New reserach shows that insulin is required for activation of lipid metabolism:

Published online before print January 6, 2005, 10.1073/pnas.0405067102
PNAS | January 18, 2005 | vol. 102 | no. 3 | 791-796

http://www.pnas.org/cgi/content/short/102/3/791

Distinct roles of insulin and liver X receptor in the induction and cleavage of
sterol regulatory elementbinding protein-1c

Bronwyn D. Hegarty, Alexandre Bobard, Isabelle Hainault, Pascal Ferré, Pascale
Bossard and Fabienne Foufelle

Abstract
Sterol regulatory element-binding proteins (SREBPs) are transcription factors
central to the regulation of lipid metabolism. The SREBPs are synthesized as
precursor proteins that require proteolytic processing to become
transcriptionally active. Whereas the regulation of SREBP-1a and -2 cleavage by
cellular sterol content is well defined, much less is known about the
regulation of SREBP-1c, the predominant SREBP isoform in the liver. Both
insulin and liver X receptor  (LXR) induce SREBP-1c transcription; however, the
respective roles of these factors and the mechanism responsible for proteolytic
cleavage of this SREBP isoform are not known. In this study, we compare the
effects of insulin and LXR agonist TO-901317 on SREBP-1c expression and
transcriptional activity in isolated rat hepatocytes. We report that full
induction of the mature and transcriptionally active form of SREBP-1c protein
requires insulin. Although activation of LXR leads to the induction of SREBP-1c
gene expression and precursor protein, it has a very poor effect in inducing
the mature nuclear form of the transcription factor. This may be due to the
induction of insulin-induced gene-2a mRNA and protein by LXR activation. The
LXR-induced SREBP-1c precursor, however, is rapidly cleaved on acute exposure
to insulin via a phosphatidylinositol 3-kinase-dependent mechanism. Finally, we
show through experiments in suckling mice that this acute action of insulin to
stimulate the proteolytic processing of SREBP-1c is functional in vivo.

Tom Billings