Question:
Lipid droplets (LDs) are crucial organelles that play a central role in lipid homeostasis.
A particular lipase associated with LDs is the patatin-like phospholipase domain containing
protein 3 (PNPLA3). The I148 M single nucleotide polymorphism of PNPLA3 has been shown
to be associated with non-alcoholic fatty liver disease and the development of hepatocellular
carcinoma. Aim of the study was to investigate the underlying molecular mechanism
driving the progression of the disease.
Methods:
To unravel the interplay between PNPLA3 and the LD-associated proteins of the perilipin-family
in the regulation of LDs, (immuno)-histochemical analysis of a collective of 47 ASH-
and 25 NASH-patients with known PNPLA3-status was undertaken. In addition, co-immunoprecipitation
experiments were performed to identify novel PNPLA3 binding partners. Finally, the
impact of the polymorphism on the lipolytic activity was determined.
Results:
Histologically, livers of I148 M carriers showed enhanced ballooning, acinar inflammation,
microgranulomas and increased fibrosis with a prominent staining for perilipin 2 at
ballooned hepatocytes. Perilipin 5 localized less to LDs, but showed a more cytoplasmic
and partially nuclear localization instead. Interestingly, hepatocytes that were strongly
positive for PNPLA3 showed diminished perilipin 1-expression.
Furthermore, we identified perilipin 5 and the lipase PNPLA2/ATGL as novel interaction
partners of PNPLA3 that co-localized at LDs. In addition, we could show that perilipin
5, PNPLA3, and PNPLA2 are part of the same complex and that perilipin 5 is enhancing
the binding of PNPLA3 with PNPLA2 dramatically. Strikingly, we could show dimerization
of PNPLA2, a process inhibited by PNPLA3. Addressing the lipolytic activity of PNPLA2
revealed an inhibitory effect of PNPLA3, which is strongest for PNPLA3(I148 M).
Conclusion:
In summary, our data indicate that PNPLA3 regulates lipolysis by repressing the lipolytic
activity of PNPLA2 in a perilipin 5-dependent manner most likely via disrupting active
PNPLA2 homo-dimers/oligomers. The enzymatically inactive PNPLA3(I148 M) even further
reduces PNPLA2-mediated lipolysis and increases toxic accumulation of lipids. We have
thereby unraveled a mechanism of PNPLA3 in the progression of steatotic liver diseases.
Concerning the long-standing debate on why and when bland steatosis progresses to
steato-hepatitis, our data point to a critical step in lipolysis rather than in lipogenesis
itself.
