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Ann Rheum Dis. Oct 2007; 66(10): 1399–1400.
PMCID: PMC1994318

Unlike ghrelin, obestatin does not exert any relevant activity in chondrocytes

Obestatin is a 23 amino acid amidated peptide identified as the product of posttranslational cleavage of the preproghrelin,1 the polypeptide precursor of ghrelin. Obestatin has been reported to have actions opposite to ghrelin,2 but its ghrelin antagonist properties are still unclear and controversial. As our group has identified ghrelin as a peptide synthesised and secreted by chondrocytes, with significant biological activity on cartilage cell metabolism,3 it was conceivable that obestatin might exert some physiological action on chondrocytes. These cells are emerging as a local producer and target of several endocrine factors including ghrelin3 and leptin.4,5 It is of note that the obestatin co‐joined hormone ghrelin has been revealed as a convergent factor between bone metabolism and energy homeostases.6,7 So, it was plausible to verify the presence of obestatin in chondrocytes and to study the potential physiological effects of this recently discovered peptide.

According to our results, specific enzyme immunoassay (EK03190; Phoenix Pharmaceuticals Inc., Belmont, California, USA, linear range 0.23–2.37 ng/ml) is able to detect immunoreactive obestatin in the supernatants of cultured murine ATDC5 chondrocytes (0.543 ± 0.032 ng/ml), suggesting that chondrocytes have been equipped with the biochemical machinery in charge of the posttranslational process of the preproghrelin peptide by generating both ghrelin and obestatin. In contrast to ghrelin,3 obestatin binds weakly to chondrocytes, and quantitative analysis of obestatin binding sites indicates the presence of less than 1000 binding sites per cell. Next, we assessed the effect of obestatin on chondrocyte metabolic activity by means of MTT colorimetric assay. These experiments have been performed in undifferentiated ATDC5 murine cells, but were further confirmed in differentiated mature ATDC5 cells,8 as well as in the immortalized human chondrocyte cell line, C28‐I2 (a kind gift of Dr Mary B. Goldring, Hospital for Special Surgery, New York, USA).9 Obestatin‐stimulated chondrocytes showed a significant decrease in their metabolic activity (fig 11).). Intriguingly, this effect was similar, and not opposite, to that exerted by ghrelin,3 suggesting that obestatin is able to modulate the mitochondrial respiratory chain or oxidative burst as ghrelin did, excluding in this way any kind of antagonism between ghrelin and obestatin.

figure ar68155.f1
Figure 1 Obestatin inhibits chondrocyte metabolic activity, evaluated by the MTT colorimetric method, in a dose‐dependent manner. ***p<0.001. As a positive control, metabolic activity was evaluated in the ...

To exclude any potential cytotoxic effect of obestatin, we analysed the chondrocyte cell cycle by propidium iodide staining flow cytometry and determined lactate dehydrogenase levels in supernatants of human cultured chondrocytes stimulated or not with obestatin. No difference between controls and obestatin‐treated cells exists, suggesting that obestatin, per se, exerts no cytotoxic effect.

Obestatin, unlike ghrelin, was unable to modulate significantly either fatty acid or glucose uptake, both macronutrients that serve as precursors for eicosanoids and glycoaminoglycans. Moreover, obestatin had no effects on the quantitative expression of matrix metalloproteases 3 and 9, as well as on enzymes involved in cellular stress such as lactate dehydrogenase, alkaline phosphatase and nitric oxide synthase type 2.

In conclusion, our study suggests that obestatin, unlike ghrelin, plays a marginal role in the regulation of chondrocyte metabolism. We cannot exclude the possibility that the unique indicative response of obestatin as a downmodulator of the chondrocyte respiratory rate might be linked to the induction, or repression, of genes whose roles in chondrocyte physiopathology have yet to be defined. The identification of factors involved in obestatin response may thus provide the basis for future studies on the molecular effects of this novel member obtained by posttranslational processing from the same ancestor peptide that generates ghrelin.

Acknowledgements

The authors gratefully acknowledge Dr Juan Viñuela and Manuel Otero, BS (Santiago University Clinical Hospital, Central Laboratory of Biochemistry and Immunology) for their contribution to this work.

Footnotes

The research described in this article has been supported by the Spanish Ministry of Health, Fondo de Investigación Sanitaria, Instituto de Salud Carlos III (FIS PI 05/0525, PI030115, PI050419) and Xunta de Galicia. O.G. and F.L. are recipients of a contract under the “Programme of stabilization of researchers” co‐funded by the Instituto de Salud Carlos III and SERGAS. R.L. and R.G. are recipients of a project‐associated fellowship from Instituto de Salud Carlos III.

References

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9. Finger F, Schorle C, Zien A. et al Molecular phenotyping of human chondrocyte cell lines T/C‐28a2, T/C‐28a4, andC‐28/I2. Arthritis Rheum 2003. 483395–3403.3403 [PubMed]

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