Our previous studies have revealed several common defects in lymphocytes including Natural Killer (NK) cells in patients with cancer [1-4]. Detected signaling abnormalities include dysregulated expression and signaling of the key cellular pathways associated with c-Myc, Notch, c-kit/SCF, STAT3, MAPK, p-53, Cdk6, Rb, C/EBPα, BACH1 and PU.1 in cells isolated from patients with gastric and lung cancer [5,6]. NK cells are recognized as important antitumor cytotoxic innate lymphoid cells not only for their ability to eliminate pre-malignant and malignant cells, but also for their capacity to diminish tumor cell spreading, formation of metastases, controlling dormancy and sculpting the tumor microenvironment by secreting cytokines and chemokines, attracting effector antitumor immune cells and maintaining their functionality [7,8]. Therefore, understanding molecular abnormalities in NK cells in patients with cancer is crucial for developing new approaches for cancer therapy by correcting and boosting NK cell interactions and functionality within tumor tissues.

c-Myc (henceforth MYC), a 62 kDa protein (439 amino acids) from the basic helix-loop-helix zipper class of transcription factors, controls transcription of specific genes linked to DNA replication, chromatin alteration and ribosome and mitochondrial biogenesis [9]. In physiological conditions, c-Myc expression, as a component of the immediate-early gene program, is controlled by growth factors and cytokines, and regulates the crucial cellular activities such as cell differentiation, metabolism, growth, division and apoptosis [10]. Although expression of c-Myc is precisely regulated in normal cells, in 60-70% of human tumors, Myc is frequently amplified due to either direct genomic rearrangements or as a result of the deviant upregulation of upstream oncogenic pathways, contributing to the initiation and maintenance of disease [11]. Furthermore, being a principal controller of immune cell metabolism [12], c-Myc dysregulation is associated with inflammatory, autoimmune and metabolic non-cancerous diseases, although it remains weakly recognized [13,14]. Moreover, analysis of Myc expression and the significance of its changes in immune cells in the tumor environment have not been yet systematically conducted.

Although immune effectors play a vital role in cancer surveillance, polarized and regulatory immune cells participate in the formation of the premalignant milieu, maintenance of the tumor microenvironment and development of the pre-metastatic niches. In addition, anomalous differentiation, activation, function and longevity of immune cells in cancer have been well documented in pre-clinical and clinical setting, which are also responsible for tumor escape from immune recognition and elimination. For instance, impaired expression of activating receptors (NKG2D) and effector molecules (granzyme B, IFN-γ), proliferation, direct and antibody-dependent cytotoxicity have been demonstrated in NK cells in different types of cancer suggesting that NK cells are functionally compromised in the tumor milieu [15-17]. Our earlier studies revealed defective expression of c-Myc mRNA and protein in NK cells isolated from patients with cancer [1,5]. One can speculate that an abnormal expression of proto-oncogenes in lymphoid and myeloid cells may be due to a general imbalance of transcription factors responsible for the lineage-specific differentiation. In fact, cancer-associated disruption of differentiation of hematopoietic progenitor cells in myeloid and lymphoid cells have been repeatedly reported [18-22], as well as the role of c-Myc in differentiation of lymphoid and myeloid cells [23,24]. These and other results allow hypothesizing the involvement of transcriptional defects in the mechanism of differentiation of immune cells as one of potential sources of immune abnormalities in cancer patients.

Transcription factors (TF) are DNA-binding proteins that control the structure of chromatin and gene transcription. The human genome encodes 2,000+ different TF, and specific patterns of TF expressed in different cell subsets comprise specific cellular programs. Importantly, mutated TF contribute to the formation of tumors, and an excessive amount of oncogenic TF can change the basic autoregulatory pattern of a cell [25]. In our studies of non-cancerous cells, we assessed the expression of the transcription factor proteins C/EBPα, BACH1 and PU.1, which play a key role in the differentiation of myeloid and lymphoid cells. We found significant reductions in C/EBPα, BACH1 and PU.1 TF in lymphoid (NK and T cells) as well as myeloid cells in the peripheral blood from patients with cancer [3]. In all cancer patients, a general tendency towards a decrease in all TF expression in examined immune cells was noted. Moreover, we found a statistically significant decrease in specific transcription factors responsible for the differentiation of various types of cells, which indicates a defect in the differentiation of these cells in cancer patients. Specifically, we observed the significantly reduction of C/EBPα in myeloid cells, BACH1 in T cells and PU.1 in NK cells.

These results might suggest that cancer development is accompanied by dysfunctions in different hematopoietic pathways. To test this, we investigated CD34⁺CD45^dim cells from the same patients, which represent a subset of hematopoietic early stem/progenitor cells [26,27], with the expectation to see similar TF alterations as we detected in differentiated leukocytes. However, the results revealed an opposite tendency; while we observed a down-regulated expression of TF in mature immune cells in all cancer patients, in progenitor cells, we noticed an increase in TF expression. Interestingly, this increase in TF in hematopoietic precursors was not observed in all evaluated patients, but only in patients with lung cancer. There were no marked changes in precursor cell TF in cancer patients with the localization of the malignant process in the gastrointestinal tract [3]. Thus, we observed a discrepancy between the expression of the same transcription factor in progenitor cells and later differentiated cell forms.

The differential expression of TF in mature and immature circulating leukocytes in different types of cancer not only supports a hypothesis about a common disturbance of immune cell differentiation and activation in carcinogenesis but rinses even a more interesting question about the mechanisms responsible for abnormal differentiation of hematopoietic cells in cancers with different locations. How does cancer-specific local and systemic microenvironment control expression and function of TF, such as C/EBPα, BACH1and PU.1, in hematopoietic stem cells and how do cancer-derived or cancer microenvironment-derived factors specific and non-specific molecules regulate leukocyte differentiation from altered precursors?

Determined abnormalities in TF in CD34⁺CD45^dim cells in lung cancer, especially in patients with detectable metastases, but not in gastric cancer may provide some clues about the tumor type and hematopoiesis malfunction. However, despite the known cell-specific role of C/EBPα as a lung tumor suppressor [28] and the role of BACH1 in lung cancer metastases [29], as wells the fact that PU.1 can serve as a stromal prognostic marker in non-small cell lung carcinoma [30], the relationship between the high expression of these factors in early hematopoietic progenitor cells and lung cancer progression is unclear.

Overall, in addition to the reported defects of C-myc and Notch1 in immune cells in cancer, our results suggest that decreased expression of C/EBPα, BACH1 and PU.1 transcription factors can be attributed to the general immune impairment in cancer patients. Our data also indicate that molecular defects in the immune system in cancer patients involve cells at different steps of their differentiation, from the early precursors to the mature peripheral cells. Further analysis of the abnormal expression of other transcription factors in early lymphoid and myeloid progenitor cells should help clarify the regulatory damage of the immune system in oncological processes.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.