Keratin 8 modulates β-cell stress responses and normoglycaemia

Abstract

SummaryKeratin intermediate filament (IF) proteins are epithelial cell cytoskeletal components that provide structural stability and protection from cell stress, among other cellular and tissue-specific functions. Numerous human diseases are associated with IF gene mutations, but the function of keratins in the endocrine pancreas and their potential significance for glycaemic control are unknown. The impact of keratins on b-cell organisation and systemic glucose control was assessed using keratin 8 (K8) wild-type (K8 +/+ ) and K8 knockout (K8 2/2 ) mice. Islet b-cell keratins were characterised under basal conditions, in streptozotocin (STZ)-induced diabetes and in non-obese diabetic (NOD) mice. STZ-induced diabetes incidence and islet damage was assessed in K8 +/+ and K8 2/2 mice. K8 and K18 were the predominant keratins in islet b-cells and K82/2 mice expressed only remnant K18 and K7. K8 deletion resulted in lower fasting glucose levels, increased glucose tolerance and insulin sensitivity, reduced glucose-stimulated insulin secretion and decreased pancreatic insulin content. GLUT2 localisation and insulin vesicle morphology were disrupted in K8 2/2 b-cells. The increased levels of cytoplasmic GLUT2 correlated with resistance to high-dose STZ-induced injury in K8 2/2 mice. However, K8 deletion conferred no long-term protection from STZ-induced diabetes and prolonged STZ-induced stress caused increased exocrine damage in K8 2/2 mice. b-cell keratin upregulation occurred 2 weeks after treatments with low-dose STZ in K8 +/+ mice and in diabetic NOD mice, suggesting a role for keratins, particularly in non-acute islet stress responses. These results demonstrate previously unrecognised functions for keratins in b-cell intracellular organisation, as well as for systemic blood glucose control under basal conditions and in diabetes-induced stress.

Decreased levels of keratin 8 sensitize mice to streptozotocin‐induced diabetes

“…The decreased islet levels of K8 and K18 in K8 +/− compared to K8 +/+ were confirmed by immunostaining (Figure 1D). Type II K7 protein levels are high in exocrine ducts, but very low in islets 20. Thus, K7 was detectable only by immunofluorescence staining and K7 appeared slightly decreased in K8 +/− compared to K8 +/+ islets (Figure S1).…”

“…Moreover, liver fragility is observed in both K8 −/− and K18 −/− as well as in K18 mutant mice, while K8 knockdown disrupts cell morphology and increases apoptosis in the thymus 7, 15. Keratins are upregulated in epithelial cells after organ‐specific stress or disease conditions, as documented in several organs such as colon, liver, kidney, β‐cells and gall bladder 10, 19, 20…”

“…The loss of K8 also leads to altered glucose metabolism seen as decreased fasting blood glucose levels,20 as well as increased glucose uptake and glycogen synthesis in hepatocytes 21. In β‐cells, K8 loss causes disturbances in the plasma membrane targeting of glucose transporter 2 (GLUT2) resulting in a delayed sensitivity to streptozotocin (STZ)‐induced T1D 20.…”

“…In β‐cells, K8 loss causes disturbances in the plasma membrane targeting of glucose transporter 2 (GLUT2) resulting in a delayed sensitivity to streptozotocin (STZ)‐induced T1D 20. K8 loss is moreover associated with a decrease in insulin vesicle size and insulin secretion, as well as mitochondrial defects, including smaller sized mitochondria, decreased mitochondrial membrane potential and ATP production in β‐cells 20, 22.…”

“…In β‐cells, K8 loss causes disturbances in the plasma membrane targeting of glucose transporter 2 (GLUT2) resulting in a delayed sensitivity to streptozotocin (STZ)‐induced T1D 20. K8 loss is moreover associated with a decrease in insulin vesicle size and insulin secretion, as well as mitochondrial defects, including smaller sized mitochondria, decreased mitochondrial membrane potential and ATP production in β‐cells 20, 22. Interestingly, K8 has been reported to be upregulated in cultured β‐cells upon stimulation with glucose,23 and ectopic expression of epidermal keratins K1 and K10 in the pancreas leads to diabetes due to loss of β‐cell insulin vesicles and decreased insulin secretion 24.…”