ADHESIVE PROPERTIES AND INTEGRIN EXPRESSION PROFILES OF 2 COLONIC-CANCER POPULATIONS DIFFERING BY THEIR SPREADING ON LAMININ

The mostly undifferentiated parental HT29 (HT29p) human colonic adenoc arcinoma cell line and a differentiated subpopulation selected by the anti-cancer drug 5-fluorouracil (HT29-Fu) (Lesuffleur et al. (1991) In t. J. Cancer 49, 721-730) display strikingly different behavior when g rown on laminin coatings: the former grows as aggregates while the lat ter grows as monolayers. In an attempt to explain this difference, we performed a comparative study of cell adhesion properties and of expre ssion, involvement and localization of the alpha6, beta1 and beta4 sub units constituting the integrin family among the two cell populations. HT29p and HT29-Fu cells exhibited a similar adhesion pattern to lamin in and laminin fragments E8 and P1. In both cell lines, cell adhesion could be blocked at about 90% with anti-alpha6 subunit antibodies and around 30-50% with anti-beta1 antibodies; no inhibition of the cell ad hesion was obvious when using anti-beta4 antibodies. Immunoprecipitati ons of iodinated membrane-solubilized proteins and immunoblotting expe riments showed that all alpha6 chains expressed in both HT29p and HT29 -Fu cell populations exist as alpha6beta4 integrins; beta1 subunits ar e associated with alpha2 and alpha3 chains. When HT29p or HT29-Fu cell s were injected subcutaneously in nude mice, a similar expression patt ern of alpha6, beta4 and beta1 integrin subunits was noticeable in the resulting tumors: alpha6 and beta4 subunits were localized at the bas al surface of the tumor cells facing the stromal elements, and to a le sser extent at the cell-cell contacts within the tumor-cell clumps; be ta1 subunits were mainly found within the cytoplasm of the tumor cells . Despite these overall similarities among the two cell lines, the fol lowing changes could account for their different behavior on laminin: less proteolytic processing of the beta4 integrin subunit occurred in HT29-Fu cells yielding peptidic fragments of 175 kDa, which are absent from the parental celts; the immunostaining pattern of the various su bunits demonstrated a segregation of alpha6, beta4 and beta1 integrin subunits on the basal side of the HT29-Fu cells when cultured on lamin in to the detriment of their lateral location, a phenomenon that was n ot obvious in the parental cells. Altogether, these results suggest th at the distinct behavior of the undifferentiated versus differentiated HT29 cell populations on laminin is not related to altered adhesion p roperties of the cells but rather to a deficient stabilization of the adhesion leading to cell spreading. This difference is highlighted by a variable basal segregation potential of the laminin-binding integrin s, which could be due to an altered form of the beta4 subunits.