Cytotoxic, microscopic and biochemical characteristics of MDA-231/R (R) and MDA-MB-231 (S) cells
The cytotoxicity results currently obtained clearly illustrate the high cDDP-resistance of R cells as compared to S cells. This evidence is accompanied by the presence of a higher number of PGCCs in the resistant cells, confirming previous reports of a relationship between the number of PGCCs with tumor chemo-resistance and aggressiveness [
45,
46]. PGCCs are cells with multiple nuclei or a single giant nucleus containing multiple complete sets of chromosomes and it is well documented that these cells are present in several solid tumors, usually in lower numbers in sensitive cells before treatment, compared to resistant cells [
45]. Moreover, a positive correlation has been formerly described between the number of PGCCs and glioma stage and grade [
47].
Furthermore, a basal phosphorylation of NF-κB is observed in S and R cells, although in the R cells a tendency to an increase in p-NF-κB is observed consistent with a slight higher activation of the NF-κB pathway and, hence, cell survival, as shown by reports of the relationship between NF-κB pathway activation, through the IKKα pathway, and cell survival under conditions of cDDP exposure [
48]. In addition, a significant decrease in the basal levels of phosphorylated ERK1/2, compared to S cells, seems to characterize the resistance of TNBC cells to cDDP. Again, this is consistent with previous reports associating cancer cells (
e.g. cervical carcinoma) (in)sensitivity to cDDP with the downregulation of ERK pathway activation [
49,
50]. Considering that the ERK1/2 pathway may crosstalk with other signaling pathways, and that reports are found on both ERK 1/2 and the NF-κB being involved in cDDP-acquired resistance in BC cells [
51,
52], we propose that the constitutive activation of NF-κB and of ERK1/2 pathways is modified by exposure of S cells to cDDP, with a slightly increase of NF-κB as well as a downregulation of ERK1/2 pathways that might be associated with TNBC survival and its more aggressive characteristics. A similar hypothesis has been advanced in cervical carcinoma, for which increased cDDP-resistance has been observed in association with reduction of activation of the MEK to ERK2 pathway (in the presence of a MEK1-selective inhibitor, 2’-amino-3’-methoxyflavone) and involving, at least in part, an increase of NF-κB activation [
50].
Two of the main metabolic features that R cells maintain overtime is lower anaerobic glycolytic activity (lower lactate levels, although increasing with time in each cell line), and lower glutaminolytic activity (lower glutamate and high glutamine levels). In S cells, both processes are more active, at all times, an expected hallmark of cancer metabolism [
53]. However, interestingly, both pathways seem subdued in resistant cells. This seems to contradict the increases in glycolytic and glutaminolytic activities reported in relation to Pt(II)-resistance in colon and ovarian cancer cells [
12,
15‐
17]. We therefore suggest that reduced glycolytic and glutaminolytic activities may be a specific characteristic of TNBC cDDP-sensitive cells. Both observations are consistent with the noted significant decrease in ERK activity. The role of ERK (and JNK) pathways in the metabolic reprogramming of highly proliferating cells (such as cancer cells) has previously been discussed in detail [
54]. These authors advance that increased ERK activity accompanies a high proliferative activity in cancer cells leading to a negative regulation of the enzyme pyruvate kinase isoform M2 (PKM2, responsible for PEP to pyruvate conversion), through phosphorylation. This low PKM2 activity induces an accumulation of upstream glycolytic intermediates, which are precursors of several biomolecules (
e.g. amino acids, nucleotides and fatty acids). Interestingly and paradoxically, a low PKM2 activity is also related with an increased pyruvate conversion into lactate, through the action of lactate dehydrogenase A, at the expense of NADH which is oxidized to NAD
+. This behavior seems to be descriptive of the cDDP-sensitive cells presently studied, whereas the R cells exhibit all opposite features: decreased ERK activity, decreased lactate formation and NAD
+ levels (compared to S cells). A similar reasoning applies to the relationship between the ERK signaling pathway and glutaminolysis [
54], as a decreased ERK activity in R cells should promote decreased expression of the c-Myc transcription factor, known to lead to lower glutaminolysis activity. Articulation with NF-κB expression (slightly higher in R cells), known to also affect glutaminolysis (as well as glycolysis and OXPHOS [
55]), may help to keep glutamine pools high. Glutamine may not only serve glutaminolysis for anaplerosis (which is presently seen to be slowed down in R cells), but is also related to cancer cell stemness [
56], a feature which has been generally related to therapy resistance, tumor dormancy and metastatic behavior [
57]. As glutamine deprivation has been observed to relate to decreased stemness properties [
56], we hypothesize that the richer glutamine pool found here in TNBC cDDP-resistant cells suggests a higher stemness capacity and, hence, a higher cell adaptability (and survival) once under cDDP exposure.
As mentioned before, low glutaminolysis and, thus, low levels of glutamate in R cells should contribute to lower TCA activity, as conversion of glutamate into α-ketoglutarate should decrease. However, it is interesting to note that a decrease in other anaplerotic amino acids, particularly at later culture times, (proline from 24 h; alanine, leucine, phenylalanine, tyrosine and particularly lysine, at 48 h) may suggest their preferential use as precursors into TCA intermediates and pyruvate precursor, respectively (Fig.
7), compared to methionine and branched-chain amino acids (BCAAs) isoleucine and valine, which exhibit higher levels in R cells. BCAA metabolism has been related to cancer resistance in general [
58], but it is interesting to note the different leucine behavior in R cells: decreased, as opposed to increased levels of isoleucine and valine. Indeed, leucine metabolism (in particular through the activity of branched-chain amino acid aminotransferase 1, BCAT1) has been recently suggested to lead to activated mTOR-mediated autophagy which, in turn, increases cDDP-resistance [
59]. We therefore propose that lower levels of leucine may be related to such mechanism.
Furthermore, the levels of TCA intermediates detected by NMR were strongly dependent on culture time, which suggests a possible modulation of TCA activity overtime. The low NAD
+ levels (at all time points, and not replenished by lactate production) could either arise from its use in an activated TCA cycle, and/or reflect the general significantly lower availability of nucleotides in R cells, as will be discussed below. Previous reports [
54] support the hypothesis that ERK activation promotes aerobic glycolysis and ATP synthesis, subsequently used for phosphorylation. Hence, in R cells, where ERK is less active, ATP synthesis is expected not to be significantly stimulated. Indeed, ATP levels are lower than in S cells at 0 h, although tending towards equivalent levels at later times (note the lower ADP/ATP ratios at 24 h due to ATP increase). We suggest that a later enhanced ATP synthesis in R cells may be a reflection not of ERK activity but, rather, of an adaptive later TCA activation, although the precise dynamics of this pathway overtime requires further investigation, at this stage. These relatively higher ATP levels may also explain the relatively elevated PCr levels (supported by increased Cr and sarcosine, Fig.
7) noted in R cells. Cr to PCr interconversion is an important energy buffer mechanism, which produces high energy PCr particularly in cells with high requirements of energy such as cancer cells [
60]. Furthermore, the PCr/Cr ratio has been related to metastasis and proliferation and we advance that a higher PCr/Cr ratios may be related to higher cDDP-resistance.
In addition, increased levels of taurine (the oxidized form of hypotaurine) and GSH are clear discriminators of R cells, indicating an interesting interplay of compounds related to antioxidant protection mechanisms, including methionine (related to taurine through cysteine, and to GSH through the transsulfuration pathway [
61] (Fig.
7). Although reactive oxygen species (ROS) were not quantified in this work, the NF-κB signaling pathway is believed to be closely related to oxidative stress [
62]. Hence, we propose that the maintenance of high, nearly-constant, GSH levels in R cells, compared to S cells, along with high increasing taurine levels (whereas taurine remains lower and constant in S cells), may indicate that the antioxidative mechanisms in R cells rely preferably on the hypotaurine/taurine pair, rather than on GSH/GSSG. Furthermore, taurine increase has also been reported in ovarian cDDP-resistant cells [
12,
13], possibly due to the overexpression of the taurine transporter (TauT) that leads to intracellular taurine accumulation, which in turn is suggested to result in the inhibition of cDDP uptake [
13,
63].
The decrease in ERK activity noted in this work may also be related to the marked overall low levels of nucleotides and several of their derivatives, mainly involving adenine, uracil and inosine (including UDP-Glc/GlcA and uridine diphosphate
N-acetyl-glucosamine UDP-GlcNAc), which make up a nitrogen-base-depleted metabolic profile in R cells, which is more stable overtime than in S cells (Fig.
7). Indeed, a decreased ERK activity and its correlation with increased PKM2 activity may explain the decreased biosynthesis of nucleotides, as described above [
54]. However, lower levels of nucleotides may also be related with their use as building blocks as dNTPs to support nucleotide excision repair (NER). This has been reported as a major resistance mechanism against cDDP in several types of cancer [
64,
65] and is believed to justify cell death when repairs are not possible [
66]. Additionally, poly(ADP-ribose) polymerase (PARP) enzymes, especially PARP1, may determine DNA damage response and maintenance of genome stability through their involvement in NER [as well as in other mechanisms such as base excision repair (BER) and homologous recombination (HR)] [
67]. Therefore, we hypothesize that PARP enzymes may have an active role in a more efficient DNA repair in R cells. Upon exposure, these enzymes may interfere with the formation of cDDP-adducts with DNA’s purine bases (adenine and guanine). We hypothesize that the setup of the R cell line, through exposure to low concentrations of cDDP, may have activated these enzymes. As PARP proteins require NAD
+ to act [
67,
68], it is possible that the low NAD
+ levels in R cells may also reflect this mechanism.
Finally, R cells are also depleted in choline and GPC, the former increasing overtime, while GPC remains stable. This reflects disturbances in membrane metabolism but the exact variation pattern contrasts with results characterizing cDDP-resistance in ovarian cancer cells, which were characterized by increased levels of GPC (although confirming decreased levels of choline as described here) [
13]. This relationship between choline compounds and the exact nuances of membrane remodeling mechanisms characterizing cDDP-resistance in TNBC remains unclear, at this stage. We furthermore suggest that the changes observed may reflect distinct lipid metabolic features (eventually detectable by lipid metabolomics) characteristic of this type of resistance in TNBC.