Beta 1 integrins in epithelial tissues: a unique distribution in the lens.

Integrins have been shown to play a role in directing and maintaining cell differentiation and polarization. The embryonic lens provides a good system in which to examine their role in epithelial cell differentiation, because all stages of lens development are represented in an individual embryonic lens. Therefore, we examined the expression and distribution of beta 1 integrin heterodimers in both the lens epithelium and the differentiated lens fiber cells. In lens epithelial cells beta 1 integrin was found to be localized to all membrane surfaces. Lens fiber cells contained beta 1 integrin all along their lateral borders as well as at the site of their attachment to the lens capsule and at their interface with lens epithelial cells. The distribution of beta 1 integrin in the lens was distinct from that observed in simple epithelia, the retinal pigment epithelium (RPE), kidney, and intestine, where it was limited to a basal lateral localization. We examined the specific beta 1 integrin heterodimers expressed in the lens by Western blot analysis for the integrin alpha subunits following beta 1 immunoprecipitation and compared them with those expressed in RPE cells. In the lens we detected alpha 3 and alpha 6 subunits but not alpha 1, alpha 5, or alpha v. When the lens was separated into epithelial and fiber cells, we found that alpha 3 was expressed at a higher level in the epithelial cells, while alpha 6 was primarily associated with the fiber cells. In the RPE the primary beta 1 integrin detected was alpha 3. Unlike in lens and kidney, alpha 6 beta 1 integrin in RPE cells was expressed only at a low level. alpha v was also expressed in RPE cells but not as a beta 1 heterodimer. As in the lens, neither alpha 5 beta 1 nor alpha 1 beta 1 integrin was detected in RPE. Both lens and RPE cells express a specific subset of beta 1 integrin heterodimers which are likely to be important to the initiation and maintenance of their differentiated phenotype.