in our recent article  sequestosome 1/p62 facilitates her2-induced mammary tumorigenesis through multiple signaling pathways  , we reported that the adaptor protein sequestosome 1 ( sqstm1 ) , also known as p62 , is required for human epidermal growth factor receptor 2 ( her2)-induced cellular transformation and tumorigenesis in allograft and transgenic mouse mammary tumor models .
our findings are similar to the results of an earlier publication on ras - induced lung cancer .
these 2 studies , taken together , implicate p62 in potent oncogene - driven epithelial tumorigenesis and suggest that p62 may be a therapeutic target in such malignancies .
p62 is a multidomain protein that governs a diverse array of signaling pathways , and its deregulation has been implicated in paget 's disease of bone ( pdb ) , neurodegenerative diseases , liver disorders , and cancer .
transcriptional activation of nf - e2-related factor-2 ( nrf2 ) and defective autophagy can both lead to p62 accumulation , whereas p62 in turn regulates nrf2 stability through its interaction with kelch - like ech - associated protein 1 ( keap1 ) . upon binding to polyubiquitinated substrates
, p62 serves as a cargo carrier to proteasomes and the autophagic machinery , where it is degraded along with the cargo .
furthermore , p62 is involved in nuclear factor kappa b ( nf-b ) activation , receptor trafficking , extracellular signal - regulated kinase 1 ( erk1 ) inhibition , and mammalian target of rapamycin ( mtor ) signaling . as a result ,
disruption of p62 homeostasis impairs disposal of toxic cellular waste , response to oxidative stress , and normal cell signaling .
our interest in the role of p62 in her2-induced mammary tumorigenesis was initially triggered by previously published studies that reported increased p62 levels in her2-positive breast tumors .
we found that her2 overexpression results in p62 accumulation at the protein level , possibly due to her2-induced autophagy suppression as described in another recent study from our laboratory .
two- and 3-dimensional culture of her2-expressing sqstm1 immortalized mouse mammary epithelial cells ( immecs ) revealed that p62 is required for cell survival upon amino acid starvation and during lumen formation in acinar morphogenesis .
as the phosphatase and tensin homolog deleted on chromosome 10 ( pten)/phosphoinositide-3-kinase ( pi3k)/akt axis is known to promote cell survival and her2-mediated mammary oncogenesis , we examined the role of p62 in this signaling pathway .
our results demonstrated that her2-induced activation of akt , glycogen synthase kinase 3  ( gsk3 ) and -catenin in mammary cells is dependent on the presence of p62 both in vitro and in vivo .
furthermore , our studies revealed a newly discovered role for p62 in pten regulation . in the presence of p62
, her2 overexpression activates the pi3k axis while reducing pten protein levels in vitro and in vivo .
in contrast , in the absence of p62 , her2-dependent activation of the pi3k pathway is suppressed in association with increased pten expression , suggesting that p62 is required for pten downregulation and resultant induction of pi3k signaling in her2-expressing mouse mammary tumor cells .
whether p62 also activates the akt / gsk3/-catenin axis independently of pten remains to be investigated .
given the established role of p62 in activation of nf-b and the nrf2-driven antioxidant response , we also examined the impact of p62 loss on these pathways and found that in vivo activation of nf-b and nrf2 signaling in response to her2 overexpression also depends on p62 .
a limitation of our study is that we did not investigate every pathway that p62 is known to be involved in , such as the mitogen - activated protein kinase ( mapk ) pathway .
a role for p62 in her2-mediated regulation of the mapk pathway would not be surprising , as p62 has been reported to inhibit erk1 phosphorylation . nevertheless , and in accordance with the complex nature of cancer
, our study shows that p62 contributes to her2-induced mammary tumorigenesis in an elaborate manner that can not be attributed to a single mechanism ( fig .
figure 1.p62 is a hub of multiple signaling pathways in her2-induced mammary tumorigenesis . in response to her2 overexpression in mammary epithelial cells
, p62 is upregulated at the protein level , which in turn negatively regulates pten and activates pi3k signaling , thus leading to tumorigenic signaling cascades .
it is also possible that p62 exerts its effects on the akt / gsk3/-catenin axis independently of pten downregulation .
her2-induced p62 accumulation alters mammary ductal morphogenesis by enhancing side - branching while suppressing lumen formation , and promotes anchorage independence and cellular transformation in vitro , as well as mammary tumorigenesis in vivo .
it is likely that p62 also plays a role in the metabolic reprogramming observed in her2-positive breast cancer cells , but how this affects mammary tumorigenesis remains to be investigated .
gsk3 , glycogen synthase kinase 3  ; her2 , human epidermal growth factor receptor 2 ; nf-b , nuclear factor kappa b ; nrf2 , nf - e2-related factor-2 ; pi3k , phosphoinositide-3-kinase ; pten , phosphatase and tensin homolog deleted on chromosome 10 . 
 
p62 is a hub of multiple signaling pathways in her2-induced mammary tumorigenesis . in response to her2 overexpression in mammary epithelial cells
, p62 is upregulated at the protein level , which in turn negatively regulates pten and activates pi3k signaling , thus leading to tumorigenic signaling cascades .
it is also possible that p62 exerts its effects on the akt / gsk3/-catenin axis independently of pten downregulation .
her2-induced p62 accumulation alters mammary ductal morphogenesis by enhancing side - branching while suppressing lumen formation , and promotes anchorage independence and cellular transformation in vitro , as well as mammary tumorigenesis in vivo .
it is likely that p62 also plays a role in the metabolic reprogramming observed in her2-positive breast cancer cells , but how this affects mammary tumorigenesis remains to be investigated .
gsk3 , glycogen synthase kinase 3  ; her2 , human epidermal growth factor receptor 2 ; nf-b , nuclear factor kappa b ; nrf2 , nf - e2-related factor-2 ; pi3k , phosphoinositide-3-kinase ; pten , phosphatase and tensin homolog deleted on chromosome 10 .
our finding that p62 mediates activation of the pi3k / akt / gsk3/-catenin and nf-b pathways in parallel with a reduction in pten levels in response to her2 overexpression led us to hypothesize that , in addition to potential direct interactions , p62 may also regulate these signaling axes indirectly through its effect on pten protein stability . since p62 participates in proteasome- and autophagy - mediated protein degradation , it is possible that her2-induced p62 accumulation promotes pten degradation by either or both mechanisms .
thorough exploration of the mechanism(s ) involved in p62-mediated pten downregulation in response to her2 activation will certainly be of great interest .
another finding of our study , which is intriguing and worthy of future investigation , is the observation that while p62 loss sensitizes her2-expressing mammary tumor cells to amino acid withdrawal , it also renders these cells more resistant to glucose deprivation .
given that autophagy is activated as a prosurvival function in response to amino acid starvation , we entertained the idea that sqstm1 her2-expressing immecs cells might be autophagy - defective compared to their wild - type counterparts .
however , and in agreement with an earlier report , our results showed that neither autophagic flux nor degradation of long - lived proteins is affected by p62 loss .
it thus appears that her2-expressing mammary tumor cells survive in the absence of amino acids via a p62-mediated , but autophagy - independent , mechanism .
a possible explanation resides in the hypothesis that p62 may be essential for glycolysis upregulation , which is known to occur in breast cancer cells in response to her2 activation ; as a consequence , amino acid starvation may compromise her2-expressing mammary tumor cell survival only when p62 is lost and glycolysis can not be induced to sustain metabolic demands .
if this is the case , her2-expressing mammary tumor cells with intact p62 and metabolism dependent on increased glycolytic rates are expected to be more sensitive to glucose deprivation than their sqstm1-null counterparts , which are probably metabolically adjusted to accommodate defective glycolysis induction under nutritional stress .
since our results are in complete agreement with this prediction , we propose that p62 plays an essential role in the metabolic reprogramming of mammary tumor cells with active her2 signaling . in summary ,
our work identifies p62 as a hub of signaling and metabolic pathways in her2-induced mammary tumorigenesis and sets the stage for future studies focusing on the pathophysiology and the discovery of novel therapeutic targets in her2-positive breast cancer .
 we recently reported that depletion of p62 in the background of human epidermal growth factor receptor 2 ( her2 ) overexpression sensitizes mammary tumor cells to amino acid deprivation , abolishes cellular transformation in vitro , and suppresses mammary tumorigenesis in vivo . 
 extensive investigation on the underlying molecular mechanisms has revealed a multifaceted role for p62 in her2-associated mammary tumorigenesis . 
Commentary
Conclusions, Implications, and Unresolved Issues
Disclosure of Potential Conflicts of Interest