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BackgroundLung adenocarcinoma is the most common subtype of Non small cell lung cancer in which the PI3K/Akt cascade is frequently deregulated. The ubiquitous expression of the PI3K and the frequent inactivation of PTEN accounts for the prolonged survival, evasion of apoptosis and metastasis in cancer. This has led to the development of PI3K inhibitors in the treatment of cancer. Synthetic PI3K inhibitors undergoing clinical and preclinical studies are toxic in animals. Hence, there is a critical need to identify PI3K inhibitor(s) of natural origin.The current study aims to explore the efficacy of the red algae Gelidiella acerosaon inhibition of cell proliferation, migration and the expression of cell survival genes in lung adenocarcinoma cell line A549.MethodsThe phytoconstituents of Gelidiella acerosa were extracted sequentially with solvents of different polarity, screened qualitatively and quantitatively for secondary metabolites and characterized by GC-MS. The in-vitro studies were performed to check the efficacy of the extract on cell proliferation (MTT assay), cell invasion (scratch assay and colony formation assay), apoptosis (fluorescent, confocal microscopy and flow cytometry) and expression of apoptosis and cell survival proteins including PI3K, Akt and GSK3β and matrix metalloproteinase MMP2 and MMP9 by Western blot method. The antitumor activity of GAE was analyzed in a tumor model of Zebrafish.ResultsThe outcomes of the in vitro analysis showed an inhibition of cell proliferation, induction of apoptosis, inhibition of cell migration and colonization by the crude extract. The analysis of protein expression showed the activation of caspases 3 and Pro apoptotic protein Bax accompanied by decreased expression of Bcl-2 and Bcl-XL. On the other hand, subsequent activation of GSK3β and down regulation of PI3K, Akt were observed. The decreased expression of MMP2 correlated with the antimetastatic activity of the extract. The in vivo studies showed an inhibition of tumor growth by GAE in Zebrafish.ConclusionThe phytoconstituents of algal extract contributed to the anticancer properties as evidenced by in vitro and in vivo studies. These phytoconstituents can be considered as a natural source of PI3K/Akt inhibitor for treatment of cancers involving the PI3K cascade.Electronic supplementary materialThe online version of this article (10.1186/s12906-018-2165-1) contains supplementary material, which is available to authorized users.