The two factors responsible for most breast cancer-related deaths are the ability of cancer cells to metastasize and to develop resistance to anti-cancer therapies. Indeed, resistance to chemotherapy is a major obstacle to successful treatment of breast cancer [1, 2]. Anthracycline antibiotics are among the most effective and commonly used anticancer drugs, and doxorubicin (DOX) in particular is often the first choice to treat mammary cancer. Nevertheless, its use has been limited due to the rapid development of chemoresistance and cardiomyopathic side effects [1, 3]. The therapeutic activity of DOX results from its intercalating into DNA, thereby inhibiting topoisomerase II and preventing DNA and RNA synthesis [4, 5]. With respect to chemoresistance, it is known that DOX-resistant cells exhibit: 1) diminished Topoisomerase II expression, 2) increased production of calcium-dependent protein transglutaminases and of integrins involved in membrane stabilization, and 3) increased expression of anti-apoptotic messengers like Bcl2, Bxl, or survivin [2, 6, 7]. The protein survivin (Surv), the smallest member of the inhibitor of apoptosis protein (IAP) family, is expressed and required for normal fetal development but is generally absent in adult tissues. However, re-expression of Surv is observed in numerous human cancers where its presence is associated with enhanced proliferation, metastasis, and a poor prognosis .
Short-term adverse effects of DOX occurring within 2–3 days of administration include nausea, vomiting, neutropenia, alopecia, and arrhythmias . A long-term side effect is the cardiomyophathy associated with congestive heart failure [10, 11]. This chronic cardiotoxicity is likely related to iron oxidation and oxygen free radical formation rather than to antineoplastic mechanisms of the drug, since cardiomyocytes are minimally replicating cells . DOX typically induces dilated cardiomyophathy with microscopic features including myofibrillar loss, mitochondrial and interstitial edema, fibroplasias, etc. Clinical serum biomarkers such as creatine kinase MB (CK-MB) or troponins may increase and serve as important, early predictors of cardiotoxicity [12, 13].
Low incidences of mammary cancer and cardiovascular disease have been associated with the high iodine intake in Asian, as compared to Western populations (5280 vs 209 μg /day). Seaweeds, such as wakame, nori, or mekabu, are widely consumed in Asian countries and contain high quantities of iodine in several chemical forms [14, 15]. In recent years several research groups including ours have demonstrated that iodine per se acts as an antioxidant in the whole organism and participates in the physiology and/or pathology of organs that capture iodine, such as thyroid, mammary gland, prostate, and stomach [16–19]. In mammary gland, supplementation with molecular iodine (I2), but not iodide (I-), alleviates human mastalgia and exerts potent antineoplastic and apoptotic effects on animal and human cancer. When administered over periods of weeks up to years, moderately high concentrations of I2 supplements (3–6 mg/day) have no deleterious effects on thyroid or general health [20–22]. Although the cellular mechanisms through which iodine exerts these effects have not been elucidated in depth, two hypotheses have been proposed: a direct action involving the antioxidant/oxidant properties of iodine, and an indirect effect through iodolipid formation. In the case of antioxidant/oxidant effects, two sets of data have been obtained showing that: a) at low or moderate concentrations, I2 significantly reduces lipid oxidation either by competing with reactive oxygen species (ROS) for various cellular components, or by neutralizing HO radicals, and b) at high concentrations, iodine acts as a direct oxidant, dissipating the mitochondrial membrane potential, thereby triggering mitochondrion-mediated apoptosis [18, 23–25]. The indirect effect was originally postulated for thyroid tissue and involves the formation of iodolipids such as 6-iodo-5-hydroxy-8,11,14-eicosatrienoic acid (also called 6-iodolactone; 6-IL) or alpha-iodohexadecanal derived from arachidonic acid (AA) or eicosapentaenoic acid, respectively . Both iodolipids exert apoptotic effects , and our group has demonstrated that 6-IL is present in mammary cancer cells after I2 supplement and is a functional ligand of peroxisome proliferator-activated receptor (PPAR) type gamma (PPARγ) [28–30]. PPARs are ligand-activated transcription factors, and three subtypes -- PPAR alpha, PPAR beta, and PPAR gamma -- have been identified . Although originally described as molecular regulators of lipid metabolism, PPARγ were recently shown to play an important role in cell proliferation, differentiation, and apoptosis in many cancer cell lines including breast, prostate, and non-small-cell lung cancer . Moreover, PPARγ activation potentiates the cytotoxic effect of chemotherapeutic agents such as DOX by inhibiting the expression of anti-apoptotic proteins like Bcl2 and Surv  or by delaying the epithelial-mesenchymal transition .
In the present work we evaluated the effect of I2 supplement in combination with DOX on the methylnitrosourea (MNU)-induced mammary cancer model. Our results show that a 7-day I2 supplement exerts a significant antineoplastic adjuvant effect with DOX as well as significant cardioprotection, whereas a 56-day iodine treatment is associated with enhanced DOX sensitivity, increased Bax and PPARγ, and decreased Bcl2 and Surv expression suggesting that iodine, through PPARγ expression/activation, induces differentiation and impairs the development of chemoresistance.