Abstract
The cholesteryl ester transfer protein (CETP) gene has been associated with a variety of phenotypes, including HDL-cholesterol levels and, more sporadically, with cardiovascular disease, obesity, and extreme longevity. Alterations of CETP activity levels can be caused by single-base polymorphisms as well as by alternative splicing. In addition to the previously characterized alternative splicing that skips exon 9, we found additional minor variants and characterized the activity of the resultant proteins. The novel variants skipped exon 9 sequences and inserted one of two in-frame exons from Alu-derived intronic sequences. None of the alternatively spliced variants are efficiently secreted, and coexpression of them inhibits wild-type CETP secretion. Expression of the alternative spliced variants causes an induction of genes linked to the endoplasmic reticulum (ER) stress response, including the neighboring HERPUD1 (homocysteine- and ER stress-inducible protein, ubiquitin-like domain-containing) gene. Unexpectedly, even though wild-type CETP is secreted much more efficiently than spliced variants, it induces the same degree of stress response as spliced variants, whereas a control secreted protein does not. CETP plays a complex role in modulating ER stress, with its expression inducing the response and its cholesteryl ester transfer activity and differential splicing modulating the response in other ways.
Highlights
The cholesteryl ester transfer protein (CETP ) gene has been associated with a variety of phenotypes, including HDL-cholesterol levels and, more sporadically, with cardiovascular disease, obesity, and extreme longevity
Splice variation in CETP had been previously identified with exon 9, so this region was assessed in more detail in a variety of CETP-expressing tissues (Fig. 1)
In one individual (BMI 5 21), sequence analysis revealed that the inserted 108 bp was derived from an Alu-like sequence present in intron 10 of CETP (Fig. 2A)
Summary
The cholesteryl ester transfer protein (CETP ) gene has been associated with a variety of phenotypes, including HDL-cholesterol levels and, more sporadically, with cardiovascular disease, obesity, and extreme longevity. Because the protein product of the major alternative splice variant of CETP is retained within the ER, there is a possibility that it could affect the ER stress or the unfolded protein response. Protein expression and secretion of CETP(FL), CETP(2Ex9), and BPI (bactericidal permeability-increasing protein) were analyzed using whole-cell lysates and unconcentrated media from the transfections.
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