ovarian cysts

E. Transforming growth factor β overproduction

[47.2] A 7-year-old child who was very small for his age began receiving treatment with growth hormone. Which one of the following metabolic alterations is most likely to be observed after beginning this treatment?

A. Inhibition of cartilage formation

B. Inhibition of gluconeogenesis

C. Inhibition of triglyceride breakdown and oxidation in adipocytes

D. Stimulation of IGF-1 secretion

E. Stimulation of protein breakdown

[47.3] The following polypeptide hormones each interact with receptors in the plasma membrane of their target cells. Which one triggers a signaling pathway that is directly stimulated by treatment of the cell with an inhibitor of cyclic AMP phosphodiesterase?

A. ACTH

B. Epidermal growth factor

C. Growth hormone

D. Insulin

E. Nerve growth factor

Answers

[47.1] B. The symptoms are consistent with acromegaly, or GH overproduction, in the adult. This condition usually is caused by a pituitary tumor. In the female, breast milk secretion is sometimes observed, either as a result of GH overproduction or an accompanying overproduction of prolactin. Growth hormone opposes insulin action resulting in decreased glucose utilization and symptoms of diabetes mellitus. Growth hormone also increases IGF-1 production by the liver, leading to stimulation of cartilage synthesis and muscle mass. Excessive bone and tissue growth lead to the characteristic coarse facial features, enlarged tongue and heart, bone thickening, and other characteristics associated with this syndrome.

[47.2] D. Growth hormone stimulation of IGF-1 secretion is an important aspect of its action.

[47.3] A. ACTH acts by activation of adenylate cyclase and production of cAMP. Inhibition of cAMP breakdown synergistically increases the intracellular response to this hormone.

BIOCHEMISTRY PEARLS

❖ Growth hormone (somatotropin) is a polypeptide synthesized and secreted by somatotrophs in the anterior pituitary.

❖ Mutations in growth hormone or its receptor lead to dwarfism.

❖ Excessive GH secretion leads to giantism, if expressed before the growth plates have closed, or acromegaly, if overproduction is initiated in the adult.

❖ The most common cause of growth hormone overproduction in the adult is a noncancerous pituitary tumor.

References

Litwack G, Schmidt TJ. Biochemistry of hormones I: polypeptide hormones. In: Devlin TM, ed. Textbook of Biochemistry with Clinical Correlations, 5th ed. New York: Wiley-Liss, 2002. 

Pathophysiology of the endocrine system. An online textbook from Colorado State University: http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/

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