A significant body of data from
in vivo experiments [
4,
26‐
29], clinical trials [
30‐
33], and epidemiological reviews [
34‐
36] support the notion that molecular iodine has a protective effect against the progression of neoplastic diseases and inflammatory/proliferative pathologies. An important fact to consider is that these beneficial effects occur only at relatively high iodine concentrations (milligrams/day) and only in those tissues that are able to capture iodine. In the case of mammary and prostate glands, both normal and tumoral cells internalize I
2 by a sodium/iodine symporter (NIS)-independent mechanism [
1,
4,
5,
8,
37] that seems to involve a facilitated diffusion process (saturable and dependent on protein synthesis) [
5]. It has not yet been determined how I
2 reacts with cell components, but I
2 is known to bind covalently to proteins and lipids [
5]. The iodination of fatty acids generates several derivatives, but those from arachidonic acid (AA), like 6IL, are especially relevant since endogenous 6IL has been detected [
38,
39], and it generates a range of biological effects [
3]. The proposal that PPARG plays a role in the antitumor effects exhibited by I
2 after its conversion to 6IL is supported by the following evidence: 1) 6IL can be identified in both
in vitro and
in vivo models after administration of I
2 [
1,
2]; 2) expression of
PPARG is increased in I
2- or 6IL-supplemented conditions (possibly through an autoregulation mechanism) [
2,
10]; 3) the antiproliferative effect of I
2 is only observed in cancer cells, which contain elevated concentrations of AA [
2]; and 4) when incubated with total protein, 6IL can activate the PPAR response element (shown by luciferase transactivation assays) and is capable of inducing MCF-7 cells to accumulate lipid droplets in the same manner as the highly specific agonist rosiglitazone [
10]. Previously, our group showed that the molecular antiproliferative/apoptotic actions of the 6IL/PPARG complex include the activation of p53 and the consequent increase of p21 expression, thereby inducing cell arrest and Bax-caspase expression, which activates the intrinsic apoptosis pathway [
1,
28,
29]. GW9662 irreversibly modifies Cys
285 in the PPARG ligand-binding domain with an IC
50 in the nanomolar range [
40], as compared to the micromolar IC
50 of 6IL [
1,
10]. This very high affinity can explain the almost complete inhibition by GW9662 observed in control cells. However, proliferation was not totally blocked in cells overexpressing
PPARG. By its nature, the adenoviral system forces the cell to maintain unregulated expression; therefore, it is possible that the GW9662 concentration was not sufficient to saturate the PPARGs due either to their overexpression from the vector or to well-established, positive, autoregulatory mechanisms that maintain a high and continuous expression of
PPARG [
41,
42]. In contrast, the 6IL-induced effect on apoptosis is totally blocked by GW9662 in both control and
PPARG-overexpressing cells. Thus, partial PPARG blockage may unmask this dual action of 6IL, in which only a few free PPARG receptors are required for the antiproliferative effect (maybe cell cycle arrest), but additional available receptors are needed to induce apoptosis. Similar dose-dependent responses of PPARG ligands have been described in the literature previously, showing induction of arrest at low concentrations, whereas at high levels they trigger apoptosis [
43,
44]. Moreover and although molecular mechanisms for these preferential responses have not been explored, differential recruitment of coactivator cannot be excluded. More than 300 cofactors (coactivators, coregulators, corepressors, etc.) have been identified for the nuclear receptor family. It has been shown that these elements interact with one another and the same ligand modify transcription of many genes [
45]. Interestingly, in the
PPARG-overexpressing cells, 6IL administration also induces increased expression of
FASN, a PPARG-regulated gene that is related to differentiation [
46,
47]. This observation is consistent with the differentiation effect of 6IL, observed as lipid accumulation in MCF-7 cells [
10].
Finally, it has been described that I
2 or 6IL induces apoptosis effects through at least two pathways: AIF/PARP1 and Bax-caspases [
1,
9]; Bonofiglio
et al. [
51] reports that PPARG is able to activate the promoter of the
FasL gene, thereby inducing extrinsic apoptosis in MCF-7 cells; this observation was reinforced recently by similar findings in human colorectal cancer cells [
52]. In the present work we explored if the extrinsic apoptotic pathway is activated by the 6IL/PPARG complex. Our results show that caspase-8 activity is not increased by I
2 or by 6IL treatment; therefore, it seems that the apoptotic effects of I
2 and 6IL are mediated exclusively by intrinsic apoptosis pathways.