Intracellular Retention of Carboxyl‐ester Lipase Caused by Single‐base Deletion in its Variable Number of Tandem Repeats

A single‐base deletion (1686delT) in the first repeat of the variable number of tandem repeats of the gene encoding carboxyl‐ester lipase (CEL) associates with exocrine and endocrine pancreatic dysfunction in humans. Because the mutation results in a frame shift of the C‐terminus of the protein (C563fsX673), we hypothesized that the mutant protein would have decreased secretion, accumulate in the cell and trigger stress responses. We compared the properties of CEL C563fsX673 to those of normal CEL in transiently transfected 293T cells. Compared to normal CEL, the secretion of CEL C562fsX673 protein was severely impaired and the secreted mutant protein was less stable; intracellular levels of C562fsX673 CEL were significantly increased and the mutant protein was less soluble. MG132 or lactacystin did not alter levels of C562fsX673 protein. Neither BIP protein expression nor Xbp1 mRNA splicing were increased. Our results demonstrate that the frame‐shift alters the secretion of C562fsX673 CEL and that the mutant protein accumulates in the cell. The intracellular protein is not largely degraded through the proteasome degradation pathway. Although the intracellular retention of C562fsX673 CEL may play a role in the pathogenesis of the exocrine and endocrine dysfunction of patients with the mutation, the mechanism does not significantly involve ER stress or UPR in transfected cells. Funded by NIH DK080820.