Reply to Liu et al.: Hypothalamic control of islets
Proceedings of the National Academy of Sciences of the United States of America
Liu et al. make several comments and suggestions (1) on our publication (2) in which we studied primarily the role of hypothalamic releasing hormones corticotropin-releasing hormone (CRH) and growth hormone-releasing hormone on islet function. The occurrence of HSD enzymes in rodent and human pancreatic islets has been shown by various groups. Although most of these groups have studied and discussed the role of 11β-HSD1 and steroid action with respect to β-cell function and insulin release, one
... study suggests a primary role of α-cells in pancreatic islets (3). Our study (2) demonstrated mRNA and protein expression of 11β-HSD1 in rodent and human islets and in β-cell line INS-1, as well as steroid regulation in INS-1 cells. Analysis of expression of mRNA for human and rat 11β-HSD1 and 11β-HSD2 was performed with sequence-specific primers and further quantified by real-time PCR. In addition, we analyzed expression of protein by using the anti-HSD1 antibody marketed by Abcam and detected a clear protein product for 11β-HSD1 in human and rat islets and a thin but detectable protein product in INS-1 cells. This protein was confirmed by immunohistochemistry. In further studies, we also could demonstrate a time-dependent down-regulation of the distinct 11β-HSD1 protein product in INS-1 cells treated with CRH in contrast to control cells. Previous studies of other groups [e.g., Liu et al. (4) ] that used several cell lines such as the preadipocyte, 3T3-L1, cell line with an acknowledged role of 11β-HSD1 have shown a very low, but detectable, expression of 11β-HSD1 protein; small changes of 11β-HSD correlated with an increase of more than 100% of the classical adipocyte cell differentiation markers, LPL or aP2. Therefore, with the 11β-HSD1 enzyme, low expression levels do correlate with highly significant physiological and pathophysiological responses. The 11β-HSD1 activity assay used in our study (2) is established and has been used by many other research groups to measure the conversion of cortisone added to cortisol, and corresponds to other radiolabel methods (5). The cross-reactivity of this immunoassay with cortisone is lower than 0.1%. Altogether, this part of our study confirms previous work of Oppermann and coworkers (5), demonstrating 11β-HSD1 in β-cells and establishing a role of glucocorticoid conversion by local 11β-HSD as well as its function in the regulation of insulin secretion in β-cells. An additional influence of CRH on 11β-HSD1 in α-cells is suggested by the work of Swali et al. (3). However, to our knowledge, no functional studies of 11β-HSD1 in α-cell lines, such as α-TC1, have been performed. Furthermore, there is an obvious natural and well established link of the key central stimulator of peripheral cortisol, CRH, to the 11β-HSD1 in various peripheral cell systems, including adipocytes and skin cells. Importantly in this context, CRH receptors were primarily expressed in β-cell lines and, to a lesser extent, in α-cell lines (6). Nevertheless, given the newly recognized role of α-cell/β-cell crosstalk in islet function, it may be useful to test the functional interaction of hypothalamic releasing hormones, CRH and growth hormone-releasing hormone, with 11β-HSD1 in α-cells.