Abnormal proliferation in tubular epithelial cells plays a crucial role in cyst formation and/or growth in PKD [10–12
]. There is also experimental evidence for activation of the mTOR signalling pathway in the kidney in PKD [5
]. Activation of the mTOR signalling pathway results in increased proliferation and decreased apoptosis [13
]. Rapamycin is a Federal Drug Administration (FDA)-approved immunosuppressive drug and is a powerful anti-proliferative drug via inhibition of mTOR signalling [13
]. Thus, there is sound rationale for the use of mTOR inhibitors, which are potent anti-proliferative agents, in PKD.
There have been four short-term studies of mTOR inhibitors in animal models of PKD. Rapamycin (0.2 mg/ kg/day) for 5 weeks from 4 to 8 weeks of age slows disease progression in Han:SPRD rats [2
]. Rapamycin (2 mg/kg/day) orally for 3 months delays the loss of function and retards cyst development in the Han:SPRD rat [3
]. Everolimus (3 mg/kg/day) by mouth for 5 weeks inhibits cystogenesis in Han:SPRD rats [4
]. In two independent mouse models of PKD, the bpk and orpk rescue mice, daily i.p. injections of 1.67 or 5 mg/kg rapamycin for 28 or 14 days, respectively, reduced renal cystogenesis [5
]. Our study establishes the long-term efficacy of rapamycin in the treatment of PKD.
Adults and children with ADPKD will likely require long-term or even life-long therapy. The effect of long-term rapamycin treatment in animal models of PKD is not known. In the present study, we treated Han:SPRD rats with rapamycin from 1 to 12 months of age and evaluated side effects, cyst growth, renal function, blood pressure and cardiac size. The only side effect of treatment was weight loss. The weight loss of 11% in the present study of long-term treatment was less than the 22% we previously reported with short-term treatment [2
]. Food intake was monitored in vehicle- and rapamycin-treated rats. The weight loss occurred without any apparent decrease in food intake.
Side effects of rapamycin in humans include (in order of frequency) hyperlipidaemia, hypertension, diarrhoea, anaemia, nausea or vomiting, thrombocytopenia, skin rash, stomatitis and malignancies (lymphoma and skin cancers). Serum lipids, haemoglobin and platelets were not measured in the present study. Both the vehicle- and rapamycin-treated rats were carefully examined during the study, and at sacrifice they did not have diarrhoea, skin rash, stomatits or malignancies.
Survival was not an end-point of the study. However, the rapamycin-treated PKD rats were very much healthier than the vehicle-treated rats at 1 year of age. The vehicle-treated PKD rats at 1 year of age had hypertension, chronic kidney disease (CKD) (mean BUN of 176 mg/dL and serum creatinine of 3.2 mg/dL) and looked extremely sick as judged by much decreased activity and abdominal swelling, which on sacrifice revealed ascites. The rapamycin-treated PKD rats did not show signs of any side effects of treatment, did not have CKD and were much healthier than the vehicle-treated PKD rats at 1 year of age.
Increasing cyst enlargement in PKD is known to cause hypertension mediated by the renin–angiotensin–aldosterone system [14
]. Hypertension occurs early in the course of PKD and is an important factor in the mortality and morbidity of PKD [15
]. Treatment of the hypertension with enalapril in patients with PKD has been shown to decrease left ventricular hypertrophy [16
]. Hypertension is a feature of PKD in Han:SPRD rats [17,18
]. The present study demonstrates for the first time a significant decrease in hypertension and heart enlargement due to rapamycin treatment in PKD. The decrease in hypertension may be related to the decrease in cyst volume as increased cyst volume has been associated with hypertension in PKD [19
]. The decrease in heart enlargement is likely due to the decrease in blood pressure.
Increased mTOR signalling has been described in the heart due to certain hypertrophic agents [20,21
], and inhibition of mTOR signalling with rapamycin reduces the myocyte cell size and attenuates load-induced cardiac hypertrophy in mice [20
]. p70S6 Kinase (p70S6K) is the best characterized downstream target of mTOR. p70S6K is phosphorylated by mTOR. Phospho-p70S6K in turn phosphorylates the S6 ribosomal protein. An increase in the phospho-S6 protein that is inhibited by rapamycin is a measure of increased mTOR signalling. [13
]. Thus, we measured the level of the phospho-S6 protein as an index of mTOR signalling in the heart. We describe the novel finding of the increased phospho-S6 protein in the heart in PKD that is inhibited by rapamycin, indicative of increased mTOR signalling. In kidney transplant patients, rapamycin has been shown to decrease left ventricular hypertrophy, independent of blood pressure [22
]. Thus, rapamycin may reduce left ventricular hypertrophy independent of blood pressure reduction in PKD rats. A future study of the effect of hypertension on mTOR signalling in the heart of PKD rats is an exciting prospect.
The male Cy/+ Han:SPRD rat develops slowly progressive renal cystic disease with interstitial fibrosis and azotaemia seen by 6 months of age and dies at approximately 17 months of age from end-stage kidney disease [23,24
]. The present study demonstrates that rapamycin treatment prevented the onset of CKD as judged by BUN and serum creatinine levels that were not different from wild-type rats.
In summary, long-term treatment with rapamycin in PKD results in a marked decrease in CKD, hypertension and heart enlargement with minimal side effects. The novel finding that rapamycin decreases hypertension, heart enlargement and mTOR signalling in the heart in PKD rats is reported. Our study supports the feasibility of long-term rapamycin treatment in PKD.