Increased skin cancer risk in organ transplant recipients has been experimentally emulated with enhanced UV carcinogenesis from administering conventional immunosuppressants. However, newer generation immunosuppressive drugs, rapamycin (Rapa) and mycophenolate mofetil (MMF), have been shown to impair angiogenesis and outgrowth of tumor implants. To ascertain the overall effect on UV carcinogenesis, Rapa and MMF were admixed into the food pellets of hairless SKH1 mice receiving daily sub-sunburn UV dosages. With immunosuppressive blood levels neither of the drugs affected onset of tumors (<2 mm), but in contrast to MMF, Rapa significantly increased latency of large tumors (!4 mm, medians of 190 vs 125 days) and reduced their multiplicity (1.6 vs 4.5 tumors per mouse at 200 days). Interestingly, tumors (>2 mm) from the Rapa-fed group showed a reduction in UV-signature p53 mutations (39% vs 90%) in favor of mutations from putative base oxidation. This shift in mutation spectrum was not essentially linked to the reduction in large tumors because it was absent in large tumors similarly reduced in number when feeding Rapa in combination with MMF, possibly owing to an antioxidant effect of MMF. Significantly fewer tumor cells were Vegf-positive in the Rapa-fed groups, but a correspondingly reduced expression of Hif1a target genes (Vegf, Ldha, Glut1, Pdk1) that would indicate altered glucose metabolism with increased oxidative stress was not found. Remarkably, we observed no effect of the immunosuppressants on UV-induced tumor onset, and with impaired tumor outgrowth Rapa could therefore strongly reduce skin carcinoma morbidity and mortality rates in organ transplant recipients.One of the most remarkable feats of modern Western medicine in the last century has undoubtedly been organ transplantation. However, as the initially leading mortality from immunologic and nonimmunologic graft failure diminished and life expectancy of organ transplant recipients increased, other complications have arisen. A major complication, which worsens as the graft survival is extended, is posttransplantation malignancy, among which skin carcinomas are most prominent. 1,2 The carcinomas tend to be more aggressive in organ transplant recipients, causing substantial mortality, 3 and the abundance of precursor lesions (actinic keratoses) can have serious blemishing effects.Skin carcinomas among Caucasians are clearly related to solar UV exposure, 4 and so are the skin carcinomas in organ transplant recipients, 2,5 as reflected by predominant occurrence in sun-exposed skin. A striking observation is that the ratio of squamous cell carcinoma (SCC) over basal cell carcinoma (BCC) in the general population ranges around 1:3, whereas the ratio is reversed in organ transplant recipients. In the Netherlands it was estimated that about 40% of the kidney transplant recipients will have contracted at least 1 skin carcinoma by 20 years after transplantation 6 ; in Australia this percentage is already exceeded after 9 years. 7 From classic animal experimen...

Because of its antitumor effect, the immunosuppressant rapamycin holds great promise for organ transplant recipients in that it may lower their cancer risk. In a mouse model, we showed previously that rapamycin inhibits the outgrowth of primary skin carcinomas induced by UV radiation. However, the tumors that did grow out showed an altered p53 mutation spectrum. Here, we investigated whether this shift in p53 mutations already occurred in the smallest tumors, which were not affected in onset. We found that rapamycin did not alter the mutational spectrum in small tumors and in preceding microscopic clusters of cells expressing mutant-p53. However, rapamycin did reduce the number of these cell clusters. As this reduction did not affect tumor onset, we subsequently investigated whether rapamycin merely suppressed expression of mutated p53. This was not the case, as we could demonstrate that switching from a diet with rapamycin to one without, or vice versa, did not affect the number of existing mutant-p53 expressing cell clusters. Hence, rapamycin actually reduced the formation of mutant-p53 cell clusters. In wild-type and p53-mutant mice, we could not measure a significant enhancement of UV-induced apoptosis, but we did observe clear enhancement in human skin equivalents. This was associated with a clear suppression of HIF1a accumulation. Thus, we conclude that rapamycin reduces the formation of mutant-p53-expressing cell clusters without affecting tumor onset, suggesting that tumors grow out of a minor subset of cell clusters, the formation of which is not affected by rapamycin.Organ transplant recipients have a high incidence of skin cancers. An important risk factor for skin cancer development appears to be immunosuppressive therapy, especially when the conventional immunosuppressants cyclosporine and azathioprine are used.1,2 The immunosuppressive drug rapamycin (sirolimus) has now been used for several years in immunosuppressive therapies in organ transplant recipients and appears promising in reducing the post-transplant cancer risk. 3Rapamycin is an immunosuppressant that exerts its effect through inhibition of the mammalian target of rapamycine (mTOR) pathway, which is a mechanism of action entirely different from that of other immunosuppressants. Rapamycin has been shown to inhibit the mTORC1 complex, preventing p70s6k from becoming activated and thereby preventing S6 phosphorylation and by inhibiting 4EBP4, both processes affecting protein synthesis differently. 4 Furthermore, rapamycin has been shown to inhibit hypoxia-induced HIF1a and VEGF expression. 5 HIF1a accumulates in nuclei of keratinocytes after UV irradiation in a biphasic manner, peaking in protein expression at 10-24 hr after irradiation. 6 Rapamycin increased apoptosis in murine embryo fibroblasts deficient in p53 after serum depletion, while infection with Ad-p53 completely protected against rapamycin-induced apoptosis by inducing G1 cell cycle arrest. 7Recent studies indicate that rapamycin decreases the rate at which skin malignancies d...

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Cyclosporine in bolus doses appears to increase skin cancer development, whereas cyclosporine administration more evenly spread over time does not. Extrapolation to transplant patients suggests that the mode of administrating cyclosporine may be crucial for the increased skin cancer risk and that this risk might be lowered with a more steady release of cyclosporine in the body.

Immunosuppressive drugs are thought to cause the dramatically increased risk of carcinomas in sunexposed skin of organ transplant recipients. These drugs differ in local effects on skin. We investigated whether this local impact is predictive of skin cancer risk and may thus provide guidance on minimizing the risk. Immunosuppressants (azathioprine, cyclosporine, tacrolimus, mycophenolate mofetil, and rapamycin) were assessed on altering the UV induction of apoptosis in human skin models and of p53 mutant cell clones (putative tumor precursors) and ensuing skin carcinomas (with mutant p53) in the skin of hairless mice. Rapamycin was found to increase apoptosis (three-fold), whereas cyclosporine decreased apoptosis (three-fold). Correspondingly, a 1.5-to five-fold reduction (P ¼ 0.07) or a two-to three-fold increase (P < 0.001) was found in cell clusters overexpressing mutant p53 in chronically UV-exposed skin of mice that had been fed rapamycin or cyclosporine, respectively. Deep sequencing showed, however, that the allelic frequency ($5%) of the hotspot mutations in p53 (codons 270 and 275) remained unaffected. The majority of cells with mutated p53 seemed not to overexpress the mutated protein. Unexpectedly, none of the immunosuppressants admixed in high dosages to the diet accelerated tumor development, and cyclosporine even delayed tumor onset by approximately 15% (P < 0.01). Thus, in contrast to earlier findings, the frequency of p53-mutant cells was not predictive of the incidence of skin carcinoma. Moreover, the lack of any accelerative effect on tumor development suggests that immunosuppressive medication is not the sole cause of the dramatic increase in skin cancer risk in organ transplant recipients. Cancer Prev Res; 6(2);