Loss of polarity is a defining characteristic of epithelial cancers. The cytoskeletal proteins, HUGL1 and HUGL2, mediate polarity in epithelial cells through diversified roles in defining membrane identity and trafficking to the basolateral membrane. Importantly, an ortholog of these molecules can inhibit tumor growth in Drosophila, although the mechanisms of their tumor suppressive functions in mammary epithelial cells are unknown. Here, we show nonredundant tumor protective roles for HUGL1 and HUGL2 in human mammary epithelial cells. Using a three dimensional culture system, we report that loss of HUGL1 or HUGL2 causes loss of apicobasal polarity, aberrant growth of multilayered epithelium, nuclear enlargement, loss of membrane identity, and cellular overgrowth. Experiments on plastic also revealed that HUGL1 or HUGL2 loss results in induction of a phenotypic EMT in breast epithelial cells and overexpression of HUGL1 in breast cancer cells reduces proliferation.In a Drosophila model of cancer driven by loss of lgl, we have discovered the consistent dysregulation of a number of miRNAs and mRNAs including the loss of let-7 and miR-9a, which are implicated in breast cancer and associated with the suppression of stem cells. Cross comparisons revealed a set of mRNAs that are both dysregulated in vivo and represent putative targets of the miRNAs changed in lgl mutants. Among these, Thrombospondin, a component of the extracellular matrix was found to be misexpressed in both flies and human cells lacking Lgl. Moreover, genetic interaction experiments showed miR-9a to be a functional effector of lgl in controlling proliferation in the wing. Taken together, the findings reported in this dissertation suggest that HUGL1 and HUGL2 function as tumor suppressors through their roles in polarity and miRNA regulation. These two proteins, functioning as modulators of cell plasticity and promoters of differentiation, are potentially able to control the transition between a differentiated epithelial cell and a cancer stem cell. This research offers new insight into the role of HUGL1 and HUGL2 in breast cancer and reveals novel targets downstream of polarity proteins for therapeutic intervention.