The phosphorylation of a previously uncharacterized
protein of apparent M(r) approximately 140,000 was found to be increased when rat adipocytes were incubated with
insulin. The sequences of
peptides generated by digesting the
protein with
trypsin matched perfectly with sequences in mouse
lipin.
Lipin is the product of the gene that is mutated in
fatty liver dystrophy (fld) mice [Peterfy, M., Phan, J., Xu, P. & Reue, K (2001)
Nat. Genet. 27, 121-124], which exhibit several phenotypic abnormalities including
hyperlipidemia, defects in adipocyte differentiation,
impaired glucose tolerance, and slow growth. When immunoblots were prepared with
lipin antibodies, both endogenous adipocyte
lipin and recombinant
lipin overexpressed in HEK293 cells appeared as bands ranging in apparent M(r) from 120,000 to 140,000. Incubating adipocytes with
insulin decreased the electrophoretic mobility and stimulated the phosphorylation of both Ser and Thr residues in
lipin. The effects of
insulin were abolished by inhibitors of
phosphatidylinositol 3-OH
kinase, and by
rapamycin, a specific inhibitor of the mammalian target of rapamcyin (mTOR). The inhibition by
rapamycin was blocked by
FK506, which competitively inhibits those effects of
rapamycin that are mediated by inhibition of mTOR. Moreover,
amino acids, which activate mTOR, mimicked
insulin by increasing
lipin phosphorylation in a
rapamycin-sensitive manner. Thus,
lipin represents a target of the mTOR pathway, and potentially links this nutrient-sensing pathway to adipocyte development.