Developmental regulation of apolipoprotein B mRNA editing is an autonomous function of small intestine involving homeobox gene Cdx1

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to ∼90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 (∼0.3%), but very high in well differentiated enterocytes (∼91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from ∼2 to ∼90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.