Palbociclib-Induced Cellular Senescence Is Modulated by the mTOR Complex 1 and Autophagy.

Despite not dividing, senescent cells acquire the ability to synthesize and secrete a plethora of bioactive molecules, a feature known as the senescence-associated secretory phenotype (SASP). In addition, senescent cells often upregulate autophagy, a catalytic process that improves cell viability in stress-challenged cells. Notably, this "senescence-related autophagy" can provide free amino acids for the activation of mTORC1 and the synthesis of SASP components. However, little is known about the functional status of mTORC1 in models of senescence induced by CDK4/6 inhibitors (e.g., Palbociclib), or the effects that the inhibition of mTORC1 or the combined inhibition of mTORC1 and autophagy have on senescence and the SASP. Herein, we examined the effects of mTORC1 inhibition, with or without concomitant autophagy inhibition, on Palbociclib-driven senescent AGS and MCF-7 cells. We also assessed the pro-tumorigenic effects of conditioned media from Palbociclib-driven senescent cells with the inhibition of mTORC1, or with the combined inhibition of mTORC1 and autophagy. We found that Palbociclib-driven senescent cells display a partially reduced activity of mTORC1 accompanied by increased levels of autophagy. Interestingly, further mTORC1 inhibition exacerbated the senescent phenotype, a phenomenon that was reversed upon autophagy inhibition. Finally, the SASP varied upon inhibiting mTORC1, or upon the combined inhibition of mTORC1 and autophagy, generating diverse responses in cell proliferation, invasion, and migration of non-senescent tumorigenic cells. Overall, variations in the SASP of Palbociclib-driven senescent cells with the concomitant inhibition of mTORC1 seem to depend on autophagy.

Platelet Activation Is Triggered by Factors Secreted by Senescent Endothelial HMEC-1 Cells In Vitro.

Aging is one of the main risk factors for the development of chronic diseases, with both the vascular endothelium and platelets becoming functionally altered. Cellular senescence is a form of permanent cell cycle arrest initially described in primary cells propagated in vitro, although it can also be induced by anticancer drugs and other stressful stimuli. Attesting for the complexity of the senescent phenotype, senescent cells synthesize and secrete a wide variety of bioactive molecules. This "senescence-associated secretory phenotype" (SASP) endows senescent cells with the ability to modify the tissue microenvironment in ways that may be relevant to the development of various physiological and pathological processes. So far, however, the direct role of factors secreted by senescent endothelial cells on platelet function remains unknown. In the present work, we explore the effects of SASP factors derived from senescent endothelial cells on platelet function. To this end, we took advantage of a model in which immortalized endothelial cells (HMEC-1) were induced to senesce following exposure to doxorubicin, a chemotherapeutic drug widely used in the clinic. Our results indicate that (1) low concentrations of doxorubicin induce senescence in HMEC-1 cells; (2) senescent HMEC-1 cells upregulate the expression of selected components of the SASP and (3) the media conditioned by senescent endothelial cells are capable of inducing platelet activation and aggregation. These results suggest that factors secreted by senescent endothelial cells in vivo could have a relevant role in the platelet activation observed in the elderly or in patients undergoing therapeutic stress.

SASP-Dependent Interactions between Senescent Cells and Platelets Modulate Migration and Invasion of Cancer Cells.

Alterations in platelet aggregation are common in aging individuals and in the context of age-related pathologies such as cancer. So far, however, the effects of senescent cells on platelets have not been explored. In addition to serving as a barrier to tumor progression, cellular senescence can contribute to remodeling tissue microenvironments through the capacity of senescent cells to synthesize and secrete a plethora of bioactive factors, a feature referred to as the senescence-associated secretory phenotype (SASP). As senescent cells accumulate in aging tissues, sites of tissue injury, or in response to drugs, SASP factors may contribute to increase platelet activity and, through this mechanism, generate a microenvironment that facilitates cancer progression. Using in vitro models of drug-induced senescence, in which cellular senescence was induced following exposure of mammary epithelial cells (MCF-10A and MCF-7) and gastric cancer cells (AGS) to the CDK4/6 inhibitor Palbociclib, we show that senescent mammary and gastric cells display unique expression profiles of selected SASP factors, most of them being downregulated at the RNA level in senescent AGS cells. In addition, we observed cell-type specific differences in the levels of secreted factors, including IL-1ß, in media conditioned by senescent cells. Interestingly, only media conditioned by senescent MCF-10A and MCF-7 cells were able to enhance platelet aggregation, although all three types of senescent cells were able to attract platelets in vitro. Nevertheless, the effects of factors secreted by senescent cells and platelets on the migration and invasion of non-senescent cells are complex. Overall, platelets have prominent effects on migration, while factors secreted by senescent cells tend to promote invasion. These differential responses likely reflect differences in the specific arrays of secreted senescence-associated factors, specific factors released by platelets upon activation, and the susceptibility of target cells to respond to these agents.

The Potential Role of Senescence As a Modulator of Platelets and Tumorigenesis.

In addition to thrombus formation, alterations in platelet function are frequently observed in cancer patients. Importantly, both thrombus and tumor formation are influenced by age, although the mechanisms through which physiological aging modulates these processes remain poorly understood. In this context, the potential effects of senescent cells on platelet function represent pathophysiological mechanisms that deserve further exploration. Cellular senescence has traditionally been viewed as a barrier to tumorigenesis. However, far from being passive bystanders, senescent cells are metabolically active and able to secrete a variety of soluble and insoluble factors. This feature, known as the senescence-associated secretory phenotype (SASP), may provide senescent cells with the capacity to modify the tissue environment and, paradoxically, promote proliferation and neoplastic transformation of neighboring cells. In fact, the SASP-dependent ability of senescent cells to enhance tumorigenesis has been confirmed in cellular systems involving epithelial cells and fibroblasts, leaving open the question as to whether similar interactions can be extended to other cellular contexts. In this review, we discuss the diverse functions of platelets in tumorigenesis and suggest the possibility that senescent cells might also influence tumorigenesis through their ability to modulate the functional status of platelets through the SASP.

Palbociclib-induced autophagy and senescence in gastric cancer cells.

Targeting cyclin D-CDK4/6 kinase complexes has recently been shown to increase the survival of breast cancer patients with estrogen receptor positive breast tumors. Based on these outcomes, CDK4/6 inhibitors are currently being tested, alone o in combination with other drugs, in the treatment of other malignancies characterized by hyper-activation of cyclin D-CDK4/6 complexes. Nonetheless, a better understanding of the cellular processes that are implemented in response to CDK4/6 inhibition is necessary to expand the therapeutic window and confront the development of drug resistance. Herein, we show that, similar to mammary cells, gastric cancer cells are sensitive to the CDK4/6 inhibitor Palbociclib. Inhibition of CDK4/6 in gastric cancer cells leads to the implementation of cellular senescence. However, whether or not this response is accompanied by induction of autophagy seems to depend on both the pRB and p53 status. In cells retaining expression of both tumor suppressive proteins (AGS gastric cancer cells), exposure to Palbociclib induces senescence and autophagy. However, the simultaneous blockade of CDK4/6 and autophagy in these cells exacerbates the senescence phenotype, an indication that autophagy in these experimental settings represents an adaptive mechanism that promotes cell survival rather than being an effector mechanism of senescence. Interestingly, knocking down p53 resulted in senescence reduction and autophagy blockade, the latter apparently involving a disruption of the degradation of autophagosome cargo.

Cyclin D1 activity regulates autophagy and senescence in the mammary epithelium.

Overexpression of cyclin D1 is believed to endow mammary epithelial cells (MEC) with a proliferative advantage by virtue of its contribution to pRB inactivation. Accordingly, abrogation of the kinase-dependent function of cyclin D1 is sufficient to render mice resistant to breast cancer initiated by ErbB2. Here, we report that mouse cyclin D1(KE/KE) MECs (deficient in cyclin D1 activity) upregulate an autophagy-like process but fail to implement ErbB2-induced senescence in vivo. In addition, immortalized cyclin D1(KE/KE) MECs retain high rates of autophagy and reduced ErbB2-mediated transformation in vitro. However, highlighting its dual role during tumorigenesis, downregulation of autophagy led to an increase in senescence in cyclin D1(KE/KE) MECs. Autophagy upregulation was also confirmed in human mammary epithelial cells (HMEC) subjected to genetic and pharmacologic inhibition of cyclin D1 activity and, similar to our murine system, simultaneous inhibition of Cdk4/6 and autophagy in HMECs enhanced the senescence response. Collectively, our findings suggest a previously unrecognized function of cyclin D1 in suppressing autophagy in the mammary epithelium.

Cyclin D1 kinase activity is required for the self-renewal of mammary stem and progenitor cells that are targets of MMTV-ErbB2 tumorigenesis.

Transplantation studies have demonstrated the existence of mammary progenitor cells with the ability to self-renew and regenerate a functional mammary gland. Although these progenitors are the likely targets for oncogenic transformation, correlating progenitor populations with certain oncogenic stimuli has been difficult. Cyclin D1 is required for lobuloalveolar development during pregnancy and lactation as well as MMTV-ErbB2- but not MMTV-Wnt1-mediated tumorigenesis. Using a kinase-deficient cyclin D1 mouse, we identified two functional mammary progenitor cell populations, one of which is the target of MMTV-ErbB2. Moreover, cyclin D1 activity is required for the self-renewal and differentiation of mammary progenitors because its abrogation leads to a failure to maintain the mammary epithelial regenerative potential and also results in defects in luminal lineage differentiation.

Impaired bone development and increased mesenchymal progenitor cells in calvaria of RB1-/- mice.

We have previously shown that the retinoblastoma protein (pRb) can activate expression of Runx2-dependent, bone-specific genes in cultured cells. We now show that pRb also plays a role early in osteogenesis, and that in primary RB1(-/-) calvarial cells there is an increased osteoprogenitor pool. To understand pRb's function in vivo, we generated a conditional RB1-KO mouse in which pRb expression is efficiently extinguished in osteoblasts. These animals display an apparent developmental defect in bones, most strikingly in the calvaria. Cultured RB1(-/-) calvarial osteoblasts fail to cease proliferation upon reaching confluence or following differentiation. Re-plating assays of primary RB1(-/-) calvarial cells after differentiation showed a clear adipogenic ability with increased multipotency. RB1(-/-) osteoblasts display a severe reduction in levels of mRNAs expressed late in differentiation. In this study, we present strong evidence that pRb has multiple regulatory roles in osteogenesis. Furthermore, in the absence of RB1(-/-) there is a larger pool of multipotent cells compared with the WT counterpart. This increased pool of osteoprogenitor cells may be susceptible to additional transforming events leading to osteosarcoma, and is therefore key to understanding RB1 as a target in malignancy.

The LxCxE pRb interaction domain of cyclin D1 is dispensable for murine development.

Cyclin D1 is a multifunctional, tumor-associated protein that interacts with pRb via a conserved LxCxE motif, activates a kinase partner, directs the phosphorylation of pRb, activates cyclin E-cyclin-dependent kinase 2 (cdk2) by titrating Cip/Kip cdk inhibitors, and modulates the activity of a variety of transcription factors. It is thought that some of the proproliferative function of cyclin D1 is exerted by LxCxE-dependent binding to the pRb pocket domain, which might interfere with the ability of pRb to repress transcription by recruiting cellular chromatin remodeling proteins to E2F-dependent promoters. To test the importance of the LxCxE domain in vivo, we have generated a "knock-in" mouse by replacing the wild-type cyclin D1 gene with a mutant allele precisely lacking the nucleotides encoding the LxCxE domain. Analysis of this mouse has shown that the LxCxE protein is biochemically similar to wild-type cyclin D1 in all tested respects. Moreover, we were unable to detect abnormalities in growth, retinal development, mammary gland development, or tumorigenesis, all of which are affected by deleting cyclin D1. Although we cannot exclude the presence of subtle defects, these results suggest that the LxCxE domain of cyclin D1 is not necessary for function despite the absolute conservation of this motif in the D-type cyclins from plants and vertebrates.