دانلود رایگان مقاله هورمون پاراتیروئید باعث پروتئولیتیک پردازش رنکل در استئوبلاست

ABSTRACT

Rankl, the major pro-osteoclastogenic cytokine, is synthesized as a transmembrane protein that can be cleaved by specific endopeptidases to release a soluble form (sRankl). We have previously reported that interleukin-33 (IL- 33) induces expression of Tnfsf11, the Rankl-encoding gene, in primary osteoblasts, but we failed to detect sRankl in the medium. Since we also found that PTH treatment caused sRankl release in a similar experimental setting, we directly compared the influence of the two molecules. Here we show that treatment of primary murine osteoblasts with PTH causes sRankl release into the medium, whereas IL-33 only induces Tnfsf11 expression. This difference was not explainable by alternative splicing or by PTH-specific induction of endopeptidases previously shown to facilitate Rankl processing. Since sRankl release after PTH administration was blocked in the presence a broad-spectrum matrix metalloprotease inhibitor, we applied genome-wide expression analyses to identify transcriptional targets of PTH in osteoblasts. We thereby confirmed some of the effects of PTH established in other systems, but additionally identified few PTH-induced genes encoding metalloproteases. By comparing expression of these genes following administration of IL-33, PTH and various other Tnfsf11-inducing molecules, we observed that PTH was the only molecule simultaneously inducing sRankl release and Adamts1 expression. The functional relevance of the putative influence of PTH on Rankl processing was further confirmed in vivo, as we found that daily injection of PTH into wildtype mice did not only increase bone formation, but also osteoclastogenesis and sRankl concentrations in the serum. Taken together, our findings demonstrate that transcriptional effects on Tnfsf11 expression do not generally trigger sRankl release and that PTH has a unique activity to promote the proteolytic processing of Rankl.

4. Discussion

Whereas the key role of Rankl in promoting osteoclastogenesis and bone resorption is undoubted, there is still a remarkable uncertainty about the relevance and mechanisms of Rankl ectodomain shedding. It is well established that Rankl exists in (at least) two forms, i.e. as a full-length transmembrane protein or as a circulating form (sRankl) lacking the N-terminal part including the transmembrane domain [24]. It is also evident that sRankl has pro-osteoclastogenic activity, not only because it is commonly utilized to induce osteoclastogenic differentiation of cultured hematopoietic progenitor cells, but also since transgenic mice over-expressing sRankl display severe osteoporosis [25,26]. On the other hand, several in vitro studies came to the conclusion that membrane-bound Rankl is more effective than sRankl in promoting osteoclastogenesis, thus suggesting that the activity of Rankl is actually inhibited by ectodomain shedding [7,24,27]. In any case, given the central importance of the Rankl/Rank/Opg system for bone biology, it is quite important to obtain a more profound understanding of the molecular mechanisms controlling the release of sRankl from osteoblasts.