The role of the biochemistry department in the diagnosis of pituitary apoplexy.

A 47-year-old man presented with severe clinical hypoglycaemia. He had long-standing insulin-dependent diabetes with previously good glycaemic control. Intense headaches and vomiting initiated hospitalization. A brain computed tomography (CT) scan was normal, and a lumbar puncture showed elevated cerebrospinal fluid (CSF) protein [0.67 g/L; normal range (NR) 0.15-0.45 g/L], suggesting resolving viral meningitis. Routine thyroid function tests were abnormal (free thyroxine 10.6 pmol/L, NR 9-22.5 pmol/L; thyroid-stimulating hormone 0.16 mU/L, NR 0.35-5 mU/L). In the absence of evident thyroid therapy, the laboratory policy required an urgent cortisol assay to be added; this was very abnormal (42 nmol/L), suggesting hypopituitarism. Later analysis showed that concentrations of gonadotrophins and adrenocorticotrophin were low. An urgent pituitary magnetic resonance imaging scan revealed an unsuspected pituitary tumour with recent haemorrhage (pituitary apoplexy). The patient was given intravenous hydrocortisone and then stabilized on oral hydrocortisone, thyroxine and mesterolone. He made a full recovery and the hypoglycaemia resolved. The normal brain CT scan was falsely reassuring and the CSF protein was not due to viral meningitis but to haemorrhage into the pituitary tumour. If laboratory policy had not required the urgent cortisol assay be added, the diagnosis of hypopituitarism would have been delayed or even missed altogether. This could have led to the death of the patient.

Fibrillar islet amyloid polypeptide (amylin) is internalised by macrophages but resists proteolytic degradation.

Pancreatic islet amyloid, formed from islet amyloid polypeptide, is found in 96% of Type II (non-insulin-dependent) diabetic patients. Islet amyloidosis is progressive and apparently irreversible. Fibrils immunoreactive for islet amyloid polypeptide are found in macrophages associated with amyloid, suggesting that deposits can be phagocytosed. To determine the mechanism for the recognition and internalisation of fibrils, mouse peritoneal macrophages were cultured with fibrillar synthetic human islet amyloid polypeptide. Fibrils did not exert a cytotoxic effect over 72 h of culture. The uptake and degradation of fibrils was analysed by quantitative light-and electron-microscopic immunocytochemistry and immunoreactivity was detectable in 86+/-3% cells within 6 h of culture. Neither polyinosinic acid (200 microg/ml) nor nocodazole (10 microg/ml) inhibited fibril uptake, suggesting that internalisation is not blocked by poly-ions and is independent of microtubule assembly. Inhibition of pseudopodia formation by cytochalasin B blocked fibriI uptake. Fibril aggregates became condensed in lysosomes to form protofilaments and were resistant to intracellular proteolysis. Fibrils can be phagocytosed by macrophages in vitro but amyloid-associated factors may block the recognition of fibrils in vivo preventing the removal of islet amyloid in diabetes.

Islet amyloid polypeptide: actions and role in the pathogenesis of diabetes.

IAPP has been postulated to have a role as a modulating factor in glucose homoeostasis and to be involved in the pathophysiology of diabetes. However, the normal physiological functions of the peptide remain obscure: exogenous IAPP acts on many experimental systems to modulate nutrient supply and metabolism but there is no evidence to suggest that circulating IAPP has an aetiological role in the onset of Type-2 diabetes. Amyloid deposits formed from polymerized IAPP progressively accumulate in the islets of Type-2 diabetic patients. These insoluble deposits do not precipitate the onset of hyperglycaemia in Type-2 diabetes, but progressive accumulation of amyloid is associated with islet cell destruction and decreased islet function in the later stages of the disease. Although the causative factors of formation of the first IAPP fibril are unknown, continued high levels of insulin and IAPP secretion as a result of nutrient stimulation or insulin resistance will promote binding to preformed fibrils and extension of the deposits. It is important that methods to identify patients susceptible to amyloid deposition are developed and therapeutic agents are produced that can reduce or prevent polymerizatin of IAPP to form amyloid and minimize severe deterioration of islet function in Type-2 diabetes.

Processing of pro-islet amyloid polypeptide (proIAPP) by the prohormone convertase PC2.

Islet amyloid polypeptide (IAPP), 'amylin', is the component peptide of islet amyloid formed in Type 2 diabetes. IAPP is expressed in islet beta-cells and is derived from a larger precursor, proIAPP, by proteolysis. An in vitro translation/translocation system was used to separately examine processing of human proIAPP by the beta-cell endopeptidases PC2, PC3 or furin. ProIAPP was converted to mature IAPP by PC2 but there was little conversion by furin or PC3. These data are consistent with processing of proIAPP in beta-cell secretory granules. Abnormal cellular proteolysis associated with type 2 diabetes could contribute to IAPP amyloidosis.

Islet amyloid in type 2 (non-insulin-dependent) diabetes.

Amyloid deposits are found in pancreatic islets of 90% of type 2 (non-insulin-dependent) diabetic subjects at postmortem. Islet amyloid is formed from islet amyloid polypeptide (IAPP). IAPP is a 37 amino acid peptide which is a normal constituent of beta cells and is co-secreted with insulin in animals and in man. The causative factors for fibrillogenesis of IAPP are unclear, but could be related to the sequence of IAPP and abnormal production of the peptide. The lack of islet amyloid in rodent models of diabetes is due to proline substitutions in the amyloidogenic region of IAPP. Amyloid fibrils are deposited between beta cells and islet capillaries: fibrils in invaginations of the plasma membrane may interfere with membrane signalling and insulin release. Amyloid fibrils are formed within 2 days in culture in islets isolated from transgenic mice expressing the gene for human IAPP, but not in vivo. Overexpression and decreased clearance of human IAPP from islet spaces may be important factors. Progressive deposition of IAPP fibrils combined with the associated reduction in the insulin-secreting beta cells is likely to contribute to deterioration of islet function in the course of type 2 diabetes.