Differential expression of Sox9 protein and proteoglycans in the epiphyseal cartilage of bromocriptine-treated pregnant and lactating rats

Several investigations have shown that pregnancy and lactation are able to induce elongation of long bone by altering epiphyseal cartilage function in a prolactin-dependent manner. Since the transcription factor Sox9 is of utmost importance for chondrocyte proliferation and differentiation and since bromocriptine, a dopaminergic D2 agonist widely used to suppress milk production, is known to disrupt the production and release of prolactin, we herein aimed to investigate whether pregnancy and lactation as well as bromocriptine could alter the expression of Sox9. Our immunohistochemical analysis showed that the Sox9 expression levels were markedly upregulated in the tibial proliferative zone of day 21 pregnant rats. In day 8 (early) and day 14 (mid) lactating rats, the Sox9 expression was enhanced only in the proliferative zone, but not in the resting and hypertrophic zones. There was no change in Sox9 expression in day 21 (late) lactating rats. Postweaning rats manifested a decreased Sox9 expression in the hypertrophic zone. Bromocriptine had no effect on Sox9 expression in the proliferative zone of day 21 pregnant rats; however, it completely prevented the Sox9 upregulation in those of early and mid-lactating rats. A differential response was observed in the proliferative and hypertrophic zones of late lactating rats, in which bromocriptine enhanced Sox9 expression. Further investigation of cartilaginous matrix revealed no change in proteoglycans accumulation in lactating rats. In conclusion, the upregulated Sox9 expression predominantly occurred in the proliferative zone during late pregnancy and early and mid-lactation, while the bromocriptine effects depended on the periods and epiphyseal zones.