Effect of Parathyroid Hormone on MAPK Signaling and Calcification Markers in Human Intervertebral Disk Cells

Abstract

Introduction

Back pain is strongly associated with degeneration of the intervertebral disk (IVD), which is associated with ongoing mineral deposition.¹ The presence of calcium deposits and type X collagen (COL X) and the level of the indicators of calcification potential (alkaline phosphatase (ALP), Ca^2 + ions and Pi) were consistently higher in degenerative and scoliotic discs. We also showed that in mesenchymal stem cells (MSCs), parathyroid hormone (PTH 1–34) inhibits the expression of COL X while promoting type II collagen (COL II) expression, thereby preventing endochondral ossification.² In this study, we investigated the effect of PTH on expression of COLII, COLX, and ALP in human IVD cells and analyzed the potential mechanisms related to its effect.

Materials and Methods

Human lumbar IVDs from a donor without spinal pathology were obtained within 24 hours after death. Nucleus pulposus (NP) and annulus fibrosus (AF) tissues from the IVDs were digested and the corresponding NP and AF primary cells were isolated as previously described.³These cells were cultured in complete DMEM to 90% confluence. Then the cells were incubated overnight in serum-free medium followed by treatment with 100 nM PTH 30 minutes to 48 hours. Protein expression was analyzed by immunoblotting using specific antibodies to COL I and COL II (Abcam, Cambridge, MA), COL X, and PTH receptor 1 (Sigma Aldrich). Expression and phosphorylation of AKT and MAPKs was assessed by using specific corresponding antibodies (Cell Signaling, Danvers, MA). Alkaline phosphatase activity was measured colorimetrically using the StemTAG kit (Cell Biolabs, San Diego, CA, USA) and Ca^2 + release from cells was measured using calcium assay kit (Cayman Chemical, Ann Arbor, MI, USA). Statistical analyses were done using one-way ANOVA, posthoc tests p < 0.05 was considered significant.

Results Effect of PTH on PTH receptor 1 We confirmed the presence of PTH receptor 1 on the AF and NP cells and showed the effect of PTH on the PTH receptor 1 activation by incubating AF and NP cells with 100 nM PTH (1–34).

Effect of PTH on collagen expression PTH increased the expression of COL-II significantly in AF cells after 6 hours of incubation and in NP cells COL-II expression increased in a time-dependent and sustained manner from 6 to 48 hours of incubation. Expression of COL-X was not altered by PTH in AF cells, whereas in NP cells it decreased significantly from 6 to 48 hours.

Effect of PTH on MAPK signaling PTH causes ERK activation by phosphorylation within 30 minutes both in AF and NP cells (Fig. 1). PTH stimulated ERK phosphorylation was found to be sustained up to 6 hours followed by a slow decrease (Fig. 1). However, PTH led to JNK phosphorylation within 30 minutes of incubation and this declined thereafter over 48 hours. PTH stimulation of p38 MAPK phosphorylation was not altered in AF cells, whereas in NP cells there was a significant activation of p38 at 30 minutes by PTH. There was a decline in p38 phosphorylation below the control levels after 24 hours of incubation with PTH.

Effect of PTH on AKT signaling Results show that PTH causes AKT (ser 473) activation by phosphorylation at 1 hour in NP cells and has no effect in AF cells.

Effect of PTH on Calcification markers The activity of alkaline phosphatase was significantly decreased by PTH after 24 hours of incubation in NP cells. In AF cells, no significant changes were observed, even though there was a trend to increase at 48 hours. PTH significantly increased calcium release into the medium at 48 hours in NP cells and no significant change was observed in AF cells

Conclusion

PTH has previously been shown to promote chondrogenesis and to inhibit the expression of COL-X in chondrocytes probably via the activation of MAPK signaling pathways.⁴ The present results demonstrate that PTH upregulates COL-II and downregulates COL-X in IVD cells, indicating that PTH has the potential of being able to stimulate disk repair and to improve nutrient supply in the degenerative disc. Our data also suggests that activation of MAPK pathway takes place much earlier than the alterations in COL-II or COL-X expression. Interestingly, COL-II expression inversely correlates with alkaline phosphatase activity in NP cells treated with PTH. Although understanding of IVD calcification would be of great value, not only for elucidation of its mechanism, but with an eye toward eventual therapeutic intervention. PTH can thus be used towards disk regeneration therapy.

I confirm having declared any potential conflict of interest for all authors listed on this abstract

Yes

Disclosure of Interest

None declared

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