Neck Solutions

Caveolin-1 expression and stress induced premature senescence in human intervertebral disc degeneration

From: Arthritis Res Ther. 2008 Aug 5;10(4):R87 [Epub ahead of print]

Chronic and debilitating low back pain is a common condition and a huge economic burden. Many cases are attributed to age related degeneration of the intervertebral disc, however, age related degeneration appears to occur at an accelerated rate in some individuals. We have previously demonstrated biomarkers of cellular senescence within the human intervertebral disc and suggested a role for senescence in intervertebral disc degeneration. Senescence occurs with ageing, but can also occur prematurely in response to stress. We hypothesised that stress induced premature senescence occurs within the intervertebral disc and here we have investigated the expression and production of caveolin-1, a protein that has been shown previously to be upregulated in stress induced premature senescence.

Caveolin-1 gene expression in human nucleus pulposus cells was assessed by conventional and quantitative real-time PCR and caveolin-1 protein expression examined within human intervertebral discs using immunohistochemistry. Correlation between caveolin-1 and p16INK4a biomarker of cellular senescence gene expression was investigated using quantitative real-time PCR.

Caveolin-1 gene and protein expression were demonstrated within the human intervertebral disc for the first time. Nucleus pulposus cells from degenerate discs exhibited elevated levels of caveolin-1 that did not relate to increasing chronological age. A negative correlation was observed between gene expression for caveolin-1 and donor age and no correlation was found between caveolin-1 protein expression and age. A positive correlation was identified between gene expression of caveolin-1 and biomarker of cellular senescence.

Our findings are consistent with a role for caveolin-1 in degenerative rather than age induced changes in the nucleus pulposus. Its expression in intervertebral disc tissue and its association with the senescent phenotype suggests that caveolin-1 and stress induced premature senescence may play a prominent role in the pathogenesis of intervertebral disc degeneration.

Low back pain is a condition that affects a significant proportion of the population, with a lifetime incidence rate in excess of 70% in industrialised nations. It not only impacts on quality of life, but also places a substantial financial burden on the National Health Service and the economy in general due to loss of working days. Many cases of low back pain are attributed to degeneration of the intervertebral disc and imaging studies have indicated a link between intervertebral disc degeneration and low back pain.

To date, no clear mechanism for intervertebral disc degeneration has been identified, although the involvement of both environmental and genetic factors has been proposed. The occurrence of intervertebral disc degeneration increases with age, however a subset of individuals appear to exhibit accelerated degeneration which is independent of age. This has led to speculation that additional factors could play a key role in the development of degeneration in some individuals.

There is increasing evidence that many features of intervertebral disc degeneration, including altered matrix synthesis and enhanced matrix degradation, originate at a cellular level. Cellular senescence is a strong candidate for the prolonged alteration in cellular activity observed during degeneration. Senescence and accompanying alterations in cell function have been implicated in ageing related, degenerative and pathological changes in a variety of tissues, including atherosclerotic plaque development within blood vessels and osteoarthritic alterations to cartilage. Two groups have shown increased staining for senescence associated β-galactosidase in cells from prolapsed and degenerate intervertebral disc cells respectively when compared to non degenerate discs. More recently presented is comprehensive evidence of senescence biomarkers in human intervertebral disc samples, demonstrating increased cellular senescence during intervertebral disc degeneration. In particular, cells from degenerate discs exhibited increased β-galactosidase activity, elevated expression of the cell cycle inhibitor p16INK4a biomarker of cellular senescence gene expression, telomere erosion and a decrease in replicative potential. Furthermore, a correlation was observed between p16INK4a biomarker of cellular senescence gene expression and the expression of matrix-degrading enzymes matrix-metalloproteinase (MMP)-13 and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS), suggesting a role for cell senescence in the molecular processes observed during intervertebral disc degeneration.

Senescence occurs naturally with ageing, but can also occur prematurely in response to stresses (such as exposure to cytokines or oxidative stress) in a number of cell types. Since telomeric erosion and p16INK4a biomarker of cellular senescence gene expression protein are increased in degenerate discs compared to non degenerate age matched samples, we hypothesised that stress induced premature senescence occurs within the intervertebral disc and may be responsible for the accelerated degeneration observed in some individuals.

Caveolae are plasma membrane compartments found abundantly in terminally differentiated cells such as fibroblasts, endothelial and muscle cells. The mammalian caveolin gene family codes for three kDa caveolin proteins, which are integral membrane proteins essential for the structural integrity and function of caveolae. Expression of caveolin-3 is muscle specific, whereas caveolin-1 and caveolin-2 are co-expressed in many cell types. Proposed functions include lipid transport, membrane trafficking and a role in intracellular signalling pathways which stems from the co-localisation of caveolins with a variety of signal transduction molecules. Interestingly, caveolin-1 has been implicated in the senescent phenotype of several cell types including human fibroblasts, lung adenocarcinoma cells, endothelial cells and articular chondrocytes. Moreover, caveolin-1 has been proposed to mediate stress induced premature senescence in murine fibroblasts and human articular chondrocytes in response to oxidative stress and the inflammatory cytokine interleukin-1β, both of which are known to be increased during intervertebral disc degeneration. Here we have investigated the expression of caveolin-1 in human intervertebral discs and correlated its expression with the cell cycle inhibitor and the biomarker of senescence p16INK4a biomarker of cellular senescence gene expression, focusing on the nucleus pulposus as this area was seen to show the most evidence of cell senescence in human intervertebral disc.

This study has shown that caveolin-1 expression in human nucleus pulposis cells is linked to intervertebral degeneration and associated with the senescent phenotype as depicted by increased p16INK4a biomarker of cellular senescence gene expression. Caveolin-1 expression was not linked to increasing chronological age, suggesting a role in accelerated degeneration which could be due to stress induced premature senescence, rather than replicative senescence.