Ribosomal protein S6 kinase beta-1 (S6K1), also known as p70S6 kinase (p70S6K, p70-S6K), is an enzyme (specifically, a protein kinase) that in humans is encoded by the RPS6KB1 gene. [5] [6] It is a serine/threonine kinase that acts downstream of PIP3 and phosphoinositide-dependent kinase-1 in the PI3 kinase pathway. [7] As the name suggests, its target substrate is the S6 ribosomal protein. [8] Phosphorylation of S6 induces protein synthesis at the ribosome.
The phosphorylation of p70S6K at threonine 389 has been used as a hallmark of activation by mTOR and correlated with autophagy inhibition in various situations. However, several recent studies suggest that the activity of p70S6K plays a more positive role in the increase of autophagy. [9] [10]
This gene encodes a member of the S6K family of serine/threonine kinases, which phosphorylate several residues of the S6 ribosomal protein. The kinase activity of this protein leads to an increase in protein synthesis and cell proliferation. Amplification of the region of DNA encoding this gene and overexpression of this kinase are seen in some breast cancer cell lines. Alternate translational start sites have been described and alternate transcriptional splice variants have been observed but have not been thoroughly characterized.
The p70S6 kinase is a downstream target of mTOR (mammalian target of rapamycin) signaling, specifically mTORC1, an mTOR-containing complex characterized by the inclusion of Raptor rather than Rictor (mTORC2). mTOR can be activated via an AND-gate-like mechanism at the lysosome, integrating signals about growth factors and bioavailability of important molecules. For instance, amino acids such as arginine and leucine can trigger lysosomal recruitment of mTORC1. Once at the lysosome, mTOR can be activated by Rheb, a small, lysosomal-resident GTPase, in its GTP-bound state. Rheb GTPase activity is stimulated (and therefore capacity to activate mTOR diminished) by the upstream TSC complex, which is inhibited by IGF signalling. Thus, the AND gate consists of proper localization by sufficiency of amino acids and activation by growth factors. Once mTOR has been properly localized and activated, it can phosphorylate downstream targets such as p70S6K, 4EBP, and ULK1 which are important for regulating protein anabolic/catabolic balance.
Physical exercise activates protein synthesis via phosphorylation (activation) of p70S6K in a pathway that is dependent on mTOR, specifically mTORC1. This has been demonstrated by using an inhibitor of mTOR, rapamycin, to block an increase in muscle mass, despite increases in load (e.g., exercise). Exercise has been shown to increase levels of IGF-1 in muscle, thus inducing the IGF-1/PI3K/Akt/p70S6K signaling pathway, and thereby increasing the protein synthesis is required to build muscle.
Inhibition of the S6K1 protein, or a lack of it, slows the production of adipose (fat) cells by disrupting and retarding the initial "commitment stage" of their formation. The study could have implications for the treatment of obesity. [11]
Amplification of the region of DNA encoding this gene and overexpression of this kinase are seen in some breast cancer cell lines.
Another pathway for which P70 has proposed involvement is in muscle lengthening and growing. P70 is phosphorylated by passive stretch in the soleus muscle. This may be one of many protein kinases involved in muscle building. [12]
In its inactive state, S6K1 is bound to eIF3 and detaches following phosphorylation by mTOR/Raptor. Free S6K1 is then able to phosphorylate a number of its targets, including eIF4B. [13]
P70-S6 Kinase 1 has been shown to interact with:
The mammalian target of rapamycin (mTOR), also referred to as the mechanistic target of rapamycin, and sometimes called FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a kinase that in humans is encoded by the MTOR gene. mTOR is a member of the phosphatidylinositol 3-kinase-related kinase family of protein kinases.
The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
In molecular biology, ribosomal s6 kinase (rsk) is a family of protein kinases involved in signal transduction. There are two subfamilies of rsk, p90rsk, also known as MAPK-activated protein kinase-1 (MAPKAP-K1), and p70rsk, also known as S6-H1 Kinase or simply S6 Kinase. There are three variants of p90rsk in humans, rsk 1-3. Rsks are serine/threonine kinases and are activated by the MAPK/ERK pathway. There are two known mammalian homologues of S6 Kinase: S6K1 and S6K2.
Mitogen-activated protein kinase 3, also known as p44MAPK and ERK1, is an enzyme that in humans is encoded by the MAPK3 gene.
Tuberous sclerosis complex 2 (TSC2), also known as tuberin, is a protein that in humans is encoded by the TSC2 gene.
Eukaryotic translation initiation factor 4E-binding protein 1 is a protein that in humans is encoded by the EIF4EBP1 gene. inhibits cap-dependent translation by binding to translation initiation factor eIF4E. Phosphorylation of 4E-BP1 results in its release from eIF4E, thereby allows cap-dependent translation to continue thereby increasing the rate of protein synthesis.
RHEB also known as Ras homolog enriched in brain (RHEB) is a GTP-binding protein that is ubiquitously expressed in humans and other mammals. The protein is largely involved in the mTOR pathway and the regulation of the cell cycle.
Ribosomal protein S6 kinase alpha-1 is an enzyme that in humans is encoded by the RPS6KA1 gene.
5'-AMP-activated protein kinase catalytic subunit alpha-1 is an enzyme that in humans is encoded by the PRKAA1 gene.
Ribosomal protein S6 kinase alpha-2 is an enzyme that in humans is encoded by the RPS6KA2 gene.
Ribosomal protein S6 kinase beta-2 is an enzyme that in humans is encoded by the RPS6KB2 gene.
Regulatory-associated protein of mTOR also known as raptor or KIAA1303 is an adapter protein that is encoded in humans by the RPTOR gene. Two mRNAs from the gene have been identified that encode proteins of 1335 and 1177 amino acids long.
Eukaryotic translation initiation factor 3 subunit B (eIF3b) is a protein that in humans is encoded by the EIF3B gene.
Serine/threonine-protein kinase Nek6 is an enzyme that in humans is encoded by the NEK6 gene.
Rapamycin-insensitive companion of mammalian target of rapamycin (RICTOR) is a protein that in humans is encoded by the RICTOR gene.
Tuberous sclerosis proteins 1 and 2, also known as TSC1 (hamartin) and TSC2 (tuberin), form a protein-complex. The encoding two genes are TSC1 and TSC2. The complex is known as a tumor suppressor. Mutations in these genes can cause tuberous sclerosis complex. Depending on the grade of the disease, intellectual disability, epilepsy and tumors of the skin, retina, heart, kidney and the central nervous system can be symptoms.
Target of rapamycin complex subunit LST8, also known as mammalian lethal with SEC13 protein 8 (mLST8) or TORC subunit LST8 or G protein beta subunit-like, is a protein that in humans is encoded by the MLST8 gene. It is a subunit of both mTORC1 and mTORC2, complexes that regulate cell growth and survival in response to nutrient, energy, redox, and hormonal signals. It is upregulated in several human colon and prostate cancer cell lines and tissues. Knockdown of mLST8 prevented mTORC formation and inhibited tumor growth and invasiveness.
mTOR inhibitors are a class of drugs used to treat several human diseases, including cancer, autoimmune diseases, and neurodegeneration. They function by inhibiting the mammalian target of rapamycin (mTOR), which is a serine/threonine-specific protein kinase that belongs to the family of phosphatidylinositol-3 kinase (PI3K) related kinases (PIKKs). mTOR regulates cellular metabolism, growth, and proliferation by forming and signaling through two protein complexes, mTORC1 and mTORC2. The most established mTOR inhibitors are so-called rapalogs, which have shown tumor responses in clinical trials against various tumor types.
mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis.
mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, consisting of seven protein subunits. The catalytic mTOR subunit, DEP domain containing mTOR-interacting protein (DEPTOR), mammalian lethal with sec-13 protein 8, and TTI1/TEL2 complex are shared by both mTORC2 and mTORC1. Rapamycin-insensitive companion of mTOR (RICTOR), mammalian stress-activated protein kinase interacting protein 1 (mSIN1), and protein observed with rictor 1 and 2 (Protor1/2) can only be found in mTORC2. Rictor has been shown to be the scaffold protein for substrate binding to mTORC2.