Premed · Premed · Cell Biology
Lecture 9: Cell Signaling III: Intracellular Receptors and Integration
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe signaling by intracellular (nuclear) receptors and steroid hormones
- Explain the Notch, Wnt, and Hedgehog signaling pathways
- Describe the NF-kB and TGF-beta/Smad signaling pathways
- Explain how cells integrate multiple signals to determine cell fate
- Discuss the concepts of signal amplification, specificity, and feedback
Lecture Content
I. Intracellular (Nuclear) Receptors
Unlike the cell-surface receptors discussed in previous lectures, intracellular receptors bind ligands that are small, hydrophobic molecules capable of crossing the plasma membrane on their own. These receptors function directly as transcription factors that are activated by ligand binding. The nuclear receptor superfamily comprises approximately 48 members in humans.
All nuclear receptors share a common structural organization. The N-terminal activation domain (AF-1) provides ligand-independent transcriptional activation. The DNA-binding domain (DBD) contains two zinc finger motifs that recognize and bind specific hormone response elements (HREs) in the DNA. The ligand-binding domain (LBD) binds the hormone and contains the AF-2 activation function. A dimerization domain mediates receptor pairing.
Nuclear receptors are divided into two major types based on their mechanism of action. Type I (cytoplasmic) receptors, including the glucocorticoid receptor (GR), estrogen receptor (ER), androgen receptor (AR), and progesterone receptor (PR), reside in the cytoplasm bound to chaperone proteins such as Hsp90. Upon ligand binding, they are released from the chaperone, dimerize, translocate to the nucleus, and bind their target DNA sequences. Type II (nuclear) receptors, such as the thyroid hormone receptor (TR), retinoic acid receptor (RAR), vitamin D receptor (VDR), and PPARs, are already present in the nucleus, bound to DNA as heterodimers with RXR. In the absence of ligand, they associate with corepressors that recruit HDAC complexes to repress gene expression. When ligand binds, the corepressors are exchanged for coactivators with histone acetyltransferase (HAT) activity, switching the gene from a repressed to an active state.
The ligands for nuclear receptors include steroid hormones (cortisol, estradiol, testosterone, progesterone, aldosterone), thyroid hormones (T3, T4), retinoids (retinoic acid), vitamin D (calcitriol), and fatty acids and lipid metabolites (for PPARs). Because these receptors regulate gene transcription, their responses are slower than those of cell-surface receptors (hours rather than seconds to minutes) but tend to be longer-lasting.
Clinically, nuclear receptors are important therapeutic targets. Tamoxifen is a selective estrogen receptor modulator (SERM) used in breast cancer treatment. Dexamethasone is a synthetic glucocorticoid with powerful anti-inflammatory properties. Anti-androgens such as enzalutamide are used in prostate cancer treatment.
<image>Mechanism of nuclear receptor signaling. Panel A (Type I - cytoplasmic): Steroid hormone (cortisol) crosses the plasma membrane, binds glucocorticoid receptor in the cytoplasm, causing Hsp90 release. Receptor dimerizes, translocates to the nucleus, binds glucocorticoid response element (GRE) on DNA, recruits coactivators, and activates target gene transcription. Panel B (Type II - nuclear): Thyroid hormone receptor (TR) is already bound to DNA as a heterodimer with RXR. Without T3: corepressor complex with HDAC represses transcription. With T3 binding: corepressor dissociates, coactivator complex with HAT is recruited, transcription is activated. Both panels show the resulting mRNA and protein product.</image>
II. The Wnt / Beta-Catenin Pathway
The Wnt/beta-catenin pathway is critical for embryonic development, stem cell maintenance, and tissue homeostasis. Its regulation centers on the stability of beta-catenin, a protein that serves dual roles in cell adhesion and transcription.
When Wnt signaling is OFF, beta-catenin is continually targeted for destruction by the destruction complex, which consists of APC (adenomatous polyposis coli), Axin, GSK3-beta, and CK1. GSK3-beta and CK1 phosphorylate beta-catenin, marking it for ubiquitination by the E3 ligase beta-TrCP and subsequent proteasomal degradation. In the nucleus, TCF/LEF transcription factors remain bound to the corepressor Groucho, and Wnt target genes are silent.
When Wnt signaling is ON, the Wnt ligand binds to the Frizzled receptor (a 7TM, GPCR-like protein) and the LRP5/6 co-receptor. This recruits and activates Dishevelled (Dvl), which in turn inhibits the destruction complex by recruiting Axin to LRP5/6 at the membrane. With the destruction complex disabled, beta-catenin is no longer phosphorylated and accumulates in the cytoplasm. It then translocates to the nucleus, where it displaces Groucho from TCF/LEF and recruits coactivators, activating target genes including c-Myc, cyclin D1, and Axin2 (the last providing negative feedback).
APC mutations are among the most important genetic alterations in colorectal cancer. Loss of APC disables the destruction complex, leading to constitutive beta-catenin signaling. Familial adenomatous polyposis (FAP) results from germline APC mutations, and approximately 80 percent of sporadic colorectal cancers carry APC mutations.
III. The Notch Signaling Pathway
Notch signaling is unique among major developmental pathways in that it requires direct cell-cell contact (juxtacrine signaling). The Notch receptor is a single-pass transmembrane protein (Notch1-4 in mammals), and its ligands -- Delta-like (Dll1, Dll3, Dll4) and Jagged (Jag1, Jag2) -- are transmembrane proteins on neighboring cells.
When a ligand on the signal-sending cell binds Notch on the receiving cell, it triggers two sequential proteolytic cleavages. The S2 cleavage is performed by an ADAM metalloprotease on the extracellular side, and the S3 cleavage is carried out by gamma-secretase (whose catalytic subunit is presenilin) within the membrane itself. This releases the Notch intracellular domain (NICD), which translocates to the nucleus and binds the transcription factor CSL/RBP-Jk. NICD converts CSL from a transcriptional repressor into an activator by recruiting the Mastermind-like (MAML) coactivator. Target genes include the Hes and Hey families of transcriptional repressors. Notch signaling functions in lateral inhibition (cell fate decisions), stem cell maintenance, and somitogenesis. Clinically, activating Notch mutations are found in T-cell acute lymphoblastic leukemia (T-ALL), and gamma-secretase inhibitors are being explored as potential therapies.
IV. The Hedgehog (Hh) Signaling Pathway
The Hedgehog pathway is essential for embryonic patterning, particularly in limb development and neural tube formation. When Hh signaling is OFF, the receptor Patched (Ptc) inhibits Smoothened (Smo), a 7TM protein. Without Smoothened activity, the Gli transcription factors are cleaved into a repressor form (Gli-R) that actively represses Hh target genes.
When Hh signaling is ON (through binding of Sonic hedgehog, Indian hedgehog, or Desert hedgehog), Hedgehog binds Patched and relieves its inhibition of Smoothened. Smoothened then becomes active and accumulates in the primary cilium. This stabilizes the full-length activator form of Gli (Gli-A), which enters the nucleus and activates target genes including Patched itself (providing negative feedback), Gli1, and cyclin D1.
Mutations in Patched that constitutively activate the pathway cause Gorlin syndrome, characterized by multiple basal cell carcinomas. Vismodegib, a Smoothened inhibitor, is used to treat advanced basal cell carcinoma. Loss of Shh signaling during development causes holoprosencephaly.
V. TGF-beta / Smad Pathway and NF-kB Pathway
The TGF-beta/Smad pathway uses a distinct receptor mechanism in which the TGF-beta superfamily ligands (TGF-beta, BMPs, activins, Nodal) bind to type I and type II serine/threonine kinase receptors. Ligand binding to the type II receptor leads to recruitment and phosphorylation of the type I receptor. The activated type I receptor then phosphorylates receptor-Smads (R-Smads): Smad2/3 for TGF-beta and activin signaling, or Smad1/5/8 for BMP signaling. The phosphorylated R-Smad binds the co-Smad (Smad4), and the complex translocates to the nucleus to activate or repress target genes. This pathway regulates growth inhibition, differentiation, apoptosis, ECM production, and immune function. Inhibitory Smads (Smad6 and Smad7) provide negative feedback. Loss of TGF-beta signaling components is common in cancer, with Smad4 mutations prominent in pancreatic cancer.
The NF-kB pathway is a key regulator of inflammation, immunity, and cell survival. NF-kB is a family of transcription factors (including p50, p65/RelA, RelB, c-Rel, and p52). In the canonical pathway, NF-kB is held in the cytoplasm by its inhibitor IkB in unstimulated cells. Stimuli such as TNF-alpha, IL-1, LPS, or viral infection activate the IKK (IkB kinase) complex, which phosphorylates IkB, marking it for ubiquitination and proteasomal degradation. Free NF-kB then translocates to the nucleus and activates target genes encoding cytokines (IL-6, TNF-alpha), anti-apoptotic proteins (Bcl-2, Bcl-xL), and IkB-alpha itself (providing negative feedback). Constitutive NF-kB activation is observed in many cancers.
<image>Comparison of the Wnt and Notch signaling pathways side by side. Panel A (Wnt OFF): Destruction complex (APC, Axin, GSK3-beta, CK1) phosphorylates beta-catenin, leading to its ubiquitination and proteasomal degradation; TCF/LEF with Groucho represses target genes. Panel A' (Wnt ON): Wnt binds Frizzled and LRP5/6, Dishevelled disrupts the destruction complex, beta-catenin accumulates and enters the nucleus to activate TCF/LEF target genes. Panel B (Notch): Delta ligand on the sending cell binds Notch receptor on the receiving cell; ADAM protease performs S2 cleavage; gamma-secretase performs S3 cleavage releasing NICD; NICD enters nucleus, binds CSL and MAML, activating Hes/Hey target genes.</image>
VI. Signal Integration and Cellular Decision-Making
Cells in a living organism never receive just one signal at a time. Instead, they constantly integrate multiple simultaneous inputs to determine their behavior. Combinatorial signaling means that different combinations of active pathways yield different cellular outcomes. The same signal, such as EGF, can drive proliferation, differentiation, or apoptosis depending on the cell type and the context of other signals present. A dramatic example comes from PC12 cells: EGF produces transient ERK activation and triggers proliferation, while NGF produces sustained ERK activation and drives differentiation.
Signal amplification through enzyme cascades ensures that even a small extracellular signal can produce a large intracellular response. Positive feedback reinforces signals and can create switch-like, bistable responses, as when ERK phosphorylates SOS to further activate the pathway. Negative feedback attenuates signals and prevents overshoot, as illustrated by ERK phosphorylating SOS to reduce Ras activation, NF-kB inducing IkB-alpha expression, and Wnt signaling inducing Axin2.
Crosstalk between pathways allows for sophisticated integration. For example, Akt can inhibit Raf, and GSK3-beta serves as a node where the Wnt and PI3K-Akt pathways converge. Ultimately, the balance and duration of signaling pathway activities determine cell fate decisions among proliferation, differentiation, survival, apoptosis, and senescence. These decisions are governed by threshold effects (sufficient signal strength is required to trigger a response) and spatial compartmentalization (signaling from the plasma membrane can produce different outcomes than signaling from an endosome).
