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Ana I Arroba, Alfonso M Lechuga-Sancho, Laura M Frago, Jesús Argente and Julie A Chowen

Poorly controlled diabetes mellitus can result in decreased prolactin production and thus problems with lactation, reproduction, and other physiological processes. This may be due to a loss of lactotrophs, as we have previously shown that long-term (8 weeks) poorly controlled streptozotocin-induced diabetes results in increased death of lactotrophs and that this most likely occurs through the activation of caspase-8 and the extrinsic cell death cascade. However, cell proliferation is also increased in the anterior pituitary at this time, although the cell type undergoing this proliferation and whether it is a response to the increased cell death remains unknown. In order to determine the time-course of increased cell death and proliferation in the anterior pituitary and if this is related to changes in tumor necrosis factor (TNF)-α, a cytokine involved in the activation of the extrinsic cell death pathway, rats were killed at 1, 4, 6, and 8 weeks after the induction of diabetes. Cell death was significantly increased after 4 weeks, as was caspase-8 activation, although circulating levels of TNF-α were increased as early as 1 week. Pituitary levels of TNF-α did not change significantly until 8 weeks after diabetes onset. Similarly, Western-blot analysis of proliferating cell nuclear antigen showed that anterior pituitary cell proliferation increased significantly 8 weeks after diabetes onset, with the majority of proliferating cells, as detected by BrdU incorporation, corresponding to lactotrophs. These results suggest that the increased death of lactotrophs in poorly controlled diabetic rats is followed by increased proliferation of this cell type, even when no treatment is given.

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Ana I Arroba, Alfonso M Lechuga-Sancho, Laura M Frago, Jesús Argente and Julie A Chowen

Cell death is increased in the anterior pituitary of poorly controlled diabetic rats, but anti-apoptotic mechanisms are also activated. We hypothesized that specific cell types are selectively protected against diabetes-induced cell death. To determine when anti-apoptotic mechanisms are activated, streptozotocin-induced diabetic rats were killed after 1, 4, 6 and 8 weeks of evolution. Anterior pituitaries were processed for western blot analysis to determine changes in the intrinsic cell death pathway and upstream kinases involved in cell protection mechanisms. An increase in cell death was detected by ELISA at 4 weeks of diabetes. TUNEL labelling demonstrated that this corresponded to death of primarily lactotrophs, a few somatotrophs, and no thyrotrophs, corticotrophs or gonadotrophs. Levels of phosphorylated (p) Akt were increased at 1 week of diabetes, while pERK1/2 levels increased at 4 weeks and pJNK at 6 weeks. Activation of caspase 3 decreased and anti-apoptotic members of the Bcl-2 protein family increased as early as 1 week after diabetes onset. These changes were coincident with increased IGF-I receptor levels. Levels of X-linked inhibitor of apoptosis protein (XIAP) increased significantly after 6 weeks of diabetes, as did activation of nuclear factor (NF)κB. Double immunohistochemistry indicated that XIAP was expressed in less than 1% of lactotrophs and gonadotrophs, approximately 50% of somatotrophs and more than 90% of corticotrophs and thyrotrophs. These results suggest that some cell survival mechanisms are rapidly activated in the anterior pituitary, even before increased cell death can be detected, while others are more delayed. Furthermore, both pituitary cell death and expression of protective mechanisms such as XIAP are cell-type specific.