This paper demonstrated the expression of PTH1R in normal bladder tissue and its downregulation in end-stage low-compliance bladders requiring augmentation. Combined with the potent relaxant effect of PTHrP peptide in rat bladder reported in our previous study, [8] these findings suggest the functional involvement of this axis in normal human bladder physiology, and its loss in severely diseased bladders with decreased compliance.
The relaxant effect of PTHrP in the bladder has been reported previously [12, 13] Yamamoto et al. focused on stretch-induced upregulation of PTHrP in the bladder in vivo, [12] which effect was replicated in cultured smooth muscle cells under stretch by Steers et al [13]. However, PTHrP showed only modest suppression of carbachol-induced contraction of bladder strips in those studies, leaving the physiological relevance of this observation unanswered. In contrast, our recent study in rats showed a potent relaxant effect of PTHrP in suppressing the spontaneous contraction of detrusor strips, and thus increasing bladder compliance in vivo [8]. Our study also demonstrated that PTH1R was expressed primarily in the muscle layer, and not in the urothelial layer. As a logical extension of this previous study, we investigated the expression of PTH1R in the human bladder, and this report supplements and advances the previous study, by investigating the clinical relevance of this axis utilizing human samples.
Detrusor muscle tissues in normal control bladders stained positive for PTH1R, especially in the cytosol. This result seems to contradict the fact that PTH1R is a membrane-anchored G protein-coupled receptor, [14] allowing binding of PTHrP at the membrane surface. However, PTH1R translocation from the plasma membrane into the cytosol after incubation with the PTH1R agonist, PTH (1–34), and positive staining in the cytosol was also seen in other organs such as acinar cells of the prostate gland, cell clusters within the adrenal cortex and cells of the epithelial hair sheath [11].
In sharp contrast, the detrusor muscle in augmented bladders showed negative staining for PTH1R. This lack of staining did not indicate a technical failure, given that blood vessels stained positively in seven out of 13 (53.8%) cases. The negative staining of vessels in the remaining six cases may have been associated with deterioration of the vessels themselves, or technical failure caused by different fixation conditions. If PTH1R is downregulated in these bladders, they may not be able to respond to endogenous PTHrP, which could function as a protective relaxant against excessive distention. Downregulation of PTH1R is thus consistent with low bladder compliance.
In regard of the functional effect in PTHrP-PTH1R axis in the bladder, not only the receptor PTH1R, but expression level of the ligand PTHrP, in normal and diseased bladder is also of a great interest. Perez-Martinez et al. reported about PTHrP immunostaining in rabbit bladder outlet obstruction model [15]. However, we did not succeed in visualizing PTHrP signal in paraffin-embedded human bladder, nor in rat bladder under various fixation conditions. Therefore we did not focus on expression of PTHrP in this study.
Further physiological experiments would have been ideal to confirm these speculations, demonstrating the unresponsiveness of diseased bladder strips to PTHrP peptide. Similarly, it may be of interest whether the difference in expression level of PTH1R may be attributed to altered production, degradation, release, or removal. Unfortunately, it was practically impossible to obtain materials at the separate time of surgery and subsequently perform physiological experiment or obtain enough materials for various biochemical assays. Such mechanistic part should be better studied in experimental model system such as cultured cells and animals, as we did in our previous report, rather than in clinical samples. Therefore it is inevitable limitation of the present study design that it is unidimensional, but supplemented by our previous mechanistic study, it addresses more direct clinical relevance.
Clinical translation of the results of this study may not be straightforward. If downregulation of PTH1R is a feature of low-compliance bladders, it would be difficult to use this axis as a target for treatment. However, it is possible that bladders still positive for PTH1R may retain the ability to relax against excessive distention, while downregulation of PTH1R could be a marker for unresponsiveness to conservative therapy, indicating the need for bladder augmentation. In addition, the PTHrP-PTH1R axis could be a target for treating milder damage in bladders retaining PTH1R expression, including overactive bladder, which affects a far larger percentage of the population than severely diseased bladder. Unfortunately, currently-available PTH1R agonists have systemic side effects that preclude their use for bladder diseases, and bladder-specific derivatives are awaited to allow the clinical translation of the PTHrP-PTH1R axis for treating bladder diseases.