Regulation of synthesis, transport and accumulation of lipids in Lipid droplets by Acetyl-CoA carboxylase and Caveolins. How cells accumulate lipids but reduce lipotoxicity

Abstract

The following sections of the thesis are divided in two parts corresponding with the two projects conducted, whose objectives are stated below.

1. Caveolin in the endoplasmic reticulum During the last years, the lipid trafficking and disease group directed by Albert Pol has focused on the role of caveolin proteins in the homeostasis of cholesterol, while they are transported to the plasma membrane (PM).

As introduced, caveolins are key components of caveolae, which are a striking feature of the PM of many cell types and tissues (adipocytes, endothelial and muscle cells), and whose participation in mechanopro-tection, endocytosis, and signalling pathways has been the focus of research (3, 5). Alternatively, important functions of caveolins have been appreciated for some time, although the relevance and mechanisms remain poorly understood (93, 94). Indeed, organisms including Caenorhabditis elegans (C. elegans) and specific mammalian cells, such as hepatocytes, neurons, lymphocytes, and some tumour cells, seem to express caveolins without forming distinguishable caveolae, suggesting caveola-independent functions of the protein.

These non-caveolar functions of caveolins could be taking place in intracellular compartments before caveolae assembly. There, they are synthesised in the endoplasmic reticulum (ER) in a signal recognition particle- dependent process (9), and progressively oligomerise and binds cholesterol during its transport through the ER and Golgi complex (GC) to finally promote caveolae formation at the PM (75). Moreover, CAV1 has been related in previous studies of the group with the transport of lipids between different ER subdomains such as cholesterol between mitochondria-associated membranes (MAMs) (101) and mitochondria (100), or with the formation of lipid droplets (LDs) (98). Although still poorly understood, these functions likely mediate key physiological responses such as the regeneration of the liver after a partial hepatectomy (98).

Intriguingly, the pool of caveolin in the ER remains very low since it exits the compartment very fast (75). Thus, we hypothesized that non-caveolar caveolin at those early timepoints is highly controlled, and we decided to characterise the function and regulation of these intracellular caveolin pools. Additionally, it could provide an insight in the mechanism of pathogenesis induced by several caveolin mutants, which present a disrupted transport to the PM and an accumulation in intracellular compartments.

The specific objectives proposed for the first part of this thesis are the following:

1. Find the mechanism by which the cell controls caveolin protein segregation either in the plasma membrane to form caveolae, or in the intracellular compartments to perform non-caveolar functions.

2. Provide an insight in the non-caveolar functions of caveolin in lipid droplets and the endoplasmic reticulum.

This work is included inside the project “Novel roles of ACSL3, CAV1, and cholesterol in the Endoplasmic Reticulum: homeostatic sensors that determine cell’s survival or death” (BFU2015-66401-R) and granted with a fellowship for the realization of this thesis (BES-2016-077490). Both has been funded by el Ministerio de Economia y Competitividad del Gobierno de España.

2. Targeting lipid droplets for the treatment of type 2 diabetes mellitus and metabolic syndrome

Our group is also focused on studying the cell biology and physiology of the LD cellular organelle. While attached with the ER, it controls the distribution of stored lipids for many cellular processes (165-168).

It is known that the accumulation of LDs originates many lipid-related diseases such as obesity. The connection with other comorbidities such as type 2 diabetes mellitus (T2DM) is not yet fully elucidated, but it is expected to be crucial.

Nowadays, the treatment of T2DM is seen from a glucose-centred point of view and focused on lowering hyperglycaemia, which is the major consequence of the progression of this disease. The main treatment is metformin which prevents liver glucose production by gluconeogenesis inhibition (310). Other second-line treatments such as sulfonylureas or thiazolidinediones are also used for decreasing the high circulating glucose levels to complement metformin treatment (361).

Nonetheless, T2DM is strongly associated with obesity, which ultimately promotes and excessive lipid accumulation in the LDs of peripheral tissues such as muscle and liver (lipid ectopic deposition) (260). A high accumulation of lipids in the cells of these tissues leads to the overloading of LDs and the accumulation of freed toxic lipid species (lipotoxicity) (169, 174, 224), which can make those tissues resistant to insulin action, and in turn, decrease glucose uptake and promote hyperglycaemia (362). Thus, T2DM is a multifaceted disease which requires a multifaceted therapy.

Using the knowledge previously described by the group in LD formation (165, 166, 180) and consumption (165, 345), a new treatment to reduce lipid accumulation and the consequent LD overloading was aimed to be discovered. Decreased lipid accumulation would be translated in reduced lipotoxicity, increased insulin sensitivity and prevention of T2DM onset. Moreover, it could be useful for the concomitant treatment of associated diseases such as obesity during the progression of metabolic syndrome.

The specific objectives proposed for the second part of this thesis are the following:

3. Find a treatment effective in targeting LDs to reduce cellular lipid accumulation and lipotoxicity.

4. Determine if the reduction of ectopic lipid accumulation in the lipid droplets of peripheral tissues is a successful therapeutic approach for the treatment of type 2 diabetes mellitus and other diseases associated with the metabolic syndrome.

This work is included inside the project “Lipid droplet overloading promotes pathogenesis and progression of type 2 Diabetes: Identifying new therapeutic targets and applying upgraded therapies” funded by la Marató de TV3 foundation (201625.10).