Since the elucidation of the structures of the three human PRHrP isoforms in 1987, information has rapidly accured which indicates that the role of PTHrP in normal physiology will prove to be crucial as well as exceedingly complex. The importance of the role of PTHrP in normal physiology is underscored by its broad tissue expression, by its intense evolutionary conservation, by its extremely early expression after fertilization of the ovum, and by the lethal consequences of PTHrP gene disruption. The complexity of the role of PTHrP in normal physiology increases almost monthly. This complexity is reflected in the broad tissue distribution of the peptide, its complex transcriptional regulation and mRNA instability motifs, and its multiple transcripts and isoforms. It is now clear that additional complexity exists at the level of posttranslational processing. Expression of the PTHrP gene leads to the tissue-specific processing and secretion of an increasingly complex family of derivative peptides, each with its own repertoire of cognate receptors, signal transduction pathways, and physiological consequences. Further elucidation of the posttranslational processing pathways and mechanisms can be anticipated in the coming years, coupled with a corresponding elucidation of multiple PTHrP receptors, their specific signal transduction pathways, and their unique physiological roles. The role of PTHrP in causing HHM is now clearly established. Work in the coming decade will focus on the normal physiological roles played by PTHrP.

Parathyroid hormone-related protein (PTHrP) is widely expressed in normal adult and fetal tissues, where it acts in an autocrine/paracrine fashion, stimulates growth and differentiation, and shares early response gene characteristics. Since recovery from renal injury is associated with release of local growth factors, we examined the expression and localization of PTHrP in normal and ischemic adult rat kidney. Male SpragueDawley rats underwent complete bilateral renal artery occlusion for 45 min, followed by reperfusion for 15 min., and 2,6, 24, 48, and 72 h. Renal PTHrP mRNA levels, when compared with sham-operated animals, increased twofold after ischemia, and peaked within 6 h after reperfusion. PTH receptor, j-actin, and cyclophilin mRNA levels all decreased after ischemia. PTHrP immunohistochemical staining intensity increased in proximal tubular cells after ischemia, changing its location from diffusely cytoplasmic to subapical by 24 h after reperfusion. In addition, PTHrP localized to glomerular epithelial cells (visceral and parietal), but not to mesangial cells. PTHrP and PTH stimulated proliferation two-to threefold in cultured mesangial cells. We conclude that PTHrP mRNA and protein production are upregulated after acute renal ischemic injury, that PTHrP is present in glomerulus and in both proximal and distal tubular cells, and that PTHrP stimulates DNA synthesis in mesangial cells. The precise functions of PTHrP in normal and injured kidney remain to be defined. (J. Clin. Invest. 1993.

The presence of parathyroid hormone-related protein (PTHrP) in human kidney vasculature and the signal transduction pathways stimulated during PTHrP-induced vasodilation of the rabbit kidney were investigated. Immunostaining of human kidney revealed the abundant presence of PTHrP in media and intima of all microvessels as well as in macula densa. In isolated perfused rabbit kidney preconstricted with noradrenaline, 10(-5) M Rp-cAMPS, a direct inhibitor of protein kinase A, produced comparable inhibition of 2.5 x 10(-7) M forskolin- and 10(-7) M PTHrP-induced vasorelaxations. Renal vasorelaxation and renal microvessel adenylyl cyclase stimulation underwent comparable desensitization following exposure to PTHrP. Nitric oxide (NO)-synthase inhibition by L-NAME (10(-4) M), NO scavenging by an imidazolineoxyl N-oxide (10(-4) M) and guanylyl cyclase inhibition by methylene blue (10(-4) M) decreased PTHrP-induced vasorelaxation by 27 to 53%, abolished bradykinin-induced vasorelaxation and did not affect forskolin-induced vasorelaxation. The effects of Rp-cAMPS and L-NAME were not additive on PTHrP-induced vasorelaxation. Damaging endothelium by treating the kidney with either anti-factor VIII-related antibody and complement, gossypol or detergent, did not affect PTHrP- or forskolin-induced vasorelaxations but reduced bradykinin-induced vasorelaxation by 53 to 92%. Conversely, endothelial damage did not alter the inhibitory action of L-NAME on PTHrP-induced vasorelaxation. In conclusion, PTHrP is present throughout the human renovascular tree and juxtaglomerular apparatus. Activation of both adenylyl cyclase/protein kinase A and NO-synthase/guanylyl cyclase pathways are directly linked to the renodilatory action of PTHrP in a way that does not require an intact endothelium in the isolated rabbit kidney.

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While the gene and mRNA transcripts encoding PTH-related peptide (PTHrP) have been well characterized, the actual secretory form(s) of the peptide is unknown. Accordingly, synthetic and recombinant PTHrPs employed to date for biological and immunological characterization have necessarily been of arbitrary lengths. No prior evidence for glycosylation of PTHrPs has been described. To define the naturally occurring form(s) of this peptide secreted by human epidermal keratinocytes, we have affinity purified, using an anti-PTHrP-(1-36) antibody column, human PTHrP secreted under conditions of protease protection. Human keratinocyte-conditioned medium collected without measures to protect against proteolytic degradation contains multiple PTHrP immunoreactive and bioactive species. In contrast, under conditions of protease protection, human keratinocyte-conditioned medium contains a single 18,000 mol wt (Mr) form of the peptide. In contrast to recombinant and synthetic PTHrPs, which migrate as distinct, well focussed bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, this 18,000 Mr PTHrP displays the broad electrophoretic profile of a glycoprotein. Treatment of this peptide with trifluoromethanesulfonic acid, an agent that deglycosylates both O- and N-linked saccharides from their core proteins, shifted the Mr of the protein to approximately 10,000. In contrast, exposure of recombinant PTHrP-(1-141) to the same agent results in no change in electrophoretic mobility. These studies indicate that the 18,000 Mr species of PTHrP secreted by human epidermal keratinocytes is a glycoprotein.