Prometheus unbound: caveolin and liver regeneration

It could be a long way off before we can take a 'caveolin pill' to fix our livers after a big night out, but a recent breakthrough by collaborating scientists in Brisbane and Barcelona has brought the possibility a little closer, as Fiona Wylie reports.

Professor Rob Parton at the University of Queensland's Institute for Molecular Bioscience (IMB) leads the Brisbane half of the collaboration, whose findings of a functional link between caveolin and liver regeneration were published recently in Science.

Carlos Enrich at the University of Barcelona heads the other half of the endeavour, which has an interesting and rather serendipitous history, according to Parton.

"It was an amazing coincidence," he says. "Albert (Pol) came to my lab from Carlos's group some years ago as a visiting scientist to work on caveolins generally ... and he found one of our caveolin mutants on these lipid-filled organelles inside the cell. We didn't even know at the time what lipid droplets were."

It seems that Enrich knew quite a bit about liver regeneration and had noticed in his work that regenerating liver cells accumulate cytosolic structures, which turned out to be lipid droplets.

"So, we worked together," Parton says. "Half the work was done here in Brisbane and half in Barcelona. This really is a very successful and nice collaboration."

Although the collaborative work is focused on the liver, Parton and his group in Brisbane are better known for their extensive work on a family of proteins called caveolins. These proteins comprise the major protein component of cup-like pits at the plasma membrane called caveolae, which occur in most cell types.

However, caveolins also reside in other places in cells and have been implicated in a variety of cellular functions from signalling to proliferation. According to Parton, this recent work now reveals "a role for caveolin in a very specific process required for liver regeneration, which is the formation of lipid droplets".

Lipid droplets are membrane-bound 'bags' of neutral lipids found in the cytosol of many eukaryotic cells, although little is known regarding their exact composition, physiology, and regulation. An association between lipid droplets and caveolin was revealed a few years ago, and concurrent work by Parton's group implicated caveolins in regulating lipid transport from these organelles.

We now know that caveolins can redistribute from their predominant site at the cell surface and move in and out of lipid droplets under physiological conditions. This association increases in response to the accumulation of intracellular lipids seen under many normal and abnormal circumstances, such as tissue regeneration.

Liver regeneration

This brings us back to the liver, which is the organ in the body with the greatest regenerative capacity, as Parton explains.

"You can remove two-thirds of a liver and, in a rodent, that liver will regenerate to exactly the same size in a week to 10 days, and it will do this efficiently over and over again."

The ancient Greeks apparently knew of this amazing capacity, illustrated by the myth of Prometheus, who unwisely got on the wrong side of the boss, Zeus, and was banished to a mountaintop where an eagle came down every day to eat his liver.

In a technique widely used to study liver regeneration, and reminiscent of this ancient tale, rodents are subjected to a partial hepatectomy to remove 70 per cent of their liver mass. Within minutes, the acute phase of liver regeneration is stimulated - more than 100 genes are turned on, the cell cycle goes into overdrive, and lipid transport is activated.

"There is a need as part of normal liver regeneration for lipid droplet formation," Parton says. "Lipid is mobilised from the adipose tissue within the first 24 to 72 hours in response to the hepatectomy.

"It goes to the bloodstream so you get this surge of fatty acids, which is taken up into the liver."

The liver needs this process to provide the energy and molecules necessary to support rapid cell cycling and tissue regrowth.

In light of the known link between lipid droplets, caveolin and liver regeneration, the Oz/Spain team decided to examine the regenerative process at a cellular level in a recently available transgenic mouse model lacking the caveolin-1 gene. Spanish PhD student Manuel Fernandez was set the task, working both here and in Barcelona.

Under normal circumstances these knockout mice had proved remarkably normal and healthy; however, it was quickly apparent that the hepatectomised caveolin null mice would not have lasted long on that mountaintop in Ancient Greece.

"We found that regeneration was very inefficient in mice lacking caveolin-1, and that lipid droplets didn't form as well in that situation, although there were some," Parton says. "Plus there was a dramatic increase in mortality with no caveolin (eight-fold higher death rate by 72 hours)."

The knockout mice showed normal lipid synthesis and uptake, but without caveolin-1, it seems that they could not store these lipids as efficiently. In addition, the liver cells in the knockout mice failed to progress normally through the cell cycle, with a block identified in the transition to S phase.

The failure to accumulate lipid droplets therefore seemed to significantly affect cell processes critical for regeneration of liver, such as signalling and proliferation, and this recovery from injury was somehow dependent on caveolin-1.

Implications

The implications of this work are exciting, not only for clinical medicine but also for our overall understanding of fat metabolism in cells. Indeed, it is known from the work of other groups that caveolin-1 null mice are relatively resistant to a high-fat diet, and stay relatively lean, and that they are also partially protected against atherosclerosis.

"We think that this research has important implications for conditions like hepatitis and cirrhosis where you get accumulation of lipid droplets and the damage to the liver exceeds its capacity to regenerate ... but also more generally when lipid accumulation occurs in other tissues under various conditions, such as atherosclerosis and obesity," Parton says.

"It is telling us more about the role of lipid droplets in storing lipids, why you get accumulation in these droplets, how lipid gets into the cell, how they are transported - all questions that really we don't understand very well at the moment, and we believe that we are now getting more cellular insights into those processes."

So, how is the caveolin actually regulating this shift in lipid metabolism? This question remains largely unanswered, with the collaboration now focused on unravelling the exact stages of lipid droplet formation affected when caveolin is not present, as well as the demonstrated links with the cell cycle machinery.

For the Parton group, this work has also revealed new and interesting avenues for their primary focus on caveolins. The group is also looking at a muscle-specific form of caveolin in models of muscle disease to see if similar links between caveolin and lipid droplet regulation might be apparent, this time in muscle, where lipid metabolism is also very important for function.

They are also embarking on a gene-array approach to understand more about what caveolin is doing in the knockout mice, particularly the kind of compensatory pathways that are going on in uninjured mice in the absence of caveolin.

"We really do think now that caveolins may be linked to regulating metabolism, which was unheard of a few years ago," he says.

So from knowing virtually nothing about lipid droplets not long ago, these balls of fat are now one of Rob Parton's favourite organelles.