RESUMO
The colonic epithelium contains three major types of mature cells, namely, absorptive, goblet, and enteroendocrine cells. These cells are maintained by a complex process of cell renewal involving progenitor and stem cells, and colon cancers develop when this process goes awry. Much is known about the genetic and epigenetic changes that occur in cancer; however, little is known as to the specific cell types involved in carcinogenesis. In this study, we expressed the SV40 Tag oncogene in the intestinal epithelium under the control of an intestinal trefoil factor (ITF) promoter. This caused tumor formation in the proximal colon with remarkable efficiency. ITFTag tumors were rapidly growing, multifocal, and invasive. ITFTag tumor cells express synaptophysin and contain dense core secretory granules, markers of neuroendocrine differentiation. The cell type involved in the early steps of ITFTag tumorigenesis was studied by examining partially transformed crypts that contained populations of both normal and dysplastic cells. The dysplastic cell population always expressed both Tag and synaptophysin. Cells expressing Tag alone were never observed; however, normal enteroendocrine cells expressing synaptophysin but not Tag were readily visualized. This suggests that ITFTag tumor cells originate from the enteroendocrine cell lineage following a transforming event that results in Tag expression. ITFTag tumors closely resemble human small cell carcinomas of the colon, suggesting the possibility that these tumors might be derived from the enteroendocrine cell lineage as well.
Assuntos
Antígenos Transformantes de Poliomavirus/genética , Antígenos Transformantes de Poliomavirus/metabolismo , Carcinoma de Células Pequenas/patologia , Neoplasias do Colo/patologia , Mucinas/genética , Proteínas Musculares/genética , Peptídeos/genética , Regiões Promotoras Genéticas/genética , Animais , Carcinoma de Células Pequenas/genética , Carcinoma de Células Pequenas/metabolismo , Carcinoma de Células Pequenas/ultraestrutura , Neoplasias do Colo/genética , Neoplasias do Colo/metabolismo , Neoplasias do Colo/ultraestrutura , Modelos Animais de Doenças , Regulação Neoplásica da Expressão Gênica , Camundongos , Camundongos Transgênicos , Oncogenes/genética , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Fator Trefoil-2RESUMO
Human intestinal mucin genes MUC3A and MUC3B are members of a membrane mucin gene family residing at chromosome 7q22. In this paper, we utilized genomic and cDNA cloning to elucidate the sequence of the 5'-region of the MUC3A gene including the gene promoter and the amino terminus coding sequence. Following its 21-residue signal peptide, the amino terminus of the mucin consists of a 233-residue Thr-, Ser-, and Pro-rich nonrepetitive sequence that is contiguous with its hypervariable domain of 375-residue repeats. RNase protection analysis and 5'-GeneRacer PCR indicated that MUC3A gene transcripts initiate from multiple start sites along a region spanning approximately 180 bases. The 5'-flanking region of the gene had promoter activity when fused to a luciferase reporter gene in all of the tested cell lines. This region contained binding sites for several transcription factors, including those implicated in the regulation of intestinal genes, but lacked a cognate TATA box. These features of the gene promoter may enable the gene to be expressed at variable levels in several cell types with different repertoires of transcription factors. We also utilized 5'-GeneRacer PCR to determine the sequence of the 5'-terminus of the MUC3B message. The amino termini of the MUC3A and MUC3B mucins are 91% conserved at the amino acid level. Thus, MUC3A and MUC3B have highly conserved amino and carboxyl termini, suggesting a recent duplication of the entire ancestral gene. It remains to be determined whether other members of the 7q22 membrane mucin gene family have amino-terminal domains similar to MUC3A and MUC3B.
