Beta cell stress and heparan sulfate proteoglycans in type 2 diabetes

Abstract

Heparan sulfate proteoglycan (HSPG) core proteins and highly sulfated heparan sulfate (HS) are expressed at unusually high levels inside islet beta cells. Intracellular HS plays a critical role in the survival of beta cells and acts as a constitutive non-enzymatic quencher of reactive oxygen species (ROS). Heparanase (Hpse)-mediated degradation of intracellular HS in the beta cells of non-obese diabetic (NOD) mice has been shown to contribute to the development of type 1 diabetes (T1D). This project examines the role of beta cell HS and HSPG core proteins in obese db/db mice during the progression of type 2 diabetes (T2D).

Endoplasmic reticulum (ER) stress contributes to beta cell failure in T2D. Our studies demonstrate that ER stress in the islets of db/db mice correlates with a progressive loss of intra-islet HSPG core proteins and HS during T2D development. The isolation of islets in vitro leads to loss of intracellular HS but not HSPG core proteins from islet beta cells. Replacement of the lost HS in the wildtype and db/db beta cells in vitro using heparin (a highly sulfated HS analogue) improved cell viability and protected the beta cells from culture- and ROS-induced death (i.e., oxidative stress). Significantly, treatment of db/db mice with the chemical chaperone TUDCA relieved ER stress, improved intra-islet HSPG core proteins, HS and insulin levels, and impaired T2D progression. Similarly, Ins2WT/C96Y Akita mice, an established model of ER stress-mediated diabetes, demonstrated a decline in intra-islet HSPG core proteins, HS and insulin in islet beta cells. In addition, in vitro induction of ER stress in MIN6 cells reduced levels of intracellular HSPG core proteins, HS and Hpse (a HS-degrading enzyme) and increased beta cell death. ER stress-induced HS loss was therefore Hpse-independent. Since HS is synthesised directly onto HSPG core proteins, the novel findings in this project indicate that ER stress in db/db beta cells downregulates the production of HSPG core proteins, leading to diminished HS synthesis. As a consequence, beta cells are rendered more highly susceptible to oxidative stress and death.

A chronic low grade inflammation is associated with obesity, insulin resistance and beta cell stress in T2D. An increased proportion of monocytes/inflammatory macrophages in white adipose tissue, confirmed the heightened inflammatory status of db/db mice, compared to wildtype controls. Importantly, increased cell surface expression of Hpse on myeloid cells was observed in wildtype white adipose tissue (homeostatic inflammation) and db/db white adipose tissue (T2D obesity-induced inflammation) compared to corresponding levels in the circulation. This finding suggests that Hpse plays an important role in the migration of leukocytes from the circulation and across sub-endothelial basement membranes to establish the enhanced adipose tissue inflammation in T2D.

This project has identified the potential for treatment with a heparin-like HS replacer/Hpse inhibitor, chemical chaperone or a combination of both for rescuing T2D beta cell HS, reducing adipose tissue inflammation and promoting beta cell survival/function.