J. Cohen, J. Prins, and B. Venkatesh
Introduction
The importance of the stress response in survival from critical illness is not in dis- pute. Adrenalectomized animals or patients with documented adrenal insufficiency have a high mortality when exposed to physiological stress [1, 2]. However, in the setting of critical illness, attempting to determine what constitutes an appropriate stress response is not straightforward.
Evidence of activation of the hypothalamo-pituitary axis is a frequently used sur- rogate for induction of the stress response. In the normal patient population, inves- tigation of the hypothalamo-pituitary-adrenal (HPA) axis centers around measure- ments of plasma cortisol, often before and following stimulation with high doses of synthetic adrenocorticotropic hormone (ACTH). Criteria have been developed based on hormonal profiles and responses to stimulation tests to define inadequate stress responses or a condition which is loosely termed relative adrenal insufficiency.
Whilst there is acceptance of the concept of relative adrenal insufficiency, the diag- nostic criteria for it, based on plasma cortisol and the response to ACTH, continue to generate controversy. These issues have been reviewed at length in a recent publi- cation by Venkatesh et al. [3].
Interest has begun to turn to free cortisol estimation, and early work in this area is encouraging [4, 5]. An area which has received little attention in critical illness, but is gaining widespread consideration in endocrine practice, is the extent of alter- ation in cortisol metabolism, which in turn will determine the exposure of tissues to adequate concentrations of cortisol.
Normal Cortisol Metabolism
Cortisol, the major glucocorticoid synthesized by the adrenal cortex plays a pivotal role in normal metabolism. Its secretion is under the control of the hypothalamic pituitary axis. There are a variety of stimuli to secretion, including stress, tissue damage, cytokine release, hypoxia, hypotension and hypoglycemia. These factors act upon the hypothalamus to favor the release of corticotropin releasing hormone (CRH) and vasopressin. These in turn stimulate the secretion of ACTH, which stim- ulates the release of cortisol, mineralocorticoids (principally aldosterone), and androgens from the adrenal cortex. During periods of stress, trauma or infection, there is an increase in CRH and ACTH secretion and a reduction in the negative feedback effect, resulting in increased cortisol levels, in amounts roughly propor- tional to the severity of the illness [6 – 8].
Fig. 1. The cortisol – cortisone shuttle
The majority of circulating cortisol is bound to an alpha-globulin called transcortin (corticosteroid-binding globulin, CBG). At normal concentrations of total plasma cortisol (e.g., 375 nmol/l or 13.5 µg/dl) less than 5 % exists as free cortisol in the plasma; however, it is this free fraction that is biologically active. In normal subjects CBG can bind approximately 700 nmol/l (i.e., 25 µg/dl) [9]. At levels greater than this, the increase in plasma cortisol is largely in the unbound fraction. CBG is a sub- strate for elastase, a polymorphonuclear enzyme that cleaves CBG, markedly decreasing its affinity for cortisol [10]. This enzymatic cleavage results in the libera- tion of free cortisol at sites of inflammation. CBG levels have been documented to fall during critical illness [4, 11, 12], and these changes are postulated to increase the amount of circulating free cortisol.
Until recently, it was believed that the major determinant of cortisol activity in vivo was the plasma free cortisol concentration. However, it has become apparent that cortisol activity inside cells is modulated by the actions of an enzyme system, the 11 q hydroxysteroid dehydrogenases, type 1 (11 q -HSD1) and type 2 (11 q -HSD2).
These enzymes are responsible for the interconversion of active cortisol and inactive cortisone (Fig. 1) [13]. The concentration of cortisol at the receptor site is critical in determining its action and is a balance between synthesis and metabolism. Whilst a number of enzymes play a role in the metabolism of cortisol (11q -HSD, 5 alpha and beta reductases, and 6 beta hydroxylase), quantitatively the activity of 11q -HSD is the most important pathway (Table 1). 11q -HSD modulates the selectivity, specific- ity, and intensity of glucocorticoid dependent processes and regulates intracellular concentrations of cortisone (inactive) and active cortisol. The enzyme was discov- ered in 1953 by Amelung and colleagues who described the interconversion of corti- sol and cortisone [14]. The 11q -HSD1 isoform has a widespread expression through- out the body, being found primarily in liver, lung, adipose tissue, vascular tissue, ovary, and central nervous system (CNS). Its primary action in vivo appears to be reductase, catalyzing the formation of active cortisol from inactive cortisone. Previ- ously it was thought the enzyme was bi-directional in action, but it now appears that it only exhibits dehydrogenase activity in disrupted cells. The reason for this is not clear, but it may be related to the specific intracellular localization of the enzyme [15]. Conversely 11q -HSD2 functions physiologically only in the dehydrogenase mode, catalyzing the formation of cortisone from cortisol. 11q -HSD2 has its major site of action in the kidney, where it functions to inactivate cortisol prior to its bind- ing and activation of the mineralocorticoid receptor. The tissue distribution of these
Table 1. Structure and activity of the 11q -HSD enzymatic system
11q -HSD1 11q -HSD2
Structure Bidirectional, but mainly reduc-
tase, NADPH cofactor
Dehydrogenase, NAD cofactor
Function Converts cortisone to cortisol Converts cortisol to cortisone
Tissue Liver, lung, gonads, pituitary,
brain
Kidney, colon, salivary glands, placenta
Inhibition by carbenoxolone Moderate Strong
enzymes plays a major role in the regulation of glucocorticoid and mineralocorti- coid receptor activation. For example, the higher concentration of 11q -HSD2 in the kidney prevent excess mineralocorticoid effects in the renal tubules from circulating cortisol. Similarly, in the placenta, 11q -HSD2 protects the fetus from the deleterious effects of maternal glucocorticoids.
The set point of total body 11q -HSD1 vs. 11 q -HSD2 activity may be estimated by measuring the serum ratio of total cortisol to total cortisone (F:E ratio) [16 – 18].
The normal plasma F:E ratio ranges between 4 – 7 [19]. Abnormalities of the HSD enzyme system and, therefore, altered metabolism of cortisol have been implicated in the pathogenesis of hypertension, obesity, vascular disease, and the metabolic syndrome [20 – 23].
Role of 11-HSD Activity in Human Disease Syndrome of Apparent Mineralocorticoid Excess
Defects in the gene encoding for 11q -HSD have been described in patients suffering from apparent mineralocorticoid excess, an extremely rare inherited hypertensive disorder [24]. The condition is characterized by low levels of renin and aldosterone, severe hypertension and hypokalemia. This syndrome is thought to be caused by defective peripheral conversion of cortisol to cortisone. Patients thus appear to be suffering from excess mineralocorticoid activity, despite measured levels being reduced. The lack of 11q -HSD2 activity in these individuals allows cortisol to acti- vate the mineralocorticoid receptor and function as a potent mineralocorticoid.
Management by blocking the mineralocorticoid receptor with spironolactone appears to be effective.
Obesity and Metabolic Syndrome
The role of 11-q HSD1 appears to be to increase glucocorticoid activity in tissues in which it is expressed. Its potential importance in the pathogenesis of a wide variety of common clinical conditions is now becoming apparent. Mice with an over expres- sion of 11q -HSD1 in adipose tissue have been shown to have higher levels of gluco- corticoids in the portal vein, and exhibit obesity, hyperglycemia, and hypertension [25]. This observation, coupled with studies in human obese subjects, has led to the suggestion that 11q -HSD1 activity may be associated with the development of the metabolic syndrome, a group of cardiovascular risk factors including insulin resis- tance, hypertension, and obesity. Early work on selective 11q -HSD1 inhibitors in
animal models has demonstrated decreases in blood glucose, triglycerides, and body weight [26].
Miscellaneous
11q -HSD1 activity has also been associated with the development of osteoporosis [27], the pathogenesis of polycystic ovary syndrome [28], and in the regulation of vascular tone [29].
Activation of 11-HSD in Stress and Critical Illness
While there has been a great deal of interest in the local tissue activity of the 11q - HSD system, there has been relatively little work into its systemic role. Relative changes in the activity of the isoenzymes could influence glucocorticoid availability at a tissue level. This would be of particular relevance in patients subjected to a sig- nificant stress response.
There are some limited human data to support the notion that an upregulation of 11q -HSD-1 occurs in response to stress. Vogeser et al. examined the serum of 15 unselected hospitalized patients having C-reactive protein (CRP) estimations. Using the serum F:E ratio as a marker of the set point of total body HSD activity they dem- onstrated a significant correlation between elevated CRP levels and the F:E ratio (r = 0.56, p ‹ 0.001); multivariate regression analysis showed that this association was independent of serum cortisol [30]. The same group used a similar methodology to determine HSD activity in cardiac surgical patients. Postoperatively, they were able to demonstrate a significant increase in the F: E ratio as compared to the preopera- tive level (11.3 vs. 5.4, p ‹ 0.001), which persisted throughout the four days of the study [31]. In contrast, the serum cortisol level doubled on the first postoperative day, but then declined. The authors hypothesized that these data suggested that after surgical stress increased activity of 11q -HSD1 would be a more chronic response, acting to increase glucocorticoid action at a tissue level, despite declining plasma cortisol concentrations.
To our knowledge there are currently no published data examining 11q -HSD activity in the critically ill. However, our group has produced some data (unpub- lished) in patients with severe sepsis, trauma, and burns which support the previous findings. Persistent elevations in F:E ratios in patients suffering stress suggest that the set point of 11q -HSD activity is shifted towards reductase activity; this implies that the tissue activity of cortisol is increased, despite steady or even declining serum cortisol concentrations.
Mechanisms Behind Activation of 11-HSD in Critical Illness
Multiple mechanisms might come into play to account for the differential regulation of the 11q -HSD enzyme systems. The observed increase in the F:E ratio could be explained by substrate overload of 11q -HSD2; however, if this were the case, plasma cortisol concentrations and the F:E ratio would be expected to change in parallel, which was not observed [31]. Direct mediation by ACTH is also unlikely given that F:E ratios have been shown to increase in non-ACTH-dependent hypercortisolism [32].
More likely mechanisms would be enhancement by inflammatory mediators, and transcriptional regulation from other hormones. Pro-inflammatory cytokines have been demonstrated in vitro to upregulate 11q -HSD1 activity. Studies on cell cultures have demonstrated that tumor necrosis factor (TNF)-[ enhances cortisol availability to the cell by enhancing the activity of 11q -HSD1 and suppressing that of 11 q -HSD2 [33, 34]. Such effects have been demonstrated in a variety of cell types including smooth muscle [35], lung epithelium [36], and adipose tissue [37].
Hormones acting on 11q -HSD1 expression include glucocorticoids, growth hor- mone, sex steroids, insulin, and thyroid hormone. Glucocorticoids increase expres- sion, whilst growth hormone decreases it; the effect of the other hormones appears to vary from tissue to tissue and among species [38].
Other potential mechanisms for altered 11q -HSD1 activity include changes in redox potential within the cell. The oxo-reductase activity seen in intact cells requires NADPH and leads to the activation of glucocorticoids. Thus, concentrations of NADPH (which in turn are determined by cytosolic redox) influence the activity of the enzyme [39]. Critically ill septic patients frequently demonstrate alterations in cellular redox potential mediated largely by endotoxin [40]. Patients with trauma and burns may also demonstrate changes in redox resulting from altered tissue per- fusion [41].
Hypotheses for Future Research
Clearly the investigation of glucocorticoid activity at a tissue level in critically ill patients is at a very early stage. However, the recognition that a significant level of control is being exerted that is not directly observable from plasma cortisol mea- surements is of interest. An intriguing possibility is that measurements of 11q -HSD activity, either by F:E ratio or direct tissue estimation, may serve as valid markers for HPA axis activity in septic patients. It could be hypothesized that downregula- tion of 11q -HSD1 activity or an increase in 11 q -HSD2 activity would lead to tissue hypocortisolism, manifesting as hypotension and inotrope dependence. This group of patients may benefit from steroid administration. Conversely, an upregulation of 11q -HSD1 activity, and/or downregulation of 11 q -HSD2, would lead to an excess of tissue cortisol activity, manifesting as hyperglycemia and insulin resistance. In these patients, selective 11q -HSD1 inhibitors, such as benzothiazole derivatives, may have a role [42]. Further studies examining the tissue activity of each 11q -HSD isoen- zyme, and of a size large enough to detect an association with outcome would be required to investigate this possibility.
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