The importance of the immune response in tumor progression and patient prognosis is becoming increasingly clear, and our group is one of those who has been dedicating intense attention to the clinical application of the several cells and immune system mediators involved in this response[37–44]. However, interpreting data is difficult since tumor microenvironment elements can play a dual role, either suppressing malignancy or cooperating with tumor growth[45, 46]. CAMs are essential for the migration of leukocytes to sites of inflammation and play an important role in stimulating intracytoplasmic signals that regulate cell differentiation, survival, and proliferation[47]. In fact, our data indicate that both ICAM1 and ITGAL are associated with lymphocytic chronic infiltration. Due to their multiple interactions in different signaling pathways, CAMs correlate with a good prognosis in some tumor types, whereas in other neoplasm types, their expression is correlated with a worse prognosis and aggressive characteristics[5, 7–9, 13–16, 20, 21, 26, 27, 48–52]. Taking advantage of a very well-characterized group of DTC patients carefully followed-up for more than 6 years by the same group in a single institution, we investigated the role of SELL, ITGAL, and ICAM1 mRNA and protein expression in the characterization of thyroid nodules and their possible clinical utility in identifying aggressive tumors.
L-selectin is a molecule that acts mainly in the capture and rolling of migratory cells, both leukocytes and metastatic cells[10, 49]. Its expression has been studied in several types of cancer, such as ovarian[50], bladder[48], and thyroid cancer[7, 9, 11], and is usually related to tumor aggressiveness. Muzza et al[9] found higher expression of L-selectin in 15 PTC than in 4 normal (contralateral) thyroid tissues. We also observed higher SELL mRNA expression in malignant tumors than in benign thyroid nodules (p = 0.0027). We also showed that SELL mRNA expression could differentiate some of the histopathology follicular patterned cases, especially FA from FTC (p = 0.0078). Despite the trend toward higher SELL expression in tumors with lymphocytic infiltrate (1.40 ± 2.16 AU) when compared with tumors without this characteristic (0.88 ± 1.44 AU, p = 0.1883), we could not confirm the findings of Muzza et al[9] on this association[9].
We observed decreased expression of both nuclear and cytoplasmic L-selectin in more aggressive histological types, suggesting that nuclear expression of this molecule decreases as neoplastic cells become less differentiated. On the other hand, Choudhary et al observed that membrane L-selectin expression was higher in PTC than in benign nodules [5]. This finding may be explained by L-selectin being cleaved after its activation, releasing its extracellular portion in soluble form[5, 48]. Hence, the activation of this molecule may contribute to the cellular metastatic process. Indeed, the literature has reported a higher serum concentration of L-selectin in several types of malignant tumors[8, 50, 53, 54]. The increase in soluble L-selectin and the possible relocation of its cytoplasmic portion may stimulate the increase in mRNA levels, explaining the higher SELL mRNA expression observed in malignant tumors alongside the lower expression of both nuclear and cytoplasmic proteins. However, note that L-selectin is a molecule participating in a process that involves a tangle of other molecules, cytokines, chemokines, transcription factors, signaling pathways, and other components that act in a systemic and integrated way. Thus, other yet undescribed factors may modulate L-selectin function.
ICAM-1 is a glycoprotein involved in both innate and adaptive immune responses[55]. It has been suggested that ICAM-1 acts as a facilitator of cancer cell spread through the recruitment of inflammatory cells that release stimulating factors for cell proliferation, angiogenesis, and invasion. This molecule also plays a role in the mechanisms of tumor cell progression, and studies have related it to the prognosis of some types of cancer[52, 56]. Hayes and Seigel[15] analyzed approximately 300 samples of normal, malignant, and metastatic tissues from various tumors, including thyroid cancer, and found overexpression of both the ICAM1 gene and ICAM-1 protein in malignant tissues. More recently, Buitrago et al[14] confirmed higher expression of ICAM1 in malignant tumors, corroborating our findings. However, despite also observing higher mRNA expression in the presence of lymphocytic infiltration, we could not associate ICAM1 with any characteristic of tumor aggressiveness or patient outcome.
LFA-1 is encoded by the ITGAL gene and is expressed in the leukocyte membrane[23] but has also been described in several types of tumor cells[20, 24–26]. LFA-1 has been associated with the progression of lymphomas[57] and colorectal cancer[58]. Papas et al[26] found a higher expression in primary tumors from patients without distant metastasis and a negative correlation between the expression of LFA-1 and the metastatic process suggested a tumor suppressor role for LFA-1 since its expression was higher in primary tumors from patients without distant metastasis suggesting a protective role in colorectal adenocarcinomas. Our findings of lower ITGAL mRNA expression in cases with lymphovascular invasion (p = 0.0427) and extrathyroidal extension (p = 0.0111) and high mRNA expression in cases with lymphocytic infiltrate (p = 0.0244), reinforce a tumor suppressive role for LFA-1 in thyroid tumorigenesis. We know that the conformational structure of LFA-1 changes to interact with other CAMs, especially ICAM-1 [18] making us speculate that LFA-1 role depends on its conformational structure. Alternatively, it may be regulated by other signaling pathway molecules.
In conclusion, our data confirm an important role of CAMs in thyroid cell proliferation and tumor progression. In general, the protein expression of these molecules decreases as the cell dedifferentiation process occurs. However, the relatively long and positive course of most thyroid cancer patients, and the complex role of CAMs at the different stages of tumor evolution hinders the correlation of these molecules with tumor aggressiveness or patient outcomes in a clinical setting. On the other hand, we demonstrated that mRNA expression of SELL and ICAM1 and protein expression of L-selectin and LFA-1 can help in the histological characterization of thyroid nodules with a follicular pattern.