Effect of varying osmotic conditions on the response of bovine nucleus pulposus cells to growth factors and the activation of the ERK and Akt pathways

Abstract

Intervertebral disc and especially nucleus pulposus is characterized by low cellularity. Additionally, extreme variations in osmolality are observed in this tissue, as a result of its specific physicochemical environment, daily activities, or degeneration. In this study, we investigated the role of osmotic fluctuations in the proliferative response of nucleus pulposus cells to exogenous growth factors. In particular, we examined the effect of platelet-derived growth factor (PDGF) and insulin-like growth factor-I (IGF-I) on the proliferation of bovine nucleus pulposus cells and on the activation of the MEK/ERK and PI-3-K/Akt pathways under varying osmotic conditions, in an effort to understand the mechanisms regulating cell proliferation in the intact and the degenerated intervertebral disc. Exposure of cells to high osmolality restrained novel DNA synthesis induced by PDGF or IGF-I in a dose-dependent manner and reduced ERK and Akt activation stimulated by serum or isolated growth factors. Our findings indicate that hyperosmolality imposes a strict control in intervertebral disc cells' proliferation, while hypo-osmotic conditions prevailing in degenerated discs may offer a more permissive environment for cellular proliferation. Keywords: intervertebral disc; growth factor; osmolality; ERK; Akt Back pain represents a major health problem and, although its exact etiology is not yet fully elucidated, it is associated with intervertebral disc degeneration.

Inflammatory and catabolic signalling in intervertebral discs: The roles of NF-B and MAP Kinases

“…This is in agreement with the effect of osmotic stress on aggrecan overexpression in chondrocytes (Peffers et al, 2010). Finally, exposure of disc cells to high osmolality decreased the phosphorylation of ERK in response to serum or to isolated growth factors, such as PDGF or IGF-I, thus inhibiting bovine NP cell proliferation (Mavrogonatou and Kletsas, 2010;Pratsinis and Kletsas, 2007).…”

“…These changes affect also growth factor-mediated ERK activation. In particular, while ERK activation is decreased under hyperosmotic conditions in bovine NP cells, its activation is enhanced in hypoosmotic conditions, indicating that the conditions of the degenerated disc are more permissive for cell proliferation and repair (Mavrogonatou and Kletsas, 2010). In contrast, in rat NP cells, hyperosmotic culture conditions induced by NaCl supplementation of the medium were shown to increase phosphorylation and activation of ERK (Tsai et al, 2007a).…”

“…Other environmental factors also seem to play a crucial role in MAPK activation: hyperbaric oxygen for example reduced phosphorylation of p38 MAPK in human IVD cells (Niu et al, 2011). High osmolarity, to which NP cells are exposed during daily activities (Urban, 2002), resulted in p38 MAPK activation in bovine NP cells, while the activation of ERK and JNK was inhibited under these conditions (Mavrogonatou and Kletsas, 2009;Mavrogonatou and Kletsas, 2010;Mavrogonatou and Kletsas, 2011). Notably, this differential regulation of MAP kinases was observed after stimulation of cells with a salt solution (NaCl/KCl) or the osmolyte sorbitol but not by urea, indicating that these effects are due to changes in osmolality and not to increased ionic strength (Mavrogonatou and Kletsas, 2011).…”

The Effect of Glucosamine Sulfate on the Proliferative Potential and Glycosaminoglycan Synthesis of Nucleus Pulposus Intervertebral Disc Cells

“…26 This already elevated osmolality becomes even higher as a result of the mechanical forces and the subsequent water loss the disc is subjected to during a diurnal cycle 26 and participates in the maintenance of the tissue's homeostasis by imposing a strict control on the proliferation of intervertebral disc cells. 22 In this study, we aimed at exploring the possible role of glucosamine-a preferred substrate for the biosynthesis of glycosaminoglycans, with some reported benefi cial effects in clinical trials of osteoarthritis-in intervertebral disc cells residing under iso-or hyperosmotic conditions. Physiological plasma glucosamine concentrations after an oral administration of 1500 mg/d have been found to range between 150 and 300 μM 27 and reach 10 mM after repeated doses.…”

Global Identification of Genes Related to Nutrient Deficiency in Intervertebral Disc Cells in an Experimental Nutrient Deprivation Model

“…Although some vascular supply is present at the ventral margin of the IVD, NP has an avascular structure encapsulated by an endplate and the annulus fibrosus [9]. Nutrients are supplied to the avascular NP cells by capillaries, which arise in the vertebral bodies, penetrate the subchondral plate, and terminate in loops at the boundary of the cartilaginous endplate [10].…”