Brain, liver, kidney, heart, and skeletal muscle from
fatty liver dystrophy (fld/fld) mice, which do not express
lipin 1 (
lipin), contained much less Mg(2+)-dependent
phosphatidic acid phosphatase (PAP) activity than tissues from wild type mice.
Lipin harboring the fld(2j) (Gly(84) --> Arg) mutation exhibited relatively little PAP activity. These results indicate that
lipin is a major PAP in vivo and that the loss of PAP activity contributes to the fld phenotype. PAP activity was readily detected in
immune complexes of
lipin from 3T3-L1 adipocytes, where the
protein was found both as a microsomal form and a soluble, more highly phosphorylated, form. Fifteen phosphorylation sites were identified by mass spectrometric analyses.
Insulin increased the phosphorylation of multiple sites and promoted a gel shift that was due in part to phosphorylation of Ser(106). In contrast,
epinephrine and
oleic acid promoted dephosphorylation of
lipin. The PAP-specific activity of
lipin was not affected by the
hormones or by dephosphorylation of
lipin with
protein phosphatase 1. However, the ratio of soluble to microsomal
lipin was markedly increased in response to
insulin and decreased in response to
epinephrine and
oleic acid. The results suggest that
insulin and
epinephrine control
lipin primarily by changing localization rather than intrinsic PAP activity.