Circulating Prouroguanylin Is Processed to Its Active Natriuretic Form Exclusively within the Renal Tubules

The intestine and kidney are linked by a mechanism that increases salt excretion in response to salt intake. The peptide uroguanylin (UGn) is thought to mediate this signaling axis. Therefore, it was surprising to find (as reported in a companion publication) that UGn is stored in the intestine and circulates in the plasma almost exclusively in the form of its biologically inactive propeptide precursor, prouroguanylin (proUGn), and, furthermore, that infused proUGn leads to natriuretic activity. Here, we investigate the fate of circulating proUGn. Kinetic studies show rapid renal clearance of radiolabeled propeptide. Radiolabel accumulates at high specific activity in kidney (relative to other organs) and urine (relative to plasma). The principal metabolites found in kidney homogenates are free cysteine and methionine. In contrast, urine contains cysteine, methionine, and three other radioactive peaks, one comigrating with authentic rat UGn15. Interestingly, proUGn is not converted to these or other metabolites in plasma, indicating that circulating proUGn is not processed before entering the kidney. Therefore, our findings suggest that proUGn is the true endocrine agent released in response to salt intake and that the response of the kidney is dependent on conversion of the propeptide to an active form after it reaches the renal tubules. Furthermore, proUGn metabolites (other than small amounts of cysteine and methionine) are not returned to the circulation from the kidney or any other organ. Thus, to respond to proUGn released from the gut, any target organ must use a local mechanism for production of active peptide.

Endocrine response after prior treatment with fulvestrant in postmenopausal women with advanced breast cancer: experience from a single centre

The pure anti-oestrogen fulvestrant has now been licensed for use in advanced breast cancer which has progressed on an anti-oestrogen. Optimal sequencing of various endocrine agents becomes very important in the therapeutic strategy. We report our experience of further endocrine response with another endocrine agent after prior fulvestrant treatment. Among all patients with advanced breast cancer who had been entered into five phase II/III trials using fulvestrant as first- to ninth-line endocrine therapy in our Unit since 1993, 54 patients who fulfilled the following criteria were studied for their subsequent endocrine response: (i) oestrogen receptor positive or unknown; (ii) having been on a subsequent endocrine therapy for ≥6 months unless the disease progressed before; and (iii) with disease assessable for response according to International Union Against Cancer criteria. Eleven patients had received an aromatase inhibitor prior to fulvestrant, which resulted in five CBs (clinical benefit = objective remission/stable disease (SD)) for ≥6 months). Twenty-eight patients achieved CB on fulvestrant. They went on subsequent endocrine therapy with two partial responses, 11 SDs and 15 PDs (progressive disease) at 6 months. The median survival from starting fulvestrant and subsequent endocrine therapy was respectively 46.6 and 18.2 months. Among the remaining 26 patients who progressed at 6 months on fulvestrant, there were three SDs and 23 PDs at 6 months on subsequent endocrine therapy. The median survival from starting fulvestrant and subsequent endocrine therapy was respectively 12.5 and 9.3 months. Of all these 54 patients, 30% (n = 16) therefore achieved CB using another (second- to tenth-line) endocrine agent (anastrozole = 26; tamoxifen = 12; megestrol acetate = 11; others = 5). It would thus appear that further endocrine response can be induced in a reasonable proportion of patients after failing fulvestrant.

Tamoxifen-citrate counteracts the antitumor effects of cytotoxic drugs in vitro.

Hormones and cytotoxic drugs are often combined in the treatment of patients with breast carcinoma to broaden the antitumor spectrum of the therapy. We found that, in vitro, the most commonly used endocrine agent, tamoxifen citrate, attenuates the cytotoxic potential of 5-fluorouracil (5-FU) and of doxorubicin. The effect was observed on estrogen receptor positive and on estrogen receptor negative breast tumor cells. Combinations of growth inhibitory hormones and cytotoxic drugs may therefore be counter-productive. For the treatment of hormone-independent tumors they may even be harmful since in these tumors tamoxifen exerts no independent cell kill that compensates for its modifying effect on the cytotoxicity of drugs.

Cause-and-effect relationship between motilin and migrating myoelectric complexes

We investigated the cause-and-effect relationship between plasma motilin levels and migrating myoelectric complexes (MMCs). Each dog was implanted with a set of eight bipolar electrodes on the small intestine. Premature phase IIIs were initiated by morphine bolus injections. Plasma samples were assayed for motilin and gastrin. All spontaneous and morphine-initiated phase IIIs were associated with peaks of plasma motilin, which always occurred after phase IIIs had started in the proximal duodenum. The plasma motilin level decreased consistently during phase I and started to increase again only after phase II had started in the duodenum. Either a meal or somatostatin infusion disrupted MMC cycling, but morphine boluses overcame this disruption and initiated phase IIIs that propagated distally. The phase IIIs thus initiated were associated with peaks in plasma motilin levels. In contrast, bolus injections of motilin did not initiate phase IIIs during the fed state or during somatostatin infusion. Our findings suggest that endogenous motilin does not initiate spontaneous MMCs. Instead, MMC contractions release motilin. The physiological role of motilin, thus released, may be to act as an endocrine agent to coordinate secretory and motor events with the start of phase III activity in the upper small intestine.