Regulation of Cell Size Through Autophagy

A human is made of 10¹³-10¹⁴ cells split between 200 different cell types capable of performing a variety of different functions. Shapes and sizes of cells can be quite distinctive with diameters ranging from 7-8 µm for erythrocytes to 40 µm for megakaryocytes and around 120 µm for a female ovocyte. The volume of immortal cancer cells can vary up to eight-fold from 500 µm³ to 4000 µm³ with the majority around the 2000 µm³ mark. For example, in HeLa cell lines, cells can spread thinly with a diameter of about 40 µm in an environment with low confluence. This diameter can, however, decrease to 20µm when grown to confluence. Using a mouse lymphoblast cell line, Dr. Tzur and colleagues from Harvard Medical School demonstrated that the size distribution is centered at around 1000 µm³ with a variance of about 300 µm³. Using a novel mathematical model, they determined that growth rate is size-dependent throughout the cell cycle. They also concluded that in spite of the potential unsettling effects of size-dependent growth, intrinsic mechanisms under the influence of developmental or physiological constraints might be responsible for setting the size and limiting size variation.

Nutrient availability, growth factor signaling, as well as macromolecule and organelle turnover can influence growth rate and proliferation. Physiological levels of autophagy can slow down aging and promote cell growth. Conversely, abnormal levels of autophagy can inhibit cell growth and augment cell death by apoptosis or necrosis. Due to its ability to maintain homeostasis and recycle material, autophagy is critical for cell survival. Hosokawa et al. (2006) showed that mouse fibroblast cells defective for autophagy are unable to reduce their size upon starvation. The authors hypothesized that autophagy and cell size were tightly connected. Since then, several seminal studies have demonstrated a link between the two. However, very little is known about the actual cellular composition associated with the changes in size and how cell size homeostasis is regulated.

Dr. Miettinen and Dr. Björklund of the University of Dundee worked on identifying the mechanisms related to the control of cell size. Using Enzo’s SCREEN-WELL® FDA-approved drug library, they studied the effects of 786 FDA-approved compounds on cell size and proliferation of Jurkat T cells. Amongst the top 25 hits, two mevalonate pathway inhibitors belonging to the family of statins were identified. The authors found that all statins, with the exception of the most hydrophilic and least permeant one, were capable of increasing cell size and slowing down proliferation. Similar observations were made in a variety of cell lines including human U2OS osteosarcoma cells, embryonic Drosophila Kc167, immortalized mouse embryonic fibroblasts and human umbilical vein endothelial cells. The mevalonate pathway plays a key role in multiple cellular processes ranging from cholesterol synthesis to growth control. By systematically inhibiting the mevalonate pathway and rescuing inhibition events, protein geranylgeranylation via RAB11 and its binding partners was shown to be directly behind cell size homeostasis and deemed to be a key connection between mevalonate pathway and cell size.

The researchers noticed that protein density was significantly enhanced in statin-treated cells. This is in contrast with previous findings demonstrating the inhibition of protein production by statins. Using Enzo’s CYTO-ID® Autophagy detection kit and PROTEOSTAT® Aggresome detection kit, they determined that physiological levels of autophagy depended on protein geranylgeranylation by RAB11 and that autophagy is inhibited by treating cells with statins, causing a surge in protein density and an accumulation of protein aggregates. The mevalonate pathway therefore plays a critical role not only in cell size homeostasis, but also in proteostasis through the process of autophagy. Altogether, these results highlight the importance of the kinetics of protein synthesis and protein degradation in cell size variations.

Enzo Life Sciences offers comprehensive tools for advancing your research in autophagy including our SCREEN-WELL® Compound library product family as well as our CELLESTIAL® portfolio of fluorescent probes and assay kits for cellular analysis, some of which can be found below.

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