Breast Tumor Cells w/PI3K Mutation or HER2 + Selectively Addicted to Akt signaling

[Rich66]

http://www.pubmedcentral.nih.gov/art...?artid=2516933

DiscussionThe PI3K/Akt signaling pathway is deregulated in the majority of human cancers and almost certainly plays an important pathogenic role in carcinogenesis and progression. Data from model systems suggest that inhibition of the pathway would both arrest growth and induce apoptosis or sensitize the cell to proapoptotic stimuli [3]. Thus, much effort has focused on developing novel anti-tumor agents that target this pathway. Up to now, however, it has been unclear whether the degree of pathway inhibition required for significant anti-tumor activity could be achieved without unacceptable toxicity. The PI3K/Akt pathway is an important regulator of many key physiologic processes and the development of several classes of Akt inhibitors has been halted because of toxicity [19], [20].
We now show that a selective and potent allosteric inhibitor of Akt1 and Akt2 effectively inhibits Akt signaling in tumor bearing mice without gross toxicity or weight loss even when administered chronically (Figures 4

and ​and55

and S2). The drug potently inhibits Akt phosphorylation in breast cancers and inhibits the proliferation of breast tumors with PI3K mutation and/or HER2 amplification but is ineffective in tumors in which PI3K and PTEN are wild-type and HER2 is expressed at normal levels. Considering that the drug treatment is well tolerated and that tumors in which the PI3K/Akt pathway is not mutationally activated are insensitive to the drug, the hypersensitivity of tumors with PI3K mutation or HER2 amplification suggests that these tumors are ‘addicted’ to PI3K/Akt signaling.
Oncogene addiction is a term given for the observation that inhibition of oncoprotein function in transformed cells often has much more profound effects than inhibition of the corresponding wild-type protein in the untransformed parental cell [35]. This phenomenon has been noted in many systems and may be a general property of oncoprotein-transformed cells, but the underlying mechanism is not clear [36]. One possibility is that survival of oncoprotein-transformed cells requires second mutations that prevent senescence or apoptosis. In the context of these other mutations, loss of function of the activated oncoprotein may be selectively toxic [36]. A second possibility is that the complex signaling network in normal cells is robust and relatively insensitive to loss of any single component of the network. Constitutive activation of the oncoprotein may feedback inhibit redundant pathways in the network and cause the cell to become hyperdependent on the single oncoprotein. This idea is consistent with the observation in Figures 2A

and ​and3A,3A

, which show that Akt inhibition is sufficient to inhibit key downstream signaling targets in tumors with PI3K mutation or HER2 amplification, but not in those tumors with wild-type PI3K and normal HER2 expression.
Akt (especially Akt2) is a key downstream mediator of the metabolic effects of insulin and hyperglycemia is a known side-effect of both PI3K and Akt inhibition [29], [37]. The Akt1/Akt2 inhibitor caused transient and moderate hyperglycemia in mice that is associated with a transient hyperinsulinemia (manuscript in preparation). Why the drug is so effective at concentrations that cause only transient hyperglycemia, but no gross toxicity, is not entirely clear. It is possible that this is a manifestation of oncogene addiction; the levels of pathway inhibition achieved are sufficient to inhibit tumor growth, but without significant deleterious impact on normal physiology. The inhibitor is equipotent against human and mouse Akt in vitro and in vivo, ruling out selective inhibition of the Akt in the xenograft tumor (unpublished data). The greater selectivity of an allosteric inhibitor as compared to ATP-competitive inhibitors may also play a role: off-target effects are less likely. It is also possible that the relative sparing of Akt3 activity is responsible, although genetic models suggest that Akt2 is most responsible for maintaining glucose homeostasis [29]. The finding that the Akt1/Akt2 inhibitor can be given chronically to animals for months without weight loss suggests that the drug does not cause severe metabolic toxicity under these conditions, at least in mice. Whereas the physiological function of Akt3 is unclear at present, the avoidance of Akt3 inhibition may allow a greater therapeutic window for Akt1 and Akt2 inhibition.
Whereas breast tumors with PI3K or HER2 deregulation may be addicted to Akt signaling, the situation is less clear for tumors with PTEN loss. ZR-75-1 (PTEN-negative, low EGFR) is extremely sensitive to AKTi-1/2, but MDA-MB-468 (PTEN-negative, high EGFR) is not. This could be due to a variety of factors, including residual Akt3 activity in the latter cell line (Figure 1

). However, this is unlikely, since we have previously shown that MDA-MB-468 cells are not effectively inhibited when AKT activity is completely inhibited in response to induction of the expression of wild type PTEN [28]. Triple negative breast tumors often have evidence for decreased PTEN function together with elevated EGFR expression [15]. In MDA-MB-468 cells, inhibition of either EGFR/MEK/ERK or PI3K/Akt signaling alone has only modest effects on growth or survival [28]. These and other data suggest that, in the context of EGFR activation, the proliferation and survival of PTEN-negative cells are not dependent on PI3K/Akt signaling alone. However, inhibition of both pathways causes synergistic apoptosis and tumor regression in vivo [28]. These data suggest that combined inhibition of Akt and EGFR signaling may be a useful strategy for the treatment of the triple-negative basal subset of breast cancers.
The results suggest that G1 progression and the survival of breast tumors with PI3K mutation and/or HER2 amplification are dependent on Akt activation. Moreover, HER2-dependent tumor models selected for resistance to the anti-HER2 antibody Herceptin retain Akt-dependence and their sensitivity to the drug. These findings and the absence of gross toxicity in vivo suggest that Akt inhibition is a feasible therapeutic strategy for breast cancers in which PI3K/Akt signaling is dysregulated by mutation. Up to now, effective inhibition of Akt signaling in tumors could only be achieved with inhibitors of growth factor receptors (such as HER2) known to drive the pathway in particular tumors. No agent is currently available to directly and specifically inhibit Akt signaling in breast and other cancers with PI3K mutation or PTEN loss. Allosteric inhibition of Akt may be useful as a single modality in the treatment of these tumors, including those that have become resistant to HER2 inhibitors. Furthermore, it would be expected to sensitize tumors to induction of apoptosis by cytotoxic agents and, perhaps, in combination with EGFR inhibitors in triple-negative breast cancer and with drugs such as rapamycin that cause the feedback activation of PI3K/Akt signaling [38].