1) Although mutations for b-catenin and APC have already described in these cell lines, you may want to check by yourself if they are still harbouring the same mutations. Depending on the origin of your cell line you may find that they are not mutated at all. 

2) Try to treat your cells with LiCl or Gsk3 inhibitors upon transduction with your 7TGC as positive control. Sometimes (but very rarely with other lentivector and never with 7TGC to my experience) proviral particles are rearranged and loose the Wnt response elements (or other parts). Since you are normalizing on mCherry expression you will only know cherry is there, but probably the 7x is not. Just in case... but as I said this is very rare. Another option is that viral copies per cells are too low to see a good GFP signal. Try to increase the MOI, or cotransduce with 7TGP/7TGC, sort for mCherry and apply Puromyicin selections. 

3) Perform an immunofluorescence for b-catenin in your cell lines in presence or absence of Gsk3 inhibitors. If the mutations are there, gsk3 inhibition should play little or no effect on b-catenin stabilization and subcellular localization. 

4) Should everything of above be fine, take in consideration negative feedback loop. Cells with a constitutive activation of the Wnt pathway may trigger the expression of negative regulators with detrimental effects on other target genes expression (including the reporter). Check for Axin2 expression for instance, and cytoplasmic/nuclear extracts of bcat protein. 

Hope it helps

Good luck

Francesco

Dear Gabriel,

Francesco have added quite interesting points to the issue.

I have been working with HCT116 cells and what I can tell you is that:

1) They have a mutation on beta-catenin, but they also rely highly on Wnt ligands to maintain self-renewal and clonogeneicity.

2) Despite high basal levels of beta-catenin, you still can activate the Wnt pathway via TOP-Flash assays using Wnt3aCM, GSK3i or by silencing repressor components (i.e. siAPC). Perhaps not to the extent as HEK-293T cells, but they are Wnt-responsive.

3) I recommend you to try LRP6 levels by WB since you won't see too much difference on the total Beta-catenin levels.