RESUMO
Mipp is a kelch-related, placental-specific gene that is ectopically expressed in many BALB/c mouse mammary carcinomas of various etiologies. The Kelch family encompasses proteins that are emerging as key links between microfilaments and a variety of cellular structures and functions. Mouse mammary tumors express two mipp transcripts (2.2 and 5.6 kb). We cloned the 2.2 kb mipp mRNA and analysed the product of its 1.7 kb ORF. The 584 residue MIPP protein has an N-terminal BTB domain and six C-terminal tandem Kelch repeats. Despite expression of two mipp RNAs, only a single MIPP protein is expressed in mammary tumors. MIPP protein binds to microfilaments in vitro and co-immunoprecipitates with actin. MIPP co-localized with concanavalin A at the endoplasmic reticulum, suggesting that MIPP might mediate interactions between microtubules and actin filaments. Because MIPP expression is widespread in mouse mammary tumors, it might contribute to tumorigenesis. Although MIPP had little effect on the growth rate of human breast cell lines following transfection, it greatly reduced the formation of duct-like structures on reconstituted basement membrane. Our results suggest that MIPP could contribute to malignant progression in the mouse mammary epithelial cells by perverting their response to cues from the extracellular matrix.
Assuntos
Actinas/metabolismo , Neoplasias Mamárias Animais/metabolismo , Monoéster Fosfórico Hidrolases/biossíntese , Monoéster Fosfórico Hidrolases/genética , RNA Mensageiro/metabolismo , Sequência de Aminoácidos , Animais , Sequência de Bases , Northern Blotting , Western Blotting , Clonagem Molecular , Concanavalina A/farmacologia , DNA Complementar/metabolismo , Retículo Endoplasmático/metabolismo , Matriz Extracelular/metabolismo , Humanos , Camundongos , Camundongos Endogâmicos BALB C , Microscopia de Fluorescência , Microtúbulos/metabolismo , Dados de Sequência Molecular , Fases de Leitura Aberta , Monoéster Fosfórico Hidrolases/química , Testes de Precipitina , Estrutura Terciária de Proteína , Transfecção , Células Tumorais CultivadasRESUMO
A placenta-specific gene, MIPP, is transcriptionally regulated in BALB/c mice by a solo long terminal repeat (LTR) of an intracisternal A-particle (IAP), an endogenous retrotransposon. Expression of IAPs, which is also promoted by LTR sequences, is a frequent aberration in many mouse mammary tumors of BALB/c mice. Given that these retroelements and the placental gene have a common promoter, we hypothesized that the tumors also express the gene. Northern blot analysis and RT-PCR revealed high expression of the placenta-specific gene in BALB/c mouse mammary preneoplasias and carcinomas of diverse etiologies, but not in normal mammary gland from virgin, pregnant and lactating mice. The preneoplasias and tumors expressed two transcripts, one of which is apparently unique to the mammary lesions. The other transcript is the same as one expressed in placenta that is not promoted by the IAP LTR. Despite the parallel expression of the placental gene and IAPs in the mammary tissues, RT-PCR showed that LTR sequences are absent from tumor-associated MIPP transcripts. Southern analysis revealed no gross mutations of the MIPP gene in mammary preneoplasias and tumors. The ectopic expression of the placenta-specific gene in BALB/c mouse mammary preneoplasias and carcinomas raises the possibility that it acts as an oncogene.
