Transcription factors play an important role in cell proliferation and differentiation, and consequently in tissue repair. We studied the expression of two GATA transcription factors in human gastrointestinal tract in detail. As our samples were derived from both children and adults, the wide age variation may have influenced the results. It is noteworthy, however, that in murine gastrointestinal tract, GATA-4 and GATA-6 expression patterns are rather stable from birth to adulthood [30
In mature gastrointestinal mucosa, new cells are continuously generated from multipotent stem cells. Although the exact location of the stem cells of the gastrointestinal mucosa has not been determined, it has been suggested that in the stomach they lie in the isthmus, whereas in the intestine they may be located deeper in the crypts. The proliferating descendants of the stem cells enter the differentiation pathway and migrate into their specific sites in the mucosa. The cells eventually undergo apoptosis and are shed into the gut lumen [31
]. It is of interest that the strongest GATA-6 expression occurs in the basal regions of the gut mucosa including cells with the highest proliferative capacity. Although GATA-6 expression has earlier been thought to decrease during enterocyte differentiation in vitro
], we found GATA-6 in all mucosal layers, including the areas of highly differentiated cells. Ihh has been suggested to be regulated in part by GATA-6 [21
], and we find that the expression patterns of these two factors partly overlap in the gut. In contrast to GATA-6, GATA-4 was localized to more differentiated cells (this study) [10
]. In the murine small intestine, GATA-4 expression diminishes towards the villus tips the low expression thus associating with areas of apoptosis [15
]. In cardiac myocytes, GATA-4 is related to anti-apoptotic factors [32
], and its down-regulation is proposed to be essential for apoptosis [33
]. We therefore suggest that the absence of GATA-4 from the villus tip enables enterocyte apoptosis and exfoliation of senescent cells.
In chronic gastrointestinal inflammation, such as esophagitis and gastritis, the renewal of normal tissue is disturbed, possibly leading to neoplastic tissue growth [34
]. Interestingly, the expression of GATA-4, GATA-6 and Ihh appear to increase in two precancerous lesions such as Barrett's esophagus and intestinal metaplasias of the stomach. GATA-4 and GATA-6 are also expressed in hyperplastic neuroendocrine cells associated with atrophic gastritis. Some researchers have linked GATA-6 to normal murine neuroendocrine cells [35
], but we detected neither GATA-4 nor GATA-6 in these cells (this study). The appearance of GATA factors in neuroendocrine cell hyperplasia raises the question of whether they contribute to the progression of neuroendocrine neoplasms. Interestingly, GATA factors have been shown to downregulate HDC
] encoding a histamine synthesizing enzyme found in both normal and, in increasing amounts, in neoplastic neuroendocrine tissues [38
In contrast to metaplasias of the proximal gastrointestinal tract, GATA-4 is not present in colon tumors, whereas GATA-6 and Ihh are moderately expressed in colon adenomas, but to a much lesser extent in carcinomas. Our results suggest that histological tumor grade does not significantly correlate with the level of expression of GATA-6 in cancer cells, although earlier in vitro
studies have suggested that GATA-6 expression is strongest in the most undifferentiated colon carcinoma cells [10
]. In our preliminary analyses, we found that intense GATA-6 immunoreactivity is characteristic of the border regions of malignant tissue and the invasive parts of the carcinoma. This may well be due to stromal signals that induce GATA-6 in the adjacent tumor regions.
The expression patterns of GATA-4 and GATA-6 in the longitudinal and crypt-villus axes are in accordance with the results of earlier studies on murine gastrointestinal tract [15
]. In human fetuses, GATA-4 was found in the small intestine [39
], and this expression is sustained in mature mucosa as well (this study). Also an earlier study based on RT-PCR analysis demonstrated the absence of GATA-4 from colon, and its presence in stomach [40
]. A gene for hydrogen/potassium adenosine triphosphatase (H+/K+ ATPase) in the stomach, responsible for acid production in the parietal cells, is regulated by the gastrointestinal GATA factors [41
]. In the parietal cells, GATA-6 positivity varied from one cell to another. This may reflect the fact that the structure and activity of the parietal cells depend on their developmental stage and location in the glands [31
]. In our study, Ihh expression was intense in normal gastric glands, whereas Fukaya et al. [24
] found Ihh only in gastric pits. The same Ihh antibodies were used in both studies, but the samples in Fukaya's study were neoplastic and their matched tissues. These differences are likely to explain the discrepancies between the two reports.
GATA proteins have been suggested to regulate genes encoding for TFF and mucin proteins [16
] which protect the mucosa from harmful exogenous agents and are related to abnormal tissue growth and carcinogenesis [42
]. It is of interest that both GATA-4 and GATA-6 proteins are present in TFF-expressing pit and surface epithelial cells. We detected GATA-6 also in human goblet cells, and others have found GATA-4 in murine goblet cells that express the mucin protein MUC2 [17
]. Furthermore, TFF and mucin proteins have been shown to be present in esophageal and gastric metaplasias [45
], similarly to GATA-4 and GATA-6 (this study). Particularly MUC2 expression in Barrett's metaplasia is considered to indicate a higher risk for carcinoma [46
]. Collectively, these data support the interrelationship of GATA proteins, TFF, and mucins in the gastrointestinal endoderm.
A previous study suggested that inactivation of the Gata4
gene by methylation could be crucial during carcinogenesis [48
]. It is tempting to speculate that GATA-4, in addition to promoting cellular differentiation in the human gastrointestinal tract, is also involved in the suppression of abnormal growth in the proximal gastrointestinal tract. When GATA-4 is inactivated by methylation, the fate of the cells may proceed towards malignant alterations. Likewise, the role of GATA-6 in neoplasias is controversial. In vascular smooth muscle cells, GATA-6 inhibits injury induced hyperplasia [49
]. It is noteworthy that GATA-6 is present in adrenocortical adenomas, but diminishes in carcinomas, suggesting that also Gata6
may become methylated during tumorigenesis [50
]. GATA-6 has been suggested to induce cell cycle arrest [51
], to inhibit apoptosis and induce malignant cell growth [52
]. Our preliminary results suggest high expression of GATA-6 in the invasive edges of the carcinomas. The enhanced GATA-6 action in these tumor areas may be linked to uncontrolled growth of the tumor cells. More detailed studies are, however, required to establish the role of GATA-6 in the gastrointestinal tumor growth.