Searching for protein partners of shortchain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) reveals keratin 8 as a novel candidate for interaction in pancreatic β-cells

Abstract
Background Short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) is a ubiquitously expressed mitochondrial
enzyme with a role in the degradation of fatty acids. Because the protein also is a negative regulator of insulin
secretion in pancreatic β-cells, inactivating mutations in the SCHAD gene (HADH) cause congenital hyperinsulinism of
infancy (CHI) and severe hypoglycemia. Here we sought to identify novel interaction partners of SCHAD that might be
particularly relevant for the endocrine pancreas.
Results Employing the SCHAD protein as bait, we performed yeast 2-hybrid screening of a cDNA library made
from human islets of Langerhans. Surprisingly, the screening revealed the intermediate filament protein keratin
8 (K8) as a putative interaction partner of SCHAD with very high confidence. Previous reports have linked K8 to
glucose homeostasis, and we confirmed the SCHAD interaction by co-immunoprecipitation in HEK293 cells. SCHAD
and K8 expression were then characterized in the human β-cell model EndoC-βH1. By using proximity ligation
assay, we demonstrated that stimulating the cells with a high level of glucose triggered a transient increase in the
interaction. However, when studying knockout mice, we found that the loss of either K8 or SCHAD did not change the
expression level of the other interaction partner. Still, when K8 knockout mice were challenged with a ketogenic diet,
upregulation of SCHAD expression was blunted compared to the upregulation observed in wildtype littermates.
Conclusions We propose that the SCHAD protein interacts with K8 in a way that might be relevant for proper
functioning of the pancreatic β-cell. Whether the SCHAD-K8 interaction influences the phenotype of CHI remains to
be demonstrated.