Although the AKT family comprises three isoforms, most studies only consider global AKT1–3 activity. However, recent investigations have now elegantly revealed the specific effects mediated by AKT1 and AKT2 isoforms in both the ErbB2-driven and the polyoma middle T (PyMT)-driven mammary adenocarcinoma transgenic mouse models [
26,
27,
37]. In these models, constitutive activation of AKT1 in the mammary epithelium accelerated the onset of tumorigenesis but drastically decreased the number of metastatic lesions [
27]. Conversely, removal of the
akt1 gene strongly delayed the onset of tumorigenesis [
37]. Furthermore, expression of a constitutive active form of AKT2 had no effect on tumor onset but strongly increased the occurrence of lung metastases [
26]. Combined, these results suggest that AKT1 and AKT2 may play opposite roles in the metastatic process and that differential AKT isoform activities require further consideration in cancer studies. The relevance of these findings in mouse models have been recently reported for human breast tumors [
29,
30]. Gene expression datasets obtained from breast cancer cell lines and clinical samples revealed a strong association between high
akt1 expression, low expression of mesenchymal markers and better patient survival. Collectively, these results strongly suggest that AKT1 activity promotes early stages of tumorigenesis but restricts the tumor cell metastatic potential. However, these results have never been extended to non-breast cancer models. Our study suggests that AKT1 specific activity is also involved in the maintenance of the epithelial phenotype of HNSCC cells. An important implication is that AKT1 may also be predictive of the invasive capacities and aggressiveness of HNSCCs. Enhanced AKT/mTOR activity is common in oral carcinomas [
38] and alterations of the PI3K/Akt/mTOR pathway are found in a large majority of HNSCCs [
39]. As the consensus from the literature is that these pathways promote cell survival and metastasis, a great effort has been placed on pharmacological targeting of the PI3K pathway in HNSCC [
34,
40]. The majority of previous in vitro studies on HNSCCs have focused on classical readouts such as association of AKT activity with cell survival and lower sensitivity to radiotherapy and chemotherapy [
41‐
44]. Other research has indicated that increased AKT activity may promote a mesenchymal phenotype [
45]. However, none of the previous in vitro (or in vivo) studies on HNSCCs have considered the influence that specific AKT isoform expression could have on the outcome of AKT inhibition. Here we have observed that in certain subtypes of HNSCCs, which predominantly express AKT1 in comparison to AKT2, AKT1 inhibition leads to a more invasive phenotype. Therefore, it appears that, as has been recently revealed in an extensive body of work for breast cancer [
26,
27,
29,
30], additional cancer types such as HNSCC may require AKT isoform analysis to predict the outcome of pan-AKT inhibitors. Despite encouraging results obtained with mTOR inhibitors [
46‐
48], most of the clinical trials involving agents targeting the PI3K/Akt/mTOR pathway have failed to pass phase II. Consistently, a phase II clinical trial with the pan-AKT inhibitor MK2206 on recurrent and metastatic HNSCC resulted in a partial response and was not moved to phase III so far (ClinicalTrials.gov identifier NCT01349933). A significant consideration is that treatments targeting the PI3K or all AKT isoforms may promote the invasive capacities of cancer cells in some cases. These counterintuitive results may explain why PI3K and AKT inhibitors are not included yet in clinical practices [
49]. The possibility that some targeted therapies may increase the invasive and thus metastatic potential despite a reduction of the tumor burden should therefore be scrutinized.
Alterations in the PI3K/AKT/mTOR pathway occur in 38% of all cancers as demonstrated on nearly 20,000 tumors of diverse origins [
52]. More specifically in HNSCCs, pooled results from several databases showed that the PIK3CA gene, encoding the p110alpha catalytic subunit of the PI3K, is amplified in approximately 70% of HNSCC cell lines and 20% of HNSCCs clinical cases analyzed [
53]. These results pinpoint the importance of the PI3K/AKT pathway dysregulation in cancers. The CAL33 and Detroit562 cell lines that adopted a mesenchymal phenotype upon AKT or mTORC1 inhibition bear a H1047R activating mutation in PIK3CA. Conversely, the CAL27 cell line does not bear this mutation and does not display comparable modifications in the same experimental conditions. Maintenance of the epithelial phenotype of CAL33 and Detroit562 cells could rely on the constitutive activity of the PI3K/AKT1 axis that is absent in CAL27 cells. In this case, the presence of both AKT1 amplification and constitutive PI3K activity would be a prognostic marker for pro-metastatic effects of pan-AKT inhibitors.