Human mammary epithelial cell transformation through the activation of phosphatidylinositol 3-kinase

Abstract

Recent studies have demonstrated that introduction of hTERT in combination with SV40 large T antigen (LT), small t antigen (st), and H-rasV12 suffices to transform many primary human cells. In human mammary epithelial cells (HMECs) expressing elevated c-Myc, activated H-Ras is dispensable for anchorage-independent growth. Using this system, we show that st activates the PI3K pathway and that constitutive PI3K signaling substitutes for st in transformation. Moreover, using constitutively active versions of Akt1 and Rac1, we show that these downstream pathways of PI3K synergize to achieve anchorage-independent growth. At lower levels of c-myc expression, activated PI3K also replaces st to complement H-rasV12 and LT and confers both soft agar growth and tumorigenicity. However, elevated c-myc expression cannot replace H-rasV12 for tumorigenesis. These observations begin to define the pathways perturbed during the transformation of HMECs.

Immortalized cells as experimental models to study cancer

“…Together, these observations provide genetic evidence for the involvement of the PI3K and c-Myc signaling pathways in transformation by ST (Zhao et al 2003;Yeh et al 2004). However, it is currently unclear which PP2A target(s) contribute to the observed phenotypes and if human tumors contain mutations in these molecules.…”

“…Due to the structural and functional complexity of PP2A, the critical cellular target(s) of PP2A perturbed by ST expression remain undefined. However, recent studies provide genetic evidence for the involvement of the phosphatidylinositol 3-kinase (PI3K) and c-Myc signaling pathways in ST-induced transformation (Zhao et al 2003;Yeh et al 2004). Late passage HMECs that express hTERT and LT do not require RAS expression for anchorageindependent growth, and ST expression alone transforms these cells (Zhao et al 2003).…”

“…However, recent studies provide genetic evidence for the involvement of the phosphatidylinositol 3-kinase (PI3K) and c-Myc signaling pathways in ST-induced transformation (Zhao et al 2003;Yeh et al 2004). Late passage HMECs that express hTERT and LT do not require RAS expression for anchorageindependent growth, and ST expression alone transforms these cells (Zhao et al 2003). Since RAS expression activates multiple signaling pathways (reviewed by Malumbres and Barbacid 2003) and is typically required for transformation of HMECs (Elenbaas et al 2001), late passage HMECs provide an opportunity to identify STspecific targets without the confounding effects of RAS expression.…”

“…Since RAS expression activates multiple signaling pathways (reviewed by Malumbres and Barbacid 2003) and is typically required for transformation of HMECs (Elenbaas et al 2001), late passage HMECs provide an opportunity to identify STspecific targets without the confounding effects of RAS expression. In these cells, expression of a constitutively-active PI3K allele (myr-p110a) functionally replaces ST in transformation (Zhao et al 2003) (Figure 1). Furthermore, the combined effects of constitutively activated Akt1 and Rac1 substitute for myr-p110a.…”

“…Recent reports implicate the c-Myc oncoprotein as one downstream target of RAS that may partially mediate RAS-induced tumorigenesis (Sears et al 2000;Wei et al 2003;Zhao et al 2003). RAS expression enhances c-Myc stability in quiescent REF52 rodent cells and the activities of the Raf and PI3K pathways mediate this effect (Sears et al 2000).…”