CEACAM6 acts as an inducer of cellular proliferation in confluent A549.

A) The expression of the proliferation marker Ki67 is limited to A549-T cells that also expressed CEACAM6 on their cell surface. Confluent CEACAM6-negative and CEACAM6-positive A549-T cells were separated by mAb 9A6 loaded magnetic protein G microbeads and μMAS magnetic sorting columns (Miltenyi Biotec) as described in Materials and Methods. Immunoblot analysis of confluent CEACAM6-negative and CEACAM6-positive A549-T cell lysates was performed applying a Ki67 specific antibody followed by HRP-coupled secondary antibody and ECL detection. Beta-actin served as a loading control. The data shown are representative of three separate experiments. B) Cell cycle analysis of CEACAM6 negative and CEACAM6 positive A549-T cells. CEACAM6-negative and CEACAM6-positive A549 cells were fixed in 80% ethanol and stained with propidium iodide, and analyzed by flow cytometry as described in “Materials and Methods.” The DNA content in the different cell fractions is given in arbitrary units on the X-axis. Cells in the G2-M phase (second peak) contained twice as much DNA as cells in the G0-G1 phase (first peak). Cells between the peaks represent cells in the S-phase. The relative proportions of cells in the various phases are shown above the DNA profiles. Filled curve, CEACAM6 negative A549 cells; thick curve, CEACAM6 positive A549 cells. C) Confluent control sh-plasmid transfected A549-T (A549-shControl) and shCEACAM6 transfected A549-T cells (A549-shCC6) were stained for CEACAM1 with mAb clone 283340 and for CEACAM6 with mAb 9A6 (thick lines). The background fluorescence was determined by incubating the cells with control IgG antibody (thin lines). Samples were analyzed by flow cytometry. Compared to A549-shControl cells, A549-shCC6 cells completely lacked CEACAM6 expression, but continued to express CEACAM1. D) Phase contrast images of (a) control sh-plasmid transfected A549-T and (b) shCEACAM6 transfected A549-T cells. Confluent control sh-plasmid transfected A549-T piled up and formed unanchored spheroidal cell aggregates on top of the monolayer revealing insufficient contact inhibition. (b) In contrast, A549-shCC6 cells formed well spread monolayers without detection of unanchored, spheroidal cell growth indicative tight contact inhibition. Bar, 50 µm.