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This version published online on March 2, 2006
Endocrinology, doi:10.1210/en.2005-1426
A more recent version of this article appeared on June 1, 2006
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Submitted on November 9, 2005
Accepted on February 22, 2006

DHCR24-knockout embryonic fibroblasts are susceptible to serum withdrawal-induced apoptosis because of dysfunction of caveolae and insulin-Akt-Bad signaling

Xiuli Lu, Fukushi Kambe*, Xia Cao, Taemi Yoshida, Sachiko Ohmori, Kohji Murakami, Takahide Kaji, Takehisa Ishii, David Zadworny, and Hisao Seo

Department of Endocrinology and Metabolism, Division of Molecular and Cellular Adaptation, Research Institute of Environmental Medicine, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; Mitsubishi Pharma Corporation, Kamoshida-cho, Aoba-ku, Yokohama 227-0033, Japan; Department of Animal Science, McGill University, St Anne de Bellevue, H9X-3V9, P-Q, Canada

* To whom correspondence should be addressed. E-mail: kambe{at}riem.nagoya-u.ac.jp.

The DHCR24 gene encodes an enzyme catalyzing the last step of cholesterol biosynthesis, the conversion of desmosterol to cholesterol. To elucidate the physiological significance of cholesterol biosynthesis in mammalian cells, we investigated proliferation of mouse embryonic fibroblasts (MEFs) prepared from DHCR24- mice. Both DHCR24- and wild-type MEFs proliferated in the presence of serum in culture media. However, the inhibition of external cholesterol supply by serum withdrawal induced apoptosis of DHCR24- MEFs, which was associated with a marked decrease in the intracellular and plasma membrane cholesterol levels, Akt inactivation, and Bad dephosphorylation. Insulin is an anti-apoptotic factor capable of stimulating the Akt-Bad cascade, and its receptor (IR) is enriched in caveolae, cholesterol-rich microdomains of plasma membrane. We thus analyzed the association of IR and caveolae in the cholesterol-depleted MEFs. Subcellular fractionation and immunocytochemical analyses revealed that the IR and caveolin-1 contents were markedly reduced in the caveolae fraction of the MEFs suggesting the disruption of caveolae, and that large amounts of IR were present apart from caveolin-1 on plasma membrane indicating the uncoupling of IR with caveolae. Consistent with these findings, insulin-dependent phosphorylations of insulin receptor substrate-1, Akt, and Bad were impaired in the cholesterol-depleted MEFs. However, this impairment was partial, because treatment of the MEFs with insulin restored Akt activation and prevented apoptosis. Cholesterol supply also prevented apoptosis. These results demonstrate that the cellular cholesterol biosynthesis is critical for the activation and maintenance of the Akt-Bad cell survival cascade in response to growth factors such as insulin.
Key words: DHCR24 • cholesterol • insulin • Akt • Bad • caveolae • apoptosis




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