Prior to their identification as PAP enzymes, the family of three mammalian lipin genes were identified through genetic studies in the mouse. The founding member, Lpin1
(encoding lipin-1), was identified as the mutated gene in the fatty liver dystrophy (fld
) mouse [29
]. The fld
mouse exhibits neonatal fatty liver, peripheral neuropathy, lipodystrophy, insulin resistance, and increased susceptibility to atherosclerosis [30
]. Lipin-2 and lipin-3 were identified by sequence similarity to lipin-1, and the three represent a dispersed gene family with three members in mammals, and single orthologues in most invertebrates and in single celled eukaryotes [29
As described in a previous section, all three mammalian lipin proteins exhibit PAP activity, albeit at different specific activities [22
]. PAP activity requires a conserved DIDGT motif that was originally described in the yeast PAH1 [21
]. In addition to providing the DAG required for the synthesis of TAG, PC and PE in mammals and yeast, lipin-1 functions as a transcriptional coactivator [33
]. In liver, lipin-1 binds to PGC-1α and PPARα to increase the expression of enzymes involved in β-oxidation [35
]. Increased lipin-1 PAP activity enables the liver to sequester as TAG the increased fatty acids that are supplied to it in starvation and diabetes. Thus, under these conditions lipin-1 may act to coordinate the oxidation and storage of fatty acids as TAG to prevent the toxic effects resulting from fatty acid accumulation. Using mutant lipin-1 constructs in an in vitro
assay, it was shown that lipin-1 coactivator activity proceeds even in the absence of PAP activity [35
]. However, studies with the yeast PAH1-encoded PAP enzyme indicate that PAP activity is required for all roles of the yeast protein, including effects on cellular lipid composition, derepression of INO1 gene expression, and maintenance of ER/nuclear membrane structure [36
]. Further studies are required to determine whether the mammalian lipin proteins differ from the yeast PAP in having a distinct role in gene regulation that is distinct from its PAP activity.
The gene expression patterns of the three mammalian lipin proteins reveal distinct tissue distributions for each protein, suggesting unique physiological roles. Lipin-1 is expressed at highest levels in adipose tissue, skeletal muscle, and testis, and it is also present in liver, peripheral nerve and other tissues [22
]. Lipin-2 expression is most prominent in liver, and is also present in several other tissues, while lipin-3 is expressed at low levels in most visceral tissues, but has substantial levels in small intestine [22
The lipodystrophy in lipin-1 deficient fld
mice reflects a key role for lipin-1 in adipocyte differentiation and lipid biosynthesis [37
]. Interestingly, lipin-1 deficiency has recently been described in rare human patients, and it causes acute myoglobinuria in childhood [40
]. In contrast to the mouse model of lipin-1 deficiency, there was no report of lipodystrophy in these individuals, which raises the possibility that another lipin protein is able to substitute for lipin-1 activity in human adipose tissue. In support of this possibility, lipin-2 has been detected in human adipose tissue [22
], and is also expressed in a mouse preadipocyte cell line [41
]. Thus, in the 3T3-L1 cell line, lipin-2 protein levels are highest in preadipocytes, but decline dramatically as adipocyte differentiation proceeds to become virtually undetectable in mature adipocytes, when lipin-1 is expressed at high levels [41
]. Further work will establish whether there is a role for both lipin-1 and lipin-2 in adipose tissue in vivo
, and whether there are species differences.
Rare patients with lipin-2 deficiency have also provided clues to its physiological function. These individuals have a complex phenotype known as Majeed syndrome, characterized by recurrent osteomyelitis, fever, and anemia [42
]. Based on the tissue distribution of lipin-2, it is not clear how its deficiency causes these symptoms, but work in mouse models of lipin-2 deficiency will likely be informative.
The study of lipin deficiencies has revealed that, in addition to the impaired production of DAG, the accumulation of phosphatidate within cells likely accounts for some of the detrimental effects observed. Indeed, the peripheral neuropathy in fld
mice is associated with PA accumulation in Schwann cells, which may in turn activate signaling through the MEK/Extracellular-signal Regulated Kinase (ERK) cascade [44
]. Furthermore, a muscle sample obtained from one patient with lipin-1 deficiency revealed elevated phosphatidate levels [40
]). These findings suggest that the lipin proteins serve an important role in regulating the balance of lipid intermediates, including PA and DAG, and maintenance of cellular lipid homeostasis.